Prăvălie (2021). Exploring the multiple land degradation pathways across the planet

om et al. (2009)

for

delimiting the planetary boundaries.

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cover 75% of its surface), which makes it the second most heavily

affected continent in the world, after Australia (over 90% affected area

by aridity conditions).

A detailed analysis of absolute statistical data in the world

’

s most

heavily affected countries showed that Australia is the most highly

threatened by this environmental issue (with ~7 million km

2

, this state

accounts for 11% of global drylands) (

b). It is followed by China

(~5.3 million km

2

, 8%), the United States (~4.3 million km

2

, 6%),

Russia (~3.7 million km

2

, 5%), Kazakhstan (~2.7 million km

2

, 4%),

Algeria (~2.3 million km

2

, 3%) and Argentina (~2.2 million km

2

, 3%)

(

b). Together, the seven countries total about 40% of worldwide

drylands. Other states with extensive drylands, ranging between 1 and 2

million km

2

(that together account for another ~30% of global aridity),

are Saudi Arabia, Sudan, India, Canada, Libya, Iran, Mongolia, Mexico,

Chad, Mali, Brazil, Niger, South Africa and Mauritania (

b).

Regarding the percentage values (the absolute drylands area in

relation to national territories), the major aridity epicenters (

>

90% of

the total national territory affected) consist of Australia and numerous

countries located in northern, eastern and southern Africa, but also in

the Middle East and Central Asia (

c). Some of these states are

extreme hotspots of global aridity, with national territories that are fully

affected by dryness conditions. Globally, it was observed that an

impressive number of countries (41) are affected on more than 90% of

their territories by this land degradation pathway, while, continentally,

Africa and Asia hold by far the largest number of countries the drylands

of which exceed this percentage threshold (

d).

Aridity is therefore a restrictive environmental process of global

proportions that currently generates dryland conditions in 126 states

worldwide. While it may seem surprising, dryland systems can generate

positive effects for certain planetary systems/ecosystems. For the cli-

matic system, dryland systems can cool/stabilize the climate via dust

particles that can decrease incoming solar radiation (

Zhao et al., 2015

)

or through the remarkable carbon stock stored in dryland soils. In the

latter case, recent estimations showed that global drylands contain 470

Pg of organic C down to a depth of 1 m, which represents about a third of

the global soil organic C pool found in the same depth profile (

Plaza

et al., 2018

). Moreover, drylands hold huge amounts of inorganic C

(generally present as carbonates, which play a more limited role in soil

–

atmosphere carbon fluxes), which equate to 578 Pg C down to a depth of

1 m or to over 80% of worldwide soil inorganic C pool in the top one

meter layer (

Plaza et al., 2018

). Also, for certain ecological systems,

drylands provide crucial nutrients (e.g. iron or phosphorus) to marine (e.

g. Caribbean coral reefs or Atlantic phytoplankton) and terrestrial (e.g.

Amazon rainforests) ecosystems, through wind dispersion of dust

sourced from deserts worldwide (e.g. Sahara Desert, one of largest dust

sources of the planet) (

;

Mahowald et al., 2018

Jewell

).

Nonetheless, the presence of these extensive critical environments

affected by aridity, which in the past decades have expanded by almost

4% globally (

), is associated in most cases with a

wide range of environmental disturbances across the planet. For

instance, through water stress, aridity affects approximately 7.8 million

km

2

of global arable lands (~55% of the worldwide arable area of about

14 million km

2

), which is alarming considering both the extent of the

affected area and the fact that arable lands are a component of agri-

cultural systems of primary importance for global food security (

). Desertification or land degradation under aridity condi-

tions (

UNCCD, 1994

) is another negative environmental consequence,

which appears to affect, in one way or another, ~70% of global agri-

cultural drylands (

Huang et al., 2017

), possibly causing annual global

costs of up to 10% of agricultural gross domestic products (

Due to its spatial extent, desertification is a relevant issue in a global

context, considering certain sources that estimate that 10

–

20% of the

total drylands area is severely affected by degradation/desertification,

impacting 250 million people directly or about 1 billion people indi-

rectly, generally in developing countries (

Reynolds et al., 2007

). Other

analyses, conducted recently, indicate that areas with high and very

high desertification risk cover 7% and 9%, respectively, of the global

land area (

Huang et al., 2020

).

Desertification, a hot topic of land degradation, can take the form of

numerous degradation processes explored in this study, if land degra-

dative pathways act singularly or synergistically in arid, semi-arid and

dry sub-humid categories of worldwide drylands. For instance, vegeta-

tion loss or degradation (and the associated increase in bare ground),

driven by human-induced climate change and direct human activities, is

an important vector of desertification that affects many arid and semi-

arid areas of global drylands, such as southern Argentina, south-

western United States, Sahel and eastern Africa, Afghanistan,

Kazakhstan, Mongolia, northeastern China, or northwestern India (

). According to a very recent study, it ap-

pears that anthropogenic climate change has degraded more than 5

million km

2

of dryland vegetation between 1982 and 2015, thus

contributing to large-scale desertification and affecting more than 200

million people (especially in developing countries) across the world

’

s

drylands (

Burrell et al., 2020

Other important issues can emerge under aridity conditions.

Frequent dryland dust storms can cause considerable negative effects

not only on human health (e.g. via PM

10

and PM

2.5

particles, linked to

respiratory problems), but also on agricultural productivity (through

mechanical damage to crops), soil quality (soil erosion) or water quality

(contamination of drinking water supplies) (

Middleton and Sternberg,

2013

Martínez-Valderrama et al., 2020

Goudie, 2014

). The decline of groundwater resources is another

critical issue, as it was already signaled that the major global aquifers in

dryland areas, such as Northwestern India, North China Plain, Arabian,

Northern Middle East or California Central Valley aquifers, have recor-

ded high groundwater depletion rates over the past decades, under the

pressure of aridity and anthropic groundwater overexploitation (

;

Dangar et al., 2021

). Decoupling of numerous nutrients in drylands (e.g.

decreasing soil organic C and total N concentrations, amid declining

plant cover, and increasing inorganic P concentrations, due to the

amplification of the mechanical rock weathering process in drier con-

ditions), is another serious ecological consequence, which seems to

already be threatening the stoichiometry stability of dryland ecosystems

(

Delgado-Baquerizo et al., 2013

).

Exacerbating certain social issues, such as food insecurity, increase of

Fig. 2.

Global spatial pattern of aridity, approached as a major land degradation pathway across the Earth

’

s terrestrial area. Spatial representation of aridity

(drylands) across the globe (a), graphic distribution of aridity areas in the world

’

s most heavily affected states (b), spatial representation, expressed in percentages, of

world states affected by aridity (c) and graphic distribution of the number of states affected globally and continentally by aridity, at different percentage thresholds

(d). Note: in figure a, the aridity data were processed through the Global Aridity Index, which delimits drylands based on values under the 0.65 mm/mm threshold,

obtained as a ratio between mean multiannual precipitation and potential evapotranspiration values; in figure b, the world

’

s most heavily affected countries were

considered those that account, in total, for

>

95% of the global aridity area (countries with an area of at least 100,000 km

2

of drylands, which were ordered from

north to south, along the continents North America, South America, Europe, Africa, Asia, and Australia and Oceania); in figure c, percentages were computed by

relating absolute aridity areas to national territories; in figure d, the statistics of world states affected by aridity at various percentage thresholds were obtained based

on the number of states featured in figure c; abbreviations: W. Sahara

–

Western Sahara, CAR

–

Central African Republic, DRC

–

Democratic Republic of the Congo,

N

&

C America

–

North and Central America, S America

–

South America, Australia

&

Oc

–

Australia and Oceania; all the data processing was based on the global aridity

database (using the equal-area Mollweide projection, Central Meridian 0.00), which was computed at an original spatial resolution of approximately 1 km

×

1 km for

the 1950

–

2000 period (

Trabucco and Zomer, 2009

;

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poverty, human migration or war conflicts escalation, is another nega-

tive impact associated in many cases with the aridity pathway of land

degradation (

Kniveton et al., 2012

). The extent of these anthropogenic im-

balances is alarming, both at present, when over 2 billion people live in

global drylands, and especially for the decades to come, when the

human population may increase in these restrictive environments by

40

–

50% by 2050 (

Stavi et al., 2021

). This expected demographic

growth, coupled with the environmental degradation that will most

likely worsen in the coming years, will amplify socio-economic issues

such as poverty, land abandonment, unemployment, armed conflicts

and food insecurity, already present in many drylands of the world

(

Hoover et al., 2020

Chimwamurombe and Mataranyika, 2021

;

Stavi

).

Consequently, all these socio-economic and environmental distur-

bances will intensify by the end of the century, when more drier con-

ditions are expected across the globe (

). According to

some reliable estimations, by 2100, a dryland expansion of up to 56% of

the Earth

’

s terrestrial surface is expected, under the pessimistic Repre-

sentative Concentration Pathway (RCP) 8.5 scenario, or of up to 50%,

considering the relatively optimistic RCP4.5 scenario (

Huang et al.,

2016

). An important consequence of higher aridity will consist of the

intensification of desertification globally, which will have profound ef-

fects, among others, on the decline of carbon sequestration, a process

that is already occurring in numerous dryland areas of the globe (

Lal,

2019a

de Nijs and Cammeraat, 2020

3.2. Land erosion (by water)

Soil erosion by water, much more widely present globally than wind

erosion, is considered one of the most important forms of land degra-

dation, and is directly associated to this global environmental issue

(

Panagos et al., 2015a

;

;

Vanmaercke

). Soil erosion, which can consist of sheet, rill or gully erosion

(

Batista et al., 2019

;

Mushi et al., 2019

de Nijs and Cammeraat, 2020

;

Ou et al., 2021

Vanmaercke et al., 2021

), is a major land degradation

pathway through soil displacement from its original formation place,

amid aggressive water action (

Poesen, 2018

). Soil erosion by water is

currently a serious threat to soil health worldwide, due to the multitude

of negative effects it generates, such as reducing the water-holding ca-

pacity and infiltration rates, decreasing the content of organic matter

and nutrients, degrading soil structure or reducing soil depth and

fertility (

Ravi et al., 2010

Panagos et al., 2015b

;

;

Poesen, 2018

Sartori et al., 2019

). These ecological consequences are all

the more profound if soils are under the action of severe water erosion,

defined by rates that exceed the generic tolerable soil erosion threshold

of 10 t/ha/yr (

;

Sartori et al., 2019

The analysis of the spatial data of this major degradation process

(

), based on processing a geospatial database freely available at

global scale (

), indicated that severe soil erosion

(

>

10 t/ha/yr) covers ~10 million km

2

(~7% of the global terrestrial

area) worldwide. As it accounts for almost half of this affected global

area, Asia is by far the most heavily affected continent in terms of water

erosion

–

it features a total area with erosion rates

>

10 t/ha/yr more

than twice as large as the next affected continent, Africa (

a). As

expected, this continent holds the world

’

s most important hotspot of

severe water erosion, China, with highly eroded lands that total almost

1.5 million km

2

(

b) or 14% of the worldwide area affected by

intense erosion. It is followed by other remarkable hotspots, namely

Brazil (almost 1 million km

2

, 9%), Russia (~900,000 km

2

, 9%), the

United States (~760,000 km

2

, 7%) and India (~736,000 km

2

, 7%)

(

b). It is therefore remarkable that these five countries alone hold

about 47% of the world

’

s highly eroded lands.

Looking at heavily affected countries in terms of the area of severely

eroded lands in relation to national territories, it can be noticed that

tropical countries are generally the most intensively eroded countries in

the world, with over 20

–

30% of national territories affected by high

levels of soil erosion (

c). In extreme cases, these percentages reach

or even exceed the 60% threshold, as seen in the South Asian state,

Bangladesh (

c). It was generally found that most countries are

affected by high rates of water erosion that cover up to 10% of the na-

tional territory, except for those located in Africa and Asia, continents

with a high number of countries that are affected by high levels of soil

erosion to an extent that exceeds 30% (

d).

Seeing as this spatial analysis of water erosion was conducted using a

coarser database compared to the other four geospatial databases used

in this study, it must be mentioned that, in this instance, results may

overestimate, to certain degree, the spatial extent of this land degrada-

tion process. In fact, recent modelling based on processing global data at

an unprecedentedly high spatial resolution, one hundred times higher

than the data used in this paper (250

×

250 m, the data was not available

for this study), showed that the global extent of severe water erosion

totals about 7.5 million km

2

(

), which indeed con-

firms an overestimation of about 25% of the worldwide area affected by

high levels of soil erosion. However, the more general estimations ob-

tained in this instance are somewhat consistent with older scientific

sources that estimated the global extent of this process at 10.9 million

km

2

(

Oldeman, 1994

Middleton and Thomas, 1997

), but without

delimiting severe erosion based on the 10 t/ha/yr threshold (which is

why this comparison must be regarded with caution).

Regardless of the actual extent of this process, which is nearly

impossible to delimit accurately, considering it not only depends on the

spatial resolution of the data, but also on the range of models used for

erosion assessment (

Batista et al., 2019

), it is clear that water erosion is

an active environmental issue at planetary scale, according to plenty

international studies that confirm (directly or indirectly) the presence of

this process all over the world (

Haregeweyn et al., 2015

Rodrigo-Comino et al., 2020a

Maltsev and Yermolaev, 2020

;

Sannigrahi et al., 2020

Pimentel and Burgess, 2013

Wuepper

et al., 2020b

). In fact, for decades, this process has been considered the

most widespread form of land degradation (

;

However, this must be regarded with caution in the context of this

analysis, where aridity approached as a land degradation process is far

more extensive globally. Compared to water erosion, even aridity-

induced desertification (which can be associated more directly to the

concept of land degradation process) could be a more widespread

pathway of global land degradation, if considering the maximum limit of

the estimated extent of the process, i.e. 20% of total drylands (

Reynolds

et al., 2007

). This percentage indicates the fact that desertification could

cover up to ~12 million km

2

across the planet, if considering an older

estimation of the global drylands

’

area (~61 million km

2

), and up to

~13 million km

2

, if considering a recent estimation (~67 million km

2

)

(

).

Soil erosion is caused by several factors of either geomorphological

(e.g. steep slopes), climatic (e.g. rainfall erosivity) or anthropogenic

(deforestation, tillage, overgrazing or agricultural intensification) na-

ture (

;

). In many cases, the

occurrence and amplification of erosion are a consequence of the syn-

ergistic action of these factors, but anthropogenic factors generally play

a more important role in the large-scale intensification of erosion. The

role of anthropogenic factors in the dynamics of this process and other

important aspects of soil erosion (e.g. strategies necessary to fight this

major land degradation process) have been intensely studies over recent

years (

Keesstra et al., 2018b, 2019

;

a and Rodrigo-Comino, 2021

;

;

). For instance, it was reported that, between 2001

and 2012, water erosion increased by 2.5% globally, especially as a

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result of land use changes (particularly cropland expansion) (

Borrelli

et al., 2017

).

But climate change seems to have an increasingly apparent role in

the occurrence of this disruptive process, by changes in the amount,

intensity and spatio-temporal distribution patterns of rainfall values

across global terrestrial areas (

Li and Fang, 2016

). Statistically, in

certain regions of the planet (East Asia), it was found that every 1%

change in precipitation has triggered a 1.3% change in water discharge,

simultaneously with a 2% change in sediment loads (

Lu et al., 2013

).

While this sensitivity may vary significantly globally, there is a

consensus on the fact that rainfall is the most direct climatic factor that

influences soil erosion (

Li and Fang, 2016

). Climate change also in-

fluences water erosion patterns through changes in vegetation cover,

caused by climate warming. Since vegetation controls erosion by

intercepting precipitation, diminishing raindrop kinetic energy and

reducing surface runoff, any decrease in vegetation cover tends to

accelerate water erosion (

Li and Fang, 2016

). It seems that, over the past

decades, climate warming has decreased erosion at high latitudes, while

increasing it at middle and low latitudes (

Ma et al., 2021

), and simu-

lations show that climate change will significantly intensify global water

erosion during this century, especially in tropical and subtropical

countries (

Borrelli et al., 2020

Unsurprisingly, soil erosion has considerable implications for agri-

cultural systems. In terms of affected areas, recent analyses showed that

~30% of global arable lands (~4.3 million km

2

) are affected by severe

soil erosion (

). In terms of the total amount of soil

displaced through water (and wind) erosion, some sources suggest losses

of 75 Pg/yr of soil eroded from global arable lands, with associated

economic costs of US$400 billion annually (

Montanarella, 2015

). Other

estimations indicate smaller global cropland soil losses, of ~17 Pg/yr,

which represents however about half of the total predicted soil erosion

across the globe (~36 Pg/yr in 2012) (

). Regardless

of the actual annual rate of this process, it is clear that soil erosion

currently is a serious threat to global food security, and has the potential

to amplify socio-economic disruptions in many countries of the world by

decreasing agricultural yields, increasing food prices or by reducing

trade in agri-food commodities (

Sartori et al., 2019

Erosion acceleration also has implications on global biogeochemical

cycles, by modifying storage, distribution and flux rates of key elements

–

carbon, nitrogen or phosphorus (

Chappell et al., 2016

;

Berhe et al.,

Martínez-Mena et al., 2020

). Changes in soil organic carbon have

attracted much attention lately, considering that the impact of soil

erosion on the terrestrial carbon cycle is still the subject of heated de-

bates (

Amundson et al., 2015

;

Doetterl et al., 2016

Sanderman and Berhe, 2017

Lugato et al., 2016

;

Wang et al., 2017

Borrelli et al., 2018

;

Naipal et al., 2018

Lal, 2019b

), as studying these changes is extremely

important for a clearer understanding of their effects on climate change.

Considering that the carbon distributed laterally by soil erosion can lead

to an effective carbon burial in some depositional environments, but at

the same time to atmospheric leakage, if the eroded carbon is largely

oxidized (

Amundson et al., 2015

), divergent scientific opinions on the

net effect of erosion in carbon sinks or sources have so far suggested

dynamics of approximately 1 Pg C/yr globally in both instances

(

Doetterl et al., 2016

). While it is difficult to draw a conclusion with

regard to the current net impact of erosion in climate change dynamics,

future developments seem more likely to feature net losses of soil car-

bon, amid erosion and other synergistic pressures onto soils, such as

climate warming (

Crowther et al., 2016

3.3. Salinization

Another widespread form of land degradation, recognized as a major

pathway of land degradation across the planet, is soil salinization (

Ivushkin et al., 2019

Kaushal et al.,

). This important environmental disruption causes significant lim-

itations to the ecological/agroecological productivity of lands, through

the direct negative effects associated to phytotoxicity, as a result of high

concentrations of dissolved salts (Ca

2

+

, Mg

2

+

, K

+

, Na

+

, Cl

−

and others)

in the soil solution (

Herbert et al., 2015

;

Munns and Gilliham, 2015

;

Payen et al., 2016

Minhas et al., 2020a

). Other indirect effects of sali-

nization, which in most cases limit plant growth, are connected to the

degradation of soil structure, the decrease of low organic matter content,

biodiversity loss (except for salt tolerant plants), reduction of water

infiltration capacity or higher difficulty in water uptake by plants, in

more intense saline conditions (

;

). The negative effects of soil

salinization depend in agricultural environments on the plants

’

various

tolerance levels to salinity (

) and can occur throughout the

world through primary (natural accumulation of salts in the environ-

ment) or secondary (occurrence or increase of salt concentrations in the

environment, due to anthropogenic activities) salinization (

With a worldwide spatial coverage of almost 2.6 million km

2

, which

equates to ~2% of the Earth

’

s terrestrial surface, soil salinization is

therefore another global-scale land degradation process (

Although the process is present on all continents, Asia is the most

heavily affected in this case as well, totaling ~1.3 million km

2

of sali-

nized lands or approximately 50% of the global area affected by sali-

nization (

a). In terms of percentages, Australia is however the most

heavily affected continent by salinization

–

6% of the continent

’

s total

area is covered by salinized lands, vs 3% for Asia. At the same time, it is

the world

’

s most heavily affected country

–

with almost 0.5 million km

2

,

this state accounts for 19% of global soils affected by salinization

(

b). Three other major hotspots of worldwide salinization are

Kazakhstan (~319,000 km

2

, 12%), China (~307,000 km

2

, 12%) and

Iran (~300,000 km

2

, 12%) (

b). In this context, it is remarkable that

these four major epicenters alone total 55% of global salinization.

By relating absolute salinization values to national territories, it can

be noticed that Iran is the world

’

s most salinized country, with 19% of

its area affected by this environmental issue (

c). Extreme salini-

zation, which can be defined as that affecting

>

10% of the national

territory, is also present in other Middle Eastern and Central Asian

countries, such as Iraq, United Arab Emirates and Kazakhstan (

c).

Other intense salinization hotspots, in terms of percentages (affecting

between 6 and 10% of the national territory), include Hungary, Somalia,

Fig. 3.

Global spatial pattern of water erosion, approached as a major land degradation pathway across the Earth

’

s terrestrial area. Spatial representation of water

erosion across the globe (a), graphic distribution of water erosion areas in the world

’

s most heavily affected states (b), spatial representation, expressed in per-

centages, of world states affected by water erosion (c) and graphic distribution of the number of states affected globally and continentally by water erosion, at

different percentage thresholds (d). Note: in figure a, the water erosion data were mapped and processed considering the

>

10 t/ha/yr threshold, which indicates

severe soil erosion across the globe; in figure b, the world

’

s most heavily affected countries were considered those that account, in total, for

>

95% of the global area

of severe water erosion (countries with an area of at least 20,000 km

2

of severe soil erosion by water, which were ordered from north to south, along the continents

North America, South America, Europe, Africa, Asia, and Australia and Oceania); in figure c, percentages were computed by relating absolute severe erosion areas to

national territories; in figure d, the statistics of world states affected by severe erosion at various percentage thresholds were obtained based on the number of states

featured in figure c; abbreviations: DRC

–

Democratic Republic of the Congo, N

&

C America

–

North and Central America, S America

–

South America, Australia

&

Oc

–

Australia and Oceania; all the data processing was based on the global water erosion database (using the equal-area Mollweide projection, Central Meridian 0.00),

which is freely available at an spatial resolution of approximately 25 km

×

25 km for the 2012 year (

https://esdac.jrc.ec.europa.eu/content/global-soil-erosion

, the data were collected using the online

platform

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Kenya, Turkmenistan, Kuwait, Qatar and Australia (

c). While most

countries worldwide have salinized soils on up to 2% of their territories,

numerous states in Asia and, in part, in Africa are however affected to a

greater extent by this major pathway of land degradation (

d).

Soil salinization is therefore an important environmental pressure

due to its considerable extent even in the database used in this study,

which indicates however a smaller spatial footprint of lands affected by

salinization, compared to other scientific sources. For instance, ac-

cording to a recent study, the total extent of soil salinization is estimated

at approximately 1 billion ha (or 10 million km

2

) globally (

Ivushkin

et al., 2019

). These figures do however include all salinization classes

(slight, moderate, high and extreme salinity of soils), as opposed to the

data processed in the present paper, which is based only on selecting

soils with high concentrations of soluble salts. Therefore, the saliniza-

tion data used in this study do not include the slight and moderate

salinity of soils, which cover by far the largest share of the roughly 1

billion ha of soils affected by salinization globally, estimated in the

previously mentioned study (

Ivushkin et al., 2019

) or in other recent

sources (

). In other words, the analysed soil salini-

zation data (

) only correspond to intense salinization processes,

assessed based on saline (Solonchaks, soils with high concentration of

soluble salts at some point in the year) and sodic (Solonetz, soils with a

dominance of Na

+

in the soil exchange complex and other specific

physical and chemical properties) soils (

FAO, 2006

), and on salt-covered

areas (far less extensive worldwide than saline and sodic soils).

Although soil salinization can have local causes, such as seawater

intrusion in many coastal areas due to the current sea level rise (

;

),

the spatial soil salinization pattern (

a) generally corresponds to

that of aridity (

a). Therefore, soil salinization is an issue typical of

global drylands (

).

The presence of this process in aridity conditions (particularly in arid

and semi-arid climatic areas) is considered irreversible, considering the

insufficient availability of fresh water in dryland systems for leaching

any accumulated salts (

Rozema and Flowers, 2008

). Over the past de-

cades, the issue of salinization in dryland areas was enhanced by the

intensification of irrigation, which lead to the accumulation of salt

(above normal concentrations) within the root zone of arable lands,

amid high rates of evapotranspiration that determined the displacement

of soluble salts from deep layers to top soil layers (

Therefore, saline soils generally occur in drylands, where the domi-

nance of evapotranspiration over precipitation determines an excessive

accumulation of salts in the root zone (

Hassani et al., 2020

). Some recent

global analyses confirmed that soil salinization is an acute ecological

imbalance in arid environments, with constantly severe values in

countries such as Australia, China, Kazakhstan or Iran (

Hassani et al.,

;

Perri et al., 2020

), which is in agreement with this paper

’

s

cartographic results. However, a very interesting recent study, focusing

on global salinization trends, showed that, after 1980, the countries with

the highest rates of annual increase in soil salinity area are Brazil

(

>

5000 km

2

/yr), Peru (

>

2000 km

2

/yr), Sudan (

>

2000 km

2

/yr),

Colombia (

>

2000 km

2

/yr) and Namibia (almost 1500 km

2

/yr), along-

side five other states that recoded the largest positive trends in soil

sodicity, namely Iran (about 3500 km

2

/yr), Saudi Arabia (

>

2000 km

2

/

yr), Argentina (

>

2000 km

2

/yr), Afghanistan (almost 1500 km

2

/yr) and

the United States (

>

1000 km

2

/yr) (

Hassani et al., 2020

). It is therefore

surprising that countries such as Brazil and Colombia, with limited

drylands across their national territories, have become significantly

vulnerable to soil salinization since 1980.

Salinization is a process that has mixed causes (natural and anthro-

pogenic) across the planet, the anthropogenic forces of which are linked

to the use of irrigation and other human activities, such as land cover/

use changes. For instance, it was found that, in Argentina, the replace-

ment of native grasslands with tree plantations has amplified soil sali-

nization in the Pampas landscapes, due to the high groundwater uptake

by tree roots, which favored the storage of more salts than grasslands

throughout the unsaturated soil profile (from groundwater level to the

surface) (

Nosetto et al., 2013

). In Central Asia, at the border between

Kazakhstan and Uzbekistan, salinization intensification due to the Aral

Sea drying is widely known, largely as a result of expanding croplands

and irrigation more than five decades ago, which drastically decreased

the discharge from the Syr Darya and Amu Darya tributary rivers (and

thus enhanced salinization conditions) (

;

). Globally, the magnitude of salinization pres-

ence in irrigated areas is alarming

–

it seems that more than 20% of the

global irrigated area is currently affected by salinization (and water-

logging), an issue that may expand to over 50% by 2050 if the necessary

measures to fight the twin disruptive processes are not implemented

(

).

The use of intensive and long-term irrigation, based on groundwater

extraction from saline aquifers in areas without/with limited drainage,

are, alongside favoring natural factors (geological composition,

seawater intrusion or low rainfall and high evapotranspiration values),

the main driving force of salinization in Iran, the main global epicenter

of this environmental issue (

Nabiollahi et al., 2017

Emadodin et al.,

2019

). The substantial expansion of agriculture, irrigation, and the

removal of perennial native vegetation across Australia (in Western

Australia or in the Murray-Darling Basin), which generated notable

changes in groundwater levels (rising of saline water table), is another

relevant example of how anthropogenic actions influence and enhance

salinization worldwide (

Soil salinization negatively affects agriculture globally, and causes

important losses in agricultural yields. It is estimated that almost 0.3

million km

2

(~2%) of global arable lands are affected by soil salinization

(high concentrations of soluble salts) (

), an area that

can however be much larger globally if also considering slight and

moderate soil salinity. Moreover, it is estimated that the total cost of salt-

induced land degradation in irrigated lands reaches approximately US

$27 billion at global scale annually, as a result of significant losses in

crop production (

Qadir et al., 2014

). In terms of percentages, agricul-

tural losses caused by salinization reach 18

–

43% across arid and semi-

arid landscapes of the world (

).

As salinization is associated with decreases in plant productivity and

biomass, it is clear that the effects are projected into soil organic carbon

storage decline and, implicitly, into climate change acceleration.

Following this reasoning, losing soil carbon could already be an

Fig. 4.

Global spatial pattern of soil salinization, approached as a major land degradation pathway across the Earth

’

s terrestrial area. Spatial representation of soil

salinization across the globe (a), graphic distribution of soil salinization areas in the world

’

s most heavily affected states (b), spatial representation, expressed in

percentages, of world states affected by soil salinization (c) and graphic distribution of the number of states affected globally and continentally by soil salinization, at

different percentage thresholds (d). Note: in figure a, the soil salinization data were mapped by extracting Solonchaks, Solonetzs and salt-covered areas across the

globe; in figure b, the world

’

s most heavily affected countries were considered those that account, in total, for

>

95% of the global soil salinization area (countries

with an area of at least 5,000 km

2

of soil salinization, which were ordered from north to south, along the continents North America, South America, Europe, Africa,

Asia, and Australia and Oceania); in figure c, percentages were computed by relating absolute soil salinization areas to national territories; in figure d, the statistics of

world states affected by soil salinization at various percentage thresholds were obtained based on the number of states featured in figure c; abbreviations: UAE

–

United Arab Emirates, N

&

C America

–

North and Central America, S America

–

South America, Australia

&

Oc

–

Australia and Oceania, soil salin.

–

soil salinization; all

the data processing was based on the global soil salinization database (using the equal-area Mollweide projection, Central Meridian 0.00), which is freely available in

vector format from FAO Digital Soil Map of the World, for the 2007 year (

FAO, 2007

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important pressure for climatic stability, considering the current global

expansion rate of soil salinization (

Ivushkin et al., 2019

), estimated at 2

million ha (20,000 km

2

) per year (

Abbas et al., 2013

). Considering the

expansion of salinization, which will most likely continue throughout

this century amid the expected intensification of global aridity, some

studies suggested world soils may lose 6.8 Pg C due to salinization by

2100 (

Setia et al., 2013

3.4. Soil organic carbon loss

The loss of soil organic carbon (SOC) is another important indicator

of land degradation (

;

). SOC is a

fundamental element of soil organic matter (SOM, which is a complex

matter consisting of plant and/or animal organic materials in various

stages of decomposition), which accounts for approximately 55

–

60% of

its mass (

;

Simpson and Simpson, 2012

). Since SOM is a key attribute of soils

that influences many physical, chemical or biological properties of soils,

such as soil structure, water retention, nutrient storage, nutrient cycling,

or soil biodiversity and biological activity (

;

Paustian et al., 2016

;

;

ogel-Knabner and Amelung, 2021

),

any losses in SOC alter the role of these beneficial properties associated

with SOM. Under these conditions, SOC decline can determine, to a

considerable extent, the perturbation of the soils

’

structural stability,

water holding capacity, water infiltration, nutrient retention, soil

biodiversity, soil fertility and, finally, of their ecological and agro-

ecological productivity (

;

). At the same time, SOC losses are an acceleration vector for

climate change, since soil carbon is a major global carbon pool

–

numerous estimations indicate a worldwide stock of ~1500 Pg C (which

equates to twice the amount of atmospheric carbon and approximately

three times the amount of carbon present in terrestrial vegetation) in the

top 1 m of soil (

Stockmann et al., 2013

Scharlemann et al., 2014

Lal,

), from which ~750 Pg C (~50%) is stored in the first 30 cm of the

soil (

Plaza et al., 2018

).

This important land degradation indicator can therefore be quanti-

fied based on the extent of the affected area. According to recent data,

sourced from

and processed in the present study

(

), there are ~2.7 million km

2

(almost 2% of the global land area)

of lands globally that have recorded decreases in SOC surface (at 0-30

cm depth, a relevant profile for analysis, since it is directly exposed to

anthropogenic pressures), since the beginning of the 21st century

(2001

–

2015). While decreases occurred on all continents (albeit to a less

extent in Australia and Oceania) (

a), only six states can be

considered major epicentres of SOC losses

–

Brazil (~412,000 km

2

or

15% of the global area affected by SOC decline), China (~300,000 km

2

,

11%), Russia (~289,000 km

2

, 11%), the United States (~210,000 km

2

,

8%), Canada (~115,000 km

2

, 4%) and, in part, Indonesia (~83,000

km

2

, 3%) (

b). Essentially, these countries account for over half of

the global area affected by SOC losses, after the year 2001.

Nevertheless, relating absolute areas of SOC losses to national areas

only singles out Brazil and Indonesia (of the six previously mentioned

states) as countries that are relatively affected by this important form of

land degradation (

c). Conversely, European countries are among

the most heavily affected by these percentage-based decreases (espe-

cially in the continent

’

s central and north-western regions, where

certain states recorded SOC losses across over 15

–

20% of their national

territories), except for Singapore, located in Asia, which holds the

largest affected area globally

–

60% of its national territory (

c).

Overall, it can be noticed that, both globally and continentally, most

countries are affected on up to 4% of their territories by this land

degradation process, with the notable exception of Europe, where

numerous countries exceed this threshold (

d).

It is generally known that the primary driver of global SOC

decreasing trends consists of land use changes. Land conversion from

natural ecosystems into croplands is the major pathway of SOC losses,

considering that cultivated soils can lose up to 50% or even more of the

SOC (

Zomer et al., 2017

;

). This significant loss is caused by

the reduction of carbon inputs in soil, by diminishing plant litter

(considering that a large fraction of cultivated plants is harvested for

human/animal use, but also that the biomass volume of agricultural

crops can be inferior to that of the original natural vegetation), and by

increasing carbon outputs, as a result of ploughing activities (

van der

Esch et al., 2017

;

). Historically, it appears that, in the past

two centuries, land use changes and unsustainable land management

practices have caused global losses of up to 176 Pg C, mainly from

agricultural topsoils (first 30 cm) (

Currently, some of the highest SOC losses induced by land use

changes are being recorded in tropical regions (Brazil and Indonesia are

the main hotspots), where the average carbon loss (due to rainforest

deforestation and cropland expansion) for each converted hectare may

be nearly twice as high compared to temperate regions (

West et al.,

2010

). In addition to agricultural expansion in tropical areas, which is a

major pathway of land use/cover changes (and, most likely, a significant

cause of SOC decreases) in this region of the planet (

Hansen et al., 2013

;

Keenan et al., 2015

), other possible common drivers of SOC losses can be

linked to urbanization in temperate zone (e.g. in temperate regions of

the United States or China) (

Seto et al., 2012

) or forestry/logging in

boreal environments (in boreal forests of Canada or Russia) (

). Other land conversion pathways, such

as the conversion of grasslands into croplands in numerous areas glob-

ally (e.g. in certain parts of the United States, Mongolia or China)

(

), may also cause significant carbon losses in the soil.

In terms of vertical profiles, the top meter of global soils is the most

heavily affected by land use changes (it is estimated that over 75% of the

topsoil SOC is directly affected by land use activities) (

Harden et al.,

), and the top 30-cm layer is even more vulnerable to these

anthropogenic pressures. By 2050 it is expected that global SOC losses

will reach 65 Pg C, due to land use changes, poor land management

practices, peatlands drainage and burning, and climate warming (

IPBES,

). In this scenario, these losses are expected to affect global agri-

cultural productivity on a spatial scale largely exceeding the current

one, when it is estimated that SOC losses affect ~0.5 million km

2

(

>

3%)

of global arable lands (

). As SOC is a crucial indi-

cator of soil nutrient availability and of soil fertility and productivity

(

Billings et al., 2021

), a possible amplification of ecological (land

degradation) and social (food insecurity, human migration, land

Fig. 5.

Global spatial pattern of soil organic carbon (SOC) loss, approached as a major land degradation pathway across the Earth

’

s terrestrial area. Spatial rep-

resentation of the SOC decline across the globe (a), graphic distribution of SOC decline areas in the world

’

s most heavily affected states (b), spatial representation,

expressed in percentages, of world states affected by SOC decline (c) and graphic distribution of the number of states affected globally and continentally by SOC

decline (loss), at different percentage thresholds (d). Note: in figure a, the SOC decline data were obtained based on negative statistically significant trends processed

from global interannual series of SOC stock; in figure b, the world

’

s most heavily affected countries were considered those that account, in total, for

>

95% of the

global SOC decline area (countries with an area of at least 4,000 km

2

of SOC decline, which were ordered from north to south, along the continents North America,

South America, Europe, Africa, Asia, and Australia and Oceania); in figure c, percentages were computed by relating absolute SOC decline areas to national ter-

ritories; in figure d, the statistics of world states affected by SOC decline at various percentage thresholds were obtained based on the number of states featured in

figure c; abbreviations: DRC

–

Democratic Republic of the Congo, PNG

–

Papua New Guinea, N

&

C America

–

North and Central America, S America

–

South America,

Australia

&

Oc

–

Australia and Oceania; all the data processing was based on the global SOC trend database (using the equal-area Mollweide projection, Central

Meridian 0.00), which was computed at a spatial resolution of approximately 1 km

×

1 km for the 2001

–

2015 period (

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abandonment, rural depopulation, heightened poverty) imbalances is

foreseeable, if considerable decreases occur in the C pool of the world

’

soils over the next decades.

Other important causes, such as erosion and climate warming, can

contribute to significant global SOC changes. Recent estimations showed

that, after 1850, global water erosion has removed 74 Pg C from the soil,

predominantly in agricultural and grassland systems (

Naipal et al.,

). Also, while other estimations indicated a current gross SOC

displacement by water erosion of up to 2.5 Pg C/yr globally (

Borrelli

et al., 2017

Borrelli et al., 2018

), there are notable national differences

and high eroded SOC rates across extensive areas, generally in the

countries identified in the present study as major hotspots of SOC losses.

Moreover, recent studies highlight the fact that wind erosion can also

play an important role in SOC losses in many countries worldwide

(

). Although all these erosion-induced SOC changes

can influence climate change, as already mentioned in the water erosion

section, the net impact of soil erosion on the climate system is still

uncertain.

Climate warming can also be a major threat to the SOC pool, due to

the imbalance it creates in the main carbon input (plant carbon pro-

duction generated from photosynthesis) and the main carbon output

(respiratory losses resulted from microbial decomposition of SOM) in

the soil (

Davidson, 2016

). While the impact of climate warming on this

balance is at present still limited/relatively limited, over the following

decades the effects of temperature increases are expected to determine

large-scale SOC losses especially in the planetary regions holding the

largest carbon stocks, currently concentrated at high latitudes in the

Northern Hemisphere (

Crowther et al., 2016

Blaikie and Brookfield, 2015

Davidson, 2016

). It ap-

pears that, by 2050, climate warming will generate a global SOC decline

of at least 30 Pg C in the upper soil horizons (largely affecting permafrost

soils), with potentially substantial consequences for positive soil car-

bon

–

climate feedbacks that can accelerate global climate change

(

Crowther et al., 2016

).

3.5. Vegetation degradation

Vegetation degradation/decline can be considered another major

pathway of land degradation, given its diverse ecological consequences,

global spatial footprint, and the effects it generates in climate system

dynamics. As vegetation is an essential component of lands (

;

), any form of alteration of this biotic system can be

interpreted as an important pathway of land degradation. Consequently,

any form of vegetation ecological damage or loss (processes that will be

approached broadly as vegetation decline/degradation) must be inter-

preted as critical in assessing land degradation, considering that it can

cause considerable effects on land productivity decrease, biodiversity

decline and the disruption of biogeochemistry of land ecosystems (

Zhao

and Running, 2010

Ivits et al., 2014, 2016

;

Schlesinger et al., 2016

;

).

The negative effects associated with vegetation degradation are all

the more serious given the fact this land degradation pathway is present

all over the world. By investigating reliable data, purchased from a

global vegetation trend database (

Ivits et al., 2016

) and processed in

terms of spatial extent (

), it was found that the worldwide area

affected by vegetation decline totals ~6.5 million km

2

or over 4% of the

global terrestrial surface. With a transcontinental presence, which is

more pronounced in South America (almost 1.8 million km

2

of lands

affected by vegetation degradation, which equates to over a quarter of

the global affected area) (

a), this process considerably affects many

countries worldwide. The detailed analysis of national data indicated

four states largely affected by plant biomass degradation (across abso-

lute areas of

>

0.5 million km

2

, which equates to 40% of worldwide

vegetation degradation), namely Argentina (~820,000 km

2

, 13% din

worldwide vegetation decline area), Brazil (~648,000 km

2

, 10%),

Australia (~588,000 km

2

, 9%) and the United States (~538,000 km

2

,

8%) (

b). Other considerably affected countries (with areas that

range between 0.2 and 0.4 million km

2

or between 3 and 6% of the

global affected area) are Russia, Kazakhstan, China, Mexico, Mongolia

and Zambia (

b).

However, with the notable exceptions of Argentina and Zambia,

other states stand out as national vegetation degradation hotspots in

terms of percentages (

c). They are mainly located in the southern

region of South America (with 48% of its national territory affected by

vegetation degradation, Uruguay seems to be the world

’

s most heavily

affected country) and in eastern and south-eastern Africa (

c). This

land degradation process affects most countries on up to 5% of their

national territories, except, in part, for Africa, where a considerable

number of states see higher percentages (

d).

While vegetation degradation can occur through various pathways

globally, such as forest die-off/dieback (

Allen et al., 2010, 2015

Bennett

et al., 2015

), vegetation fires (

Brando et al., 2014

Jolly et al., 2015

) or

plant net primary productivity decrease (

Zhao and Running, 2010

Chen

et al., 2013

), in this analysed case, the changes (decline) in vegetation

biomass are essentially a response of ecosystems to long-term drought

conditions (

Ivits et al., 2016

). Due to water and/or heat stress, this

response can occur through different pathways in various regions across

the world, such as biomass loss or the limitation of vegetation growth,

decreasing photosynthesis rates or devitalization/withering of vegeta-

tion (

Zhao and Running, 2010

Anderegg et al., 2013

;

). These vegetation degradation forms can

therefore be deemed to be typical of the vegetation decline areas map-

ped globally (

).

Various studies confirm biomass degradation in the vegetation

decline hotspots identified worldwide. For instance, the relatively recent

effects of long droughts on vegetation degradation in the Chaco and

Pampas regions of Argentina were well documented, especially in the

case of the intense drought recorded in 2008

–

2009 (connected to the

2008

–

2009 La Ni

˜

na event), which appears to be one of the country

’

s

worst in the past century (

Barros et al., 2015

;

Naumann et al., 2019

Other representative and documented examples of drought-induced

vegetation decline, featured in the global vegetation trend database

Fig. 6.

Global spatial pattern of vegetation decline (degradation), approached as a major land degradation pathway across the Earth

’

s terrestrial area. Spatial

representation of vegetation decline across the globe (a), graphic distribution of vegetation decline areas in the world

’

s most heavily affected states (b), spatial

representation, expressed in percentages, of world states affected by vegetation decline (c) and graphic distribution of the number of states affected globally and

continentally by vegetation decline, at different percentage thresholds (d). Note: in figure a, the vegetation decline data were derived by correlating FAPAR (Fraction

of Absorbed Photosynthetically Active Radiation, a reliable indicator of plant ecological productivity) anomalies with the negative values of SPEI (Standardized

Precipitation and Evaporation Index, a suggestive indicator of drought events), and the negative statistically significant trends resulting from the correlations

indicated decline/degradation of biomass in drought-affected areas; in figure b, the world

’

s most heavily affected countries were considered those that account, in

total, for

>

95% of the global vegetation decline area (countries with an area of at least 10,000 km

2

of vegetation decline, which were ordered from north to south,

along the continents North America, South America, Europe, Africa, Asia, and Australia and Oceania); in figure c, percentages were computed by relating absolute

vegetation decline areas to national territories; in figure d, the statistics of world states affected by vegetation decline at various percentage thresholds were obtained

based on the number of states featured in figure c; abbreviations: DRC

–

Democratic Republic of the Congo, PNG

–

Papua New Guinea, N

&

C America

–

North and

Central America, S America

–

South America, Australia

&

Oc

–

Australia and Oceania, veg. decl.

–

vegetation decline; all the data processing was based on the global

vegetation trend database (using the equal-area Mollweide projection, Central Meridian 0.00), which was computed at an original spatial resolution of approximately

8 km

×

8 km for the 1982

–

2011 period (

Ivits et al., 2016

).

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(

a), include areas located at the border between the United States

and Mexico (

Allen et al., 2010

), Central Asia (especially border areas

between Kazakhstan and Russia) (

de Beurs et al., 2015

Horion et al.,

2016

) or north-eastern China (

Pei et al., 2018

Global studies based on remote sensing data (NDVI) also signaled

some of the global hotspots in vegetation degradation (

a) (e.g.

Argentina or Central Asia), although these satellite-based monitoring

studies indicate an overall vegetation greening at global scale, and thus

an improvement in vegetation quality over the past decades, possibly

due to certain major driving factors like atmospheric CO

2

fertilization

(

Ding et al., 2020

). Nevertheless, other studies based on similar satellite

observations recently reported contrasting browning (degradation)

trends, which dominated the global dynamics of the ecological state of

vegetation especially after the year 2000, most likely due to intensified

drought stress (

Pan et al., 2018

). Also, agro-ecosystems in numerous

regions worldwide seem to be vulnerable, to a certain extent, to the

degradation of agricultural vegetation

–

recent estimations showed that

drought conditions affect worldwide arable landscapes across ~1.2

million km

2

, which equates to more than 8% of global arable lands

(

).

The danger associated to drought-induced vegetation degradation is

all the greater considering that this biotic disturbance increases the

susceptibility of certain ecosystems to other disturbances such as fires or

insect outbreaks (

Seidl et al., 2017

), which widens the specter of vege-

tation degradation and, implicitly, that of land degradation pathways.

For instance, forest ecosystems in western North America were affected

after 1980 by a considerable increase in tree mortality rates, amid

synergistic effects of more intense droughts and higher temperature

stress, but also of escalating insect outbreaks (bark beetle outbreaks) and

wildfires, which were facilitated by the intensification of drought and

heat conditions in the past decades (

;

).

Another telling example consists of the Amazonian forests, affected

extensively by intense droughts in the past decade (e.g. major droughts

in 2010 and 2015), which created favorable conditions for widespread

fires in many parts of the Amazon region (

). Another highly suggestive example consists of the

widespread fires of 1997

–

1998, during the El Ni

˜

no-induced drought

event, which devastated extensive areas of tropical forests (in Indonesia)

and generated forest fire emissions of over 40% of the global anthro-

pogenic carbon emissions recorded in this time interval (

).

It seems that many global areas have become more vulnerable to

vegetation fires over the past decades, due to the complex interaction

between drought, dryness and anthropogenic (human ignitions, land

clearance, urbanization and other causes) conditions (

van Wees et al., 2021

). With a mean global

area affected annually of approximately 4

–

5 million km

2

(400

–

500

million hectares), vegetation fires can trigger or amplify numerous land

degradation pathways in various regions globally, even though this

ecological process plays an important role in the Earth system

’

s func-

tionality via biogeochemical processes (

Bowman et al., 2020

). Accord-

ing to an interesting review paper, land degradation effects associated

with vegetation fires can include soil erosion intensification, aquatic

system eutrophication (as a result of the increase in fluvial sediment and

nutrient transport

–

nitrogen, phosphorus

–

in lakes, for example, which

can trigger algal blooms) or permafrost thawing (

Bowman et al., 2020

).

The consequences of vegetation fires also have a direct anthropogenic

dimension, affecting life, human health and property, and triggering

major economic costs in numerous regions worldwide, such as the

United States (California), Australia or Indonesia

–

e.g. the 2017

–

2018

fires in California alone caused immense economic damages, estimated

at US$40 billion (

Bowman et al., 2020

There are a series of uncertainties regarding the response of vege-

tation to future climate change, linked to certain important variables

that are difficult to estimate, such as ecosystem resilience to distur-

bances, the possible interference of other environmental stressors and

even climate dynamics over the following decades. In the latter case, it is

however highly likely that the acceleration of drier and hotter condi-

tions (through changes in drought and aridity patterns) will continue

throughout this century in numerous regions worldwide (

;

), which will determine further

vegetation decline and land degradation. In this scenario, any form of

vegetation degradation in the following decade will not only alter the

state of lands, but also the Earth

’

s climatic stability, considering that

terrestrial vegetation is a major global carbon pool

–

it stores about

610Pg C above ground, an amount of carbon that is almost comparable

to that present in the atmosphere (~750Pg C) (

Petrokofsky et al., 2012

;

Pütz et al., 2014

). This potential danger for climate stability was

confirmed during the major 2010 drought in South America that caused

losses of 2.2 Pg C in the Amazonian forest vegetation, which equates to

the impressive share of 22% of global annual carbon emissions (

). More recent negative implications of vegetation degradation for

the climate system can be highlighted by the major forest fires of 2019 in

the Amazon, Australia and Indonesia, which accounted for large

amounts of carbon dioxide emissions at global scale (

Folke et al., 2021

4. Other important land degradation pathways

As previously mentioned, approximately 12 other land degradation

pathways can currently be identified, which can synthetically be called

biological invasions, coastal erosion, land erosion (by wind), land

pollution, land subsidence, landslides, permafrost thawing, soil acidifi-

cation, soil biodiversity loss, soil compaction, soil sealing and water-

logging (

Fig. 1

). These multiple land degradation forms occur in diverse

regions worldwide and were intensely studied in numerous scientific

papers, but generally in a singular and isolated manner. Therefore, as

scientific papers that approach all these multidimensional pathways of

land degradation simultaneously are still rare/lacking internationally,

this study also briefly assesses these mechanisms that deteriorate land

quality in numerous regions of the world. While this analysis completes

the full picture of global land degradation, it does so in a very brief

manner, considering the secondary priority attributed to these twelve

processes compared to the five land degradation processes that were

selected and analysed extensively considering a series of relevant

criteria, previously mentioned in the present paper.

4.1. Biological invasions

These are intentional or unintentional introductions of exotic (non-

native) species in a new environment, through several stages of the in-

vasion process, namely transport, establishment, spread and impact

(

Diez et al., 2012

). This ecological process can negatively affect all three

components of lands

–

soils (e.g. via invasive earthworms) (

Jochum

Liebhold et al., 2017a, 2017b

), vegetation (e.g. via tree insect pests) (

Boyd et al., 2013

)

and inland waters (e.g. via toxic microorganisms) (

Sukenik et al., 2015

),

and this is the reason why biological invasions can be in and of them-

selves considered a land degradation pathway. Of the three terrestrial

systems, vegetation is probably the most visible affected component of

lands.

The Earth

’

s forest ecosystems are particularly relevant and note-

worthy in the context of biological invasions, considering certain

extremely important characteristics such as the role of major hotspot of

terrestrial biodiversity or holding the highest share of the planet

’

s total

plant biomass (

). Therefore, these attributes and other key

attributes of forest ecosystems can be disturbed globally, since global

forests are particularly affected by biological invasions with species

from nearly every taxon, including woody and herbaceous plants, ver-

tebrates, invertebrates or microorganisms (such as fungi or bacteria)

(

Morin and Liebhold, 2015

;

;

Krivak-Tetley et al., 2021

). Therefore, the

presence of all these categories of non-native species in forest environ-

ments affects the productivity, stability and economic value of forests

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around the globe.

Soils are also affected by many invasive species (

), which

are however less visible (in the belowground environment) compared to

biological invasions of forests and inland waters. In this case, special

attention was given to earthworm invasions, which seem to be among

the most widely studied groups of soil invasive organisms across the

globe (

Xiang et al., 2015

;

Coyle et al., 2017

Ferlian et al., 2018, 2020

;

Frelich et al., 2019

Phillips et al., 2019

). Generally, all these exempli-

fied studies indicated either directly or indirectly that earthworm in-

vasions are an increasing threat to soils worldwide, due to multiple

disruptive ecological effects in soil processes, soil properties and plant

communities (

).

In addition to soils, biological invasions also account for a series of

negative ecological consequences in inland waters, the quality of which

is altered around the world by this disturbance (

;

The invasive species issue in worldwide forests or in other terrestrial

environments (soils, inland waters) was exacerbated over the past de-

cades amid the intensification of trade, transport, globalization (

Essl

et al., 2015

;

Faulkner et al., 2016

Seebens et al., 2021

) and climate

change (

Diez et al., 2012

;

Hulme, 2017

;

Pecl et al., 2017

). These two

categories of factors contributed to the biotic homogenization of eco-

systems worldwide (

Tabarelli et al., 2012

;

Folke et al., 2021

), the

degradation of ecosystem services (

;

), including the ecosystems

’

capacity to store carbon

(

Seidl et al., 2018

Folke et al., 2021

), decreasing soil and water quality

(

Paerl and Paul, 2012

;

Frelich et al., 2019

) and finally to the deterio-

ration of the general state of lands throughout the world (

).

These environmental disturbances will most likely intensify and expand

spatially by mid-21st century, as recent modeling shows future consid-

erable developments of biological invasions across the continents, for a

large number of taxonomic groups (

Seebens et al., 2021

4.2. Coastal erosion

This is a form of land degradation along coastal areas, which consists

of a long-term loss of sediments and rocks due to natural (e.g. waves,

currents, tides or storms) and anthropic (e.g. building dams and irriga-

tion systems that change the flux of sediments and coastal morphody-

namics or clearing of coastal ecosystems, like mangrove forests) factors

(

Mentaschi et al., 2018

). Although coastal erosion can be considered

borderline a land degradation pathway, as it generally implies a

smaller/much smaller removal of soils compared to sediments and rocks

along coastal lands, certain reliable sources (

Olsson et al., 2019

) include

this process in the specter of land degradation, which is why it was also

featured in the present study. Consequently, coastal erosion can be

considered a degradative process, which affects the interference area

between land and seas/oceans.

Coastal erosion affects various coastal landforms worldwide, and

some of the most heavily affected are, for instance, sandy coasts (

Lui-

jendijk et al., 2018

Le Cozannet et al., 2019

) and river deltas (

), which were massively subjected to the

previously mentioned mixed natural and anthropogenic influences.

Overall, it was found that, in the past three decades (1984

–

2015), the

eroded land along the global coastline totaled about 28,000 km

2

, which

is twice the area of gained land (14,000 km

2

) through accretion (

Men-

taschi et al., 2018

). Surprisingly, in the case of deltas, it seems that over

the past three decades there has been a less intense effect of coastal

erosion, considering that many deltas have experienced land area gains

due to some positive effects associated to tidal influence (in the redis-

tribution and consolidation of deltaic sediment and in slightly gaining

land area) in tide-dominated deltas (unlike wave- and river-dominated

deltas that are more vulnerable to erosion) (

Nienhuis et al., 2020

).

Additionally, even if river damming drastically reduced sediment loads

from rivers in numerous deltaic regions, in some cases it appears that net

anthropogenic effects were positive, through human-driven soil erosion

(mainly via deforestation) which caused even more than 50% increase in

sediment fluxes in some deltas of the world (

Nienhuis et al., 2020

However, there is a general consensus on the fact that erosion is

generally intensifying across the global coastlines (

Luijendijk et al.,

Mentaschi et al., 2018

Williams et al., 2018

;

Smith and Barnard, 2021

), with

negative environmental effects in most cases, although certain partic-

ular ecological benefits of this process can also be brought up

–

for

instance, the erosion of the Eurasian coastlines represents a major input

of nutrients (e.g. nitrogen) in the Arctic Ocean, which is vital for the

ocean ecosystem productivity (

Terhaar et al., 2021

). The coastal erosion

threat is all the greater since coastal zones across the world are heavily

populated and highly developed (

Hallegatte et al., 2013

), with 15 of the

20 megacities worldwide (

>

10 million people) located in coastal areas

(

Luijendijk et al., 2018

). In the future, the global coastline will undergo

even greater pressure of severe erosion, according to a prediction that

indicates that the combined action of the aforementioned natural/

anthropogenic factors and of sea level rise could determine, for instance,

the loss of almost half of the worldwide sandy coasts (beaches) by the

end of the century (

Vousdoukas et al., 2020

). The costs associated to the

aggravation of this process along global coastlines are immense

–

for

instance, through the decline in coastal ecosystem services, coastal

erosion will cause in Europe alone possible annual losses of up to 42

billion euros (

Paprotny et al., 2021

). By the end of the century, studies

confirm that direct anthropogenic activities and especially climate

change (with an increasing influence over the next decades) will

continue to escalate this problem of coastal zones (

Hinkel et al., 2013

;

Reimann et al., 2018

;

Paprotny et al., 2021

), which may in turn amplify

climate change through positive feedbacks, triggered by permafrost

eroding (in the Arctic coastlines) and carbon releasing into the atmo-

sphere (

Wegner et al., 2015

;

Tanski et al., 2019

;

Jong et al., 2020

).

4.3. Land erosion (by wind)

It is a severe environmental issue that occurs especially in global

drylands (

), where three environmental conditions

generally acknowledged to be driving forces of wind erosion (process

that moves coarse/fine particles across various landscapes by wind

power) are present

–

high wind speed, soil characteristics that are highly

susceptible to erosion, and a general lack of protective natural vegeta-

tion (

Borrelli et al., 2016

;

Jarrah et al., 2020

). Recent modeling showed

that the intensity and extent of wind erosion reach peak values in

numerous deserts (components of drylands) worldwide, such as the

Great Basin, Mojave (the United States), Atacama (Chile), Patagonia,

Monte (Argentina), Sahara (Northern African countries), Namib, Kala-

hari (Namibia, Angola, South Africa, Botswana), Arabian (countries of

the Arabian Peninsula, and partially Iraq and Jordan) and Syrian (Syria,

Iraq and Jordan) deserts, Karakum (Turkmenistan), Kyzylkum (Uzbe-

kistan, Kazakhstan), Taklamakan (China), Gobi (China, Mongolia) or

Australian deserts (

;

Borrelli et al., 2021

). In many

parts of these desert regions, the mean wind erosion largely exceeds the

1 t/ha/yr threshold (

).

Numerous national studies indicate the same global hotspots of wind

erosion across the continents. For instance, in North America it is widely

known that the United States are historically vulnerable to wind erosion,

considering the extreme wind erosion episode called Dust Bowl, which

occurred in the 1930s and was triggered by the simultaneous action of

certain climatic, socio-economic and agricultural driving forces (

Lee and

Gill, 2015

). In Africa, wind erosion is an acute issue not only in the

countries from Sahara Desert (the world

’

s most intensely wind-eroded

region), but also in other continental countries dominated to a lesser

extent by desert landscapes, such as the ones in Africa

’

s eastern region

(

Fenta et al., 2020

). Another example is Asia, with significant aeolian

erosion rates in Central (e.g. Uzbekistan or Turkmenistan) (

Li et al.,

) or Eastern (e.g. China) Asia (

Chi et al., 2019

A particular major problem of this process in dryland systems is

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linked to sand and especially dust storms, which appear to be directly

affecting over 150 countries worldwide (

Middleton and Kang, 2017

).

While sand storms can generate a negative impact, it is generally limited

in terms of spatial extent, unlike dust storms which are far more prob-

lematic, considering that finer dust particles can be transported thou-

sands of kilometers from the source, in many cases across international

boundaries (

Middleton and Kang, 2017

). It is estimated that global dust

emissions from deserts range from 1 to 3 billion tons annually, Sahara

contributing (mostly through the Bod

´

el

´

e Depression) probably 50% of

this total and generating dust transport over thousands of kilometers to

Europe, Americas and Asia (

Goudie, 2008

Middleton and Sternberg, 2013

Middleton, 2017

). Anthro-

pogenic dust emissions due to land use/cover changes or urbanization

can also accelerate the impact of global dust emissions (

), which is generally considered to be

negative, although dust particles can also generate beneficial effects on a

planetary scale by contributing to the overall functioning of the Earth

system (

Jewell et al., 2021

).

In a more general perspective, there are multiple effects associated

with intense wind erosion in drylands. Many negative effects are linked

to the wind-blown sands and dusts and include damage to natural

vegetation and crops by abrasion, disease transmission (eroded particles

can contain a large variety of microorganisms that can cause diseases to

plants, animals and people), air pollution (atmospheric particulate

matter that can cause, among others, human respiratory diseases),

drinking-water contamination, disruptions of power and transportation

infrastructure, soil carbon losses or some important climate feedbacks

(

Goudie, 2014

Gherboudj et al., 2017

;

Zhao et al., 2017

;

;

). Essentially, erosive winds are a major vector of soil fertility

decrease, agricultural decline and desertification in many drylands of

the globe (

IPCC, 2019

), and these issues will probably accentuate

throughout this century, considering the aggravation potential of wind

erosion under the effects of future warming (

Ma et al., 2021

4.4. Land pollution

This is another land degradation vector which, through various

anthropogenic driving forces (agricultural, industrial, mining activities

etc.) that introduce harmful substances into the environment, can affect

land quality in a number of different ways across the world. For instance,

soil pollution is a complex issue that can occur in many regions world-

wide through the considerable/excessive accumulation of substances in

the soil, such as fertilizers (nitrogen, phosphorus) (

Sun et al., 2012

;

Bruulsema, 2018

), pesticides (insecticides, fungicides, herbicides)

(

UNCCD, 2017

Tang et al., 2021

), heavy metals (cadmium, mercury and

others) (

Liu et al., 2021

), radionuclides (

90

Sr,

137

Cs and

other radioactive isotopes) (

Lelieveld et al., 2012

;

),

persistent organic pollutants (polychlorinated biphenyls, DDT and many

other toxic substances that were or are still being used in some parts of

the world) (

van den Berg, 2009

), plastic and microplastics

(plastic fragments

<

5 mm in size) (

Lau et al., 2020

;

Rochman and

Hoellein, 2020

) or electronic waste (which contain a variety of toxic

substances, like heavy metals, polychlorinated biphenyls, poly-

brominated biphenyls and others) (

).

Many of the substances mentioned above, as well as others, also

affect many freshwater systems (lakes, rivers) worldwide (

), which, in the context of

heightened anthropogenic pollution and progressive climate warming,

have become more vulnerable to certain forms of secondary pollution,

such as pollution with toxic microorganisms (e.g. cyanobacteria devel-

opment in phosphorus-enriched water environments) (

). Also, vegetation (e.g. forest ecosystems) can

be affected to a considerable extent by certain forms of regional or local

pollution, through certain pathways such as atmospheric nitrogen

deposition (

Sicard et al., 2016

;

Du et al., 2019

) or contamination with

radionuclides (e.g. radiocesium) resulting from nuclear weapons tests

(

) or the major nuclear accidents Chernobyl and

Fukushima (

Koarashi et al., 2016

alie and Bandoc, 2018

The exemplified pollutants and others affect not only land systems

themselves (soils, vegetation and inland water resources), but also the

interference area between the land and sea, which consists of coastal

ecosystems. Studies conducted in recent years showed that numerous

coastal areas are currently threatened (in addition to climate change) by

a wide range of pollutants, which have the highest concentrations in

Eastern Atlantic (Western Europe) and Western Pacific (East Asia) (

Lu

et al., 2018

). Moreover, pollution is an environmental disruption that is

threatening land health and productivity worldwide not only latitudi-

nally and longitudinally, but also vertically, considering that many

inland water resources are currently vulnerable to groundwater pollu-

tion (e.g. due to mining activities), which can affect surface waters (

Lall

et al., 2020

The negative consequences of land degradation through the broad

spectrum of land pollution are complex and include various ecosystem

damages, eutrophication, biodiversity decline, changes in biogeochem-

ical cycles, and an overall degradation of ecosystem services (

;

;

;

). All these categories of

negative effects have been studied in thousands of scientific papers,

most of which were conducted locally. Therefore, global studies tackling

in one way of another the negative ecological consequences of the

various forms of land pollution are generally rare, and worldwide

cartographic representations of this land degradation dimension are

even more rare/non-existent. For instance, there is currently still no

global map of soil pollution pathways (

Temmerman and Kirwan, 2015

), even though it is

urgently needed for a spatial understanding of pollution (through all its

relevant forms of manifestation) in relation with other soil degradation

issues, or for ensuring a more efficient protection of soils against

pollution throughout the world. However, there is some promising

progress in terms of understanding certain particular pathways of global

land pollution

–

for instance, a very recent study identified, quantified

and mapped the global risk of pesticide pollution for the first time,

suggesting that ~31% of worldwide agricultural land is currently at high

risk of pesticide pollution (

Tang et al., 2021

).

4.5. Land subsidence

The process essentially refers to the sinking of land as a result of soil

compaction (

) and can be briefly defined

as a vertical downward displacement of the ground surface (

Kok and

Costa, 2021

). The causes of this environmental process are both natural

and anthropogenic. Natural causes of land subsidence are linked to

climate change (e.g. the direct effect of droughts on groundwater

decrease, hydrodynamics that can lead to subsidence), tectonic subsi-

dence, karst processes or permafrost thawing (

;

Anthropogenic causes are however more important, according to a

global study that recently confirmed that human-induced land subsi-

dence is predominant around the world (

Bagheri-Gavkosh et al., 2021

Bagheri-Gavkosh et al., 2021

They include groundwater, gas and oil extraction, coal mining,

anthropogenic drainage or urbanization (

;

). Groundwater overexploitation is the main driving force of land

subsidence worldwide (

Kok and Costa,

) and can be caused by excessive water extraction for agricultural

irrigation (

Jeanne et al., 2019

) or for urban water use (

Bierkens and

Wada, 2019

). This cause has become increasingly prominent over the

past decades, as groundwater extraction has intensified dramatically

due to world population growth and to the expansion of irrigated agri-

culture (

Bierkens and Wada, 2019

).

In recent years, the land subsidence process has drawn attention

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especially in deltaic regions worldwide, as deltas are highly susceptible

to land subsidence and densely populated areas (

Hallegatte et al., 2013

)

–

it is estimated that over half a billion people live on deltas, a significant

share of which in megacities (

Giosan et al., 2014

Higgins et al., 2014

).

The presence of this issue in these coastal landforms is due to the high

natural subsidence rates of deltas, but especially to human activities that

exacerbate the sinking of these areas by considerably/drastically

reducing sediment loads from rivers (by building dams or irrigation

channels), or by accelerating soil compaction through subsurface mining

(of groundwater, gas or oil) (

Syvitski et al., 2009

Tessler et al., 2015

). In

the first case, notable examples of sediment trapping due to dams and/or

irrigation systems include the Nile (98% smaller sediment loads), Indus

(94%), Rh

ˆ

one (85%), Mississippi (69%) or Danube (60%) deltas (

Giosan

et al., 2014

). In the second case, relevant examples of subsurface mining

and accelerated compaction are the Po delta in Italy (which sunk 3

–

5 m

in the 20th century, mainly due to methane extraction) (

Giosan et al.,

2014

), Chao Phraya delta in Thailand (with subsidence rates of 5

–

15

cm/yr due to intense groundwater use) (

Syvitski et al., 2009

), Yellow

delta in China (with recent subsidence rates of up to 25 cm/yr in certain

areas, mainly as a result of groundwater extraction) (

Higgins et al.,

2013

), or other important cases across the world (

Higgins, 2016

Climate change will most likely aggravate this process, for instance

through longer droughts that will accelerate domestic, agriculture or

industry water demand, which will, in turn, result in the increasing of

groundwater depletion (and implicitly in land subsidence) in many

cases, both in deltaic and non-deltaic areas worldwide (

Hu et al., 2019

;

Minderhoud et al., 2020

). In deltaic landforms, subsidence and the

presence of other vulnerability factors, such as a generally low altimetry

or high rates of sea-level rise (

Syvitski et al., 2009

), will determine an

enhancement in land degradative conditions, especially by increasing

exposure to coastal flooding. The perspective of accelerated coastal

flooding due to land surface subsidence and sea-level rise is alarming,

considering the massive populations present in these coastal areas,

currently estimated (in 2017) at 339 million people living in global river

deltas alone (

Edmonds et al., 2020

).

4.6. Landslides

These are another land degradation pathway, which generally occurs

locally in many parts of the world (

Kirschbaum et al., 2015

Froude and

Petley, 2018

). This process occurs on unstable slopes and consists of the

detachment and downslope movement (under the influence of gravity)

of a mixed mass (rock, various debris and/or soil), as a consequence of

some natural (intense rainfall, seismic activity or vegetation cover loss

after a fire event) and/or anthropogenic (deforestation, removal of

vegetation cover or slope cutting by transport infrastructure) triggering

factors (

Kirschbaum and Stanley, 2018

Reichenbach et al., 2018

). Although this

process generally has a limited spatial extent in the affected regions,

specialized studies showed that landslides are present on all continents,

especially in hilly and mountainous regions (

Kirschbaum and Stanley, 2018

More specifically, recent global studies based on susceptibility

models showed that about 3% of the worldwide land area is very highly

susceptible to landslides, with major landslide susceptibility hotspots

mainly grouped along the great mountain ranges, such as Himalaya,

Caucasus, Alps, Andes, the Rocky Mountains and others (

Stanley and

Kirschbaum, 2017

;

). Although, at pre-

sent, this form of land degradation is probably most heavily influenced

by direct human disturbances (

Li and Mo, 2019

), future climate change

may have a significantly stronger influence (through the increase in the

number of severe rainfall events or through other mechanisms such as

ice and mountain snow melting or permafrost thawing) on the occur-

rence of landslide events across the world (

Gariano and Guzzetti, 2016

;

Turetsky et al., 2019

Wood et al., 2020

). In fact, through rainfall pat-

terns determined by ocean-atmosphere teleconnections (e.g. El Ni

˜

no

Southern Oscillation), climate change seems to already be playing an

important role in triggering landslides in many parts of the world

(

Emberson et al., 2021

den Eeckhaut et al., 2012

Given the context that landslides generate annually thousands of

fatalities at global scale (

Haque et al., 2019

), it is not surprising this

hazard has been given heightened attention in recent years, in many

studies conducted from global/continental (

Günther et al., 2013

;

van

;

) to national/local scales (

;

Zhang and Huang, 2018

;

). However,

the negative ecological effects of this process are also noteworthy. While

less widely studied compared to socio-economic effects, the ecological

effects of landslides can be important and are linked to the creation of

degraded depositional environments, SOC losses, removal or displace-

ment of upper fertile soil layers, habitat loss and degradation for soil

biota, or the reduction of soil biodiversity and its overall functioning

(

4.7. Permafrost thawing

This process can lead to land degradation through frozen soil

thawing in boreal and high-altitude (mountains) regions, which gener-

ates significant negative environmental effects, such as the reduction of

vegetation cover, higher exposure of soil to erosion or releasing large

amounts of carbon into the atmosphere (

). This land degradation issue is extensively

present in the Northern Hemisphere (especially at high latitudes), where

permafrost covers broad regions (almost 23 million km

2

, of which

approximately 11 million km

2

consist of continuous permafrost, with

high carbon content) in Eurasia (for the most part) and North America

(

Zhang et al., 2008

). In addition to their immense extent, the even

greater importance of these northern frozen soils resides in the fact that

permafrost soils store the largest amount of terrestrial organic carbon on

the planet, which is estimated up to almost 1600 Pg C (

Schuur et al.,

2015

Yedoma permafrost thawing in northeastern Siberia and Central

Alaska seems to currently be the most dangerous form of this land

degradation pathway, as the region, with an area of over 1 million km

2

and with a massive and deep carbon pool (estimated at ~500 Pg C)

(

Zimov et al., 2006

Strauss et al., 2017

), has started to destabilize,

especially due to accelerated climate warming in the Arctic region

(

IPCC, 2013

). Besides the threat to critical infrastructure (e.g. oil and gas

pipelines) (

Hjort et al., 2018

), the major danger of permafrost thawing

in the Yedoma region and in other frozen regions in the Northern

Hemisphere consists of the release of large amounts of CO

2

and espe-

cially CH

4

, gases with a strong greenhouse effect that can significantly

amplify climate warming over the course of the following decades

(

;

;

;

Permafrost is already thawing at a global scale, as permafrost ground

temperatures increased by 0.29 (

±

0.12)

◦

C globally, in only one decade

(2007

–

2016) (

Biskaborn et al., 2019

). But the major concerns are linked

to the massive permafrost destabilization in the coming decades (due to

climate warming and Arctic Amplification), which will cause 23

–

174 Pg

C emissions from permafrost soils by 2100, under the RCP 8.5 scenario,

or 6

–

33 Pg C, under the RCP 4.5 scenario, thus contributing (via positive

feedbacks) to the acceleration of climate warming in this century (

An-

thony et al., 2018

). In a secondary manner, soil degradation and possible

feedbacks in climate warming also result from permafrost thawing at

lower latitudes such as in the Tibetan Plateau, which holds the largest

permafrost area in China and in the mid- and low-latitude regions of the

globe (

). This lower

latitude permafrost carbon pool, which currently contains 37 Pg C, is

also vulnerable to climate warming by 2100, and could release up to

almost 4 Pg C or nearly 2 Pg C under scenarios RCP 8.5 or RCP 4.5,

respectively (

Wang et al., 2020

).

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Permafrost destabilization is not only problematic in terms of global

warming, but also in terms of actual environmental degradation.

Permafrost thawing can trigger landslides, rapid erosion or the degra-

dation/removal of forest vegetation (due to the flooding of large areas of

trees) (

Turetsky et al., 2019

), thus contributing to the amplification of

other land degradation processes at high latitudes in the Northern

Hemisphere. The only beneficial short-term consequence of this process

is probably linked to the enhancement of the groundwater discharge to

rivers and thus to streamflow increasing in some high latitude regions,

such as the Ob, Yenisei and Lena river basins, although even this positive

effect will probably be offset in the medium and long term by the

intensification of evapotranspiration due to future warming (

Wang

et al., 2021a

). The disturbing effects of permafrost destabilization will

be frequent especially in unstable areas with abrupt permafrost thawing

(which can affect several meters of permafrost soil within days, weeks or

several months/years, as opposed to gradual thawing, which affects soils

much slower, by a few centimeters over years or decades), which is

estimated will occur in this century in ~20% of frozen soils located at

high latitudes (

Turetsky et al., 2020

). Other unexpected consequences of

permafrost thawing are linked to the release of notable amounts of

mercury into the atmosphere (

Schaefer et al., 2020

), keeping in mind

that, surprisingly, permafrost regions contain nearly twice as much Hg

as the rest of all other soils of the planet, the atmosphere, and even ocean

combined (

Schuster et al., 2018

4.8. Soil acidification

This is a process that occurs in regions with high precipitation and

leaching conditions, which consists of accumulating hydrogen and

aluminum cations in the soil and/or leaching base cations such as cal-

cium, magnesium, potassium and sodium from the soil (

;

). In addition to natural causes, soil acidification can be a

process that is triggered/amplified by anthropogenic activities, for

instance through acid deposition as a consequence of acid rain, which

occurs when certain anthropogenic gases such as sulphur dioxide and

nitrogen oxides (NO

x

) react in the atmosphere with precipitation water

(

Liang et al., 2013

;

Lu et al., 2014

). Essentially, the pH values of topsoil

lower than 7 are acidic, but only the soils below the 5.5 threshold are in

most cases unsuitable for agricultural crops (

)

–

it is esti-

mated that roughly half of the worldwide potentially arable soils are

acidic (

Kochian et al., 2015

;

UNCCD, 2017

). Moreover, soils below the

4.5 pH threshold are classified as severely acidic and cover extensive

areas globally, such as the Labrador peninsula, Amazon region, Congo

Basin, Scandinavian peninsula or vast Siberian regions (

Delgado-Baquerizo et al., 2019

).

While soil acidification can be triggered under natural conditions

(

), anthropogenic soil acidification has

become (as early as the beginning of the Industrial Revolution) an

increasingly important cause of this environmental issue, via sulphur

and nitrogen atmospheric deposition (

). Over the past de-

cades, a decrease in nitrogen and particularly in sulphur deposition has

been however noticed especially in North America and Europe, as a

result of atmospheric pollution control policies implemented in these

regions (

Greaver et al., 2012

;

Vet et al., 2014

;

Tan et al., 2018

).

Nevertheless, the emission and further deposition of nitrogen remains

(alongside the usage of N fertilizers for crop growth) one of the main

causes of soil acidification worldwide (

Wang et al., 2021b

), as global

nitrogen deposition increased over the past decades (in contrast with

sulphur deposition, which decreased globally), especially in East Asia

(mainly China) and South Asia (India), where NO

x

emissions have

remained high (

Tan et al., 2018

). Historically, it seems that nitrogen

deposition has reduced, on average, the soil pH by 0.24

–

0.26 units on a

global scale (

Tian and Niu, 2015

Meng et al., 2019

), or by up to

0.49

–

0.63 units in some terrestrial (grassland) ecosystems (

).

The consequences of soil acidification are alarming, especially given

the fact that ~50% of global arable landscapes are acidic (

Wang et al.,

2021b

). The major negative effects of this process include a lower

availability of soil inorganic nutrients, high toxicity for plants (which

results from the dissolved aluminum into the soil solution), disruption of

soil microorganism activity, reducing soil biodiversity and an overall

decline of soil fertility and productivity (

;

Soil acidification can also lead to the hindrance of leaf litter decompo-

sition, a vital ecological process for humus formation, nutrient cycling

and for the biogeochemical cycles of carbon (

). Although, surprisingly, there are some studies that suggest

this process could have certain beneficial ecological effects by consoli-

dating SOC in some croplands of the world (in China), by reducing

organic matter decomposition and increasing SOC protection against

mineralization (

Zhang et al., 2020

), it appears this conclusion is

complicated or even erroneous, and that soil acidification does not, in

fact, increase SOC accumulation and fertility of soils (

Kuzyakov et al.,

4.9. Soil biodiversity loss

Soil biodiversity decline can be considered another land degradation

pathway, which essentially consists of reducing the diversity of organ-

isms in the soil (bacteria, fungi or invertebrates), amid the local- to

global-scale action of multiple stressors (

). It is

generally acknowledged that the soil biota plays a crucial ecological role

in maintaining multiple ecosystem functions and services (multi-

functionality), through the high degree of diversity of bacteria (

Delgado-

Baquerizo et al., 2016

), fungi (

Tedersoo et al., 2014

) or earthworms

(

Phillips et al., 2019

). Essentially, soil organisms enhance multi-

functionality across the globe (e.g. through nutrient cycling, litter

decomposition, primary production or climate regulation) (

Delgado-

Baquerizo et al., 2016

Phillips et al., 2019

), thus generating direct

benefits for humans as well, such as influencing the quality of air, water

and food (

Wall et al., 2015

At present, a wide range of environmental threats impact soil-living

communities (and, implicitly, ecosystem multifunctionality) and are, in

many cases, connected to multidimensional forms of land degradation

(

Delgado-Baquerizo et al., 2019

). For instance, while soil acidification can be an

important constraining factor for the diversity of soil organisms in many

circumstances (

), many other land

degradation pathways, such as soil pollution, biological invasions or

even soil erosion, seem to be affecting/reducing soil biodiversity (via

various mechanisms, such as through the toxicity of pesticides or of

other chemical stressors in the case of soil pollution) across numerous

regions worldwide (

Beaumelle et al., 2021

;

;

lanyo and Jiangfeng, 2021

). Overall, planetary areas with the highest

soil biodiversity loss risk are, as expected, the ones with the greatest

exposure to various human activities, which determine serious threats

(in terms of intensity, variety and spatial extent) to soil biodiversity

stability across the world (

).

All major threats to soil biodiversity were very recently synthesized

in a highly complex global analysis (

), according to which soil

biodiversity is currently under the pressure of several main threats

(resulting from direct or indirect effects of anthropogenic and climatic

pressures) that act in various ecoregions worldwide, such as tropical

regions (where threats are linked to deforestation, agricultural intensi-

fication, SOC losses, fires or erosion), dryland environments (SOC losses,

salinization, fires, climate change or desertification), temperate zones

(urbanization, agricultural intensification, SOC losses, invasive species

or fires), tundra (climate change, SOC losses or invasive species) or

boreal (deforestation) regions (

). These major threats are

confirmed, in one way or another, by numerous studies conducted

continentally (e.g. Europe) or globally (

;

which highlighted the fact that global and regional changes put

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considerable pressures on soil biodiversity and its ecological

functionality.

4.10. Soil compaction

This is currently an important land degradation issue throughout the

world, and is caused by several compressive forces, such as wheels,

tillage machinery or excessive animal trampling on grazing lands (

;

). Essentially, this process causes an increase of

soil bulk density and/or a decrease in soil porosity, as air is removed

from the pores of soil grains (

Beylich et al., 2010

Smith et al., 2016

).

This densification of the soil/subsoil material significantly reduces long-

term productivity of soils and is considered among the most persistent

pathways of land degradation, with durations that range from decades to

centuries (

Batey, 2009

UNCCD, 2017

).

Globally, very little is known about the extent of this degradation

process. Estimations dating back about three decades indicated that soil

compaction could be affecting approximately 68 million ha (i.e. under 1

million km

2

) worldwide (

Oldeman et al., 1991

). It is very likely that this

estimated value is still relevant (considering the persistent nature of this

process) or even considerably higher presently (

Lamand

e et al., 2018

).

However, regionally (e.g. across Europe), there is more data available on

the spatial impact of this environmental process (

Lamand

e et al., 2018

).

Europe appears to be a major hotspot of soil compaction worldwide, as it

is estimated that this process impacts about 33 million ha in the region,

mostly in Eastern Europe (

Oldeman et al., 1991

;

Alaoui and Diserens,

).

Soil compaction strongly affects many croplands (

Shah et al., 2017

),

grasslands (

) or even forestlands (

Cambi et al., 2015

), and is

therefore present in many terrestrial environments. Generally, in all

environments affected by this degradation process there is a wide range

of ecological consequences. The multiple negative effects of the process

include disrupting the soil pore system by decreasing the macropores

(

Lamand

e et al., 2018

), reducing the water supply and oxygen to plants

and soil microorganisms (

Cambi et al., 2015

), altering soil properties

(

Batey, 2009

), disrupting soil biota and soil biological processes (

Beylich

et al., 2010

), reducing agricultural productivity (

Stoessel et al., 2018

),

increasing floods (

Alaoui et al., 2018

) or exacerbating other land

degradation processes, such as erosion (

Alaoui et al., 2018

), which can

be amplified by increasing surface runoff conditions (

The magnitude of these transformations is considerable

–

for

instance, crop yield losses due to soil compaction are estimated to reach

20

–

25%, but can exceed 50% if the soil is affected simultaneously by

compaction and other degradation processes (

Correa et al., 2019

). These

decreases in agricultural yields are explained by the reduction of crop

root growth and development, as a result of various stress conditions in

compacted soils, like mechanical impedance to penetration, low levels

of water and nutrient supply or low oxygen availability (

Correa et al.,

2019

). All the aforementioned effects, as well as others, were studied in

depth simultaneously in complex specialized studies (

;

;

),

which confirmed that compaction is an important threat to soil health

and productivity worldwide.

4.11. Soil sealing

It consists of permanently covering the soil surface with a persistent

and impermeable material (

Gardi et al., 2015

;

Smith et al., 2016

). This

process, which has major negative effects on soil quality, can have

various natural causes, such as raindrop impact, rapid wetting of dry

soil, soil crusting, soil compaction or deposition of a fine and imper-

meable material to the soil surface (

;

). However, anthropogenic causes are

far more important as they entail extensive and permanent artificial

sealing conditions (with asphalt, concrete, metal and other materials),

primarily as a consequence of urbanization and infrastructure devel-

opment (

Li et al., 2018b

).

Soil sealing is therefore a major form of land degradation in urban-

ized areas worldwide (

Abd-Elmabod et al., 2019

), where soils are also

affected by other degradation processes simultaneously, such as pollu-

tion, acidification or compaction (

Li et al., 2018b

;

Rodríguez-Espinosa

). Over the past decades, this process has expanded at an

alarming rate especially in states experiencing dramatic urbanization

after 2001, namely China, the United States, Nigeria, India, Indonesia,

Russia or Mexico (

Sun et al., 2020

). Europe is also a major global hotspot

of this process, with a soil sealing rate of approximately 1000 km

2

every

year (while estimated over the 1990

–

2000 period, the rate remained

stable around the value of 900 km

2

per year after 2000), especially due

to excessive urbanization (

Fini et al., 2017

Ronchi et al., 2019

). When

soils are sealed in urban regions, the dramatic effects are ecological (e.g.

isolation/destruction of entire belowground ecosystems) (

Pennisi,

), but also socio-economic (flooding increases, heat island inten-

sification or higher water contamination risk in urban areas) (

Pereira

et al., 2020

Global urban expansion will continue aggressively in the coming

decades (

Chen et al., 2020

), which will cause a profound impact on soil

sealing. For instance, it is highly likely that global urbanized areas will

almost triple as early as 2030 (

>

1.8 million km

2

), compared to the year

2000 (~650,000 km

2

) (

Seto et al., 2012

). Other recent modelling shows

that the global urban area may reach between 1.1 million km

2

and 3.6

million km

2

by 2100, depending on different Shared Socioeconomic

Pathways (

Gao and O

Neill, 2020

). While it is impossible to accurately

predict the future urbanization expansion, it is clear that soil sealing will

intensify across this century.

The negative implications of the soil sealing process relate to the loss

of soil ecological and economic multifunctionality, such as the soils

’

water retention and purification capacity, food and fibre production,

carbon sequestration, biodiversity conservation or nutrient cycling

(

;

). Essentially, this process affects the

soil

’

s nutrient cycles, hydrological cycle, climate regulation capacity,

causes habitat loss for soil organisms, affects soil biodiversity and

generally all soil services and functions (

Li et al., 2018b

), except for its

capacity to support infrastructure (

4.12. Waterlogging

This is a process through which soil reaches water saturation due to

excessive input and/or inadequate drainage (

). In many

cases it is a consequence of excessive use of irrigation in numerous

agricultural regions worldwide and generally coincides with soil sali-

nization in terms of spatial extent (

). In fact, the two pro-

cesses are regarded as a double threat to irrigated environments (

Singh,

2015, 2020, 2021

;

Vlotman, 2020

), affecting extensive irrigated areas

and threatening food security in some parts of the world (

Singh, 2018

But there are also other causes of this process, which are linked to the

intensification of extreme precipitation events (amid climate change),

soil compaction (which reduces water infiltration in the vertical soil

profile), shallow water table (which maintains excessive humidity in the

top soil profile), land topography (which can determine increasing

surface runoff from upslope topographic areas to lowland zones with

poor drainage conditions) or poor soil structure (which diminishes the

downward movement of water and the supply of the shallow water

table) (

Kaur et al., 2020

). The increase in frequency, intensity and

severity of extreme precipitation events is however an increasingly

important cause determining this disruptive environmental process, due

to climate warming that triggered the increase of available atmospheric

moisture necessary for the production of extreme rainfall events in many

parts of the globe (

Liu et al., 2020b

). The agricultural consequences of

this atmospheric imbalance are related not only to the occurrence of

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waterlogging (which essentially consists of the saturation of soil pores

with water), but also to that of flooding events, when agricultural plants

are submerged under water partially or completely (

Kaur et al., 2020

In a broader context, water excess (in soil pores or on the surface) can

worsen the state of lands by creating acidity conditions, depleting soil

oxygen, creating anoxic conditions in soil, or through denitrification and

emission of N

2

O into the atmosphere, a major greenhouse gas that can

considerably accelerate global warming (

Voesenek and Sasidharan,

). Essentially, waterlogging causes a

series of physical, chemical and biological changes in soil, which can

aggravate other degradative processes (e.g. acidification, compaction or

biodiversity loss) and diminish the overall biological productivity of soil

(

Geisen et al., 2019

Bailey-Serres et al., 2012

Manik et al., 2019

). In a particular context, agri-

cultural effects can be significant. As expected, the combined effects of

waterlogging and associated processes (salinization and flooding)

strongly erode crop yields and the sustainability of agricultural lands

(

;

In agricultural landscapes, water excess is a serious issue in irrigated

lands, the spatial footprint of which totals between approximately 3 and

4 million km

2

globally (

Thenkabail et al., 2009

;

Salmon et al., 2015

;

Meier et al., 2018

). It is estimated that at least 10% (at least 0.3 million

km

2

or 30 million ha) of the worldwide irrigated area is affected by

waterlogging and the related salinity issue (

), especially in arid and semiarid regions of the world (

). With such a global extent, it is not sur-

prising that waterlogging stress can cause significant losses in agricul-

tural yields worldwide, as a result of the negative effects generated to

agricultural crops, such as decreases in plant height, leaf area and leaf

greenness, disruption of stomatal conductance and photosynthetic rate,

or decreases in total carbohydrate content (

;

).

5. Major international actions for combating the worldwide

land degradation processes

As this is a multidimensional problem, fighting land degradation

requires multiple international actions that must address the ecological

stability of land systems, soil, inland water resources and vegetation.

Historically, a series of international initiatives have been launched,

which have directly or indirectly contributed to fighting land degrada-

tion by ensuring the protection (prevention of land degradation),

restoration (the full recovery of land ecosystems to their pre-degradation

state) or rehabilitation (the partial recovery or stabilization of lands in a

state that is not necessarily consistent with pre-existing ecological con-

ditions) of lands (

). Therefore, over the past decades, in-

ternational policies aimed at fighting land degradation have been

strengthened and, by promoting/implementing various sustainable land

management actions worldwide (

Fig. 7

), have created a strong global

land governance in recent years.

The first international initiative directly aimed at global land

degradation was the United Nations Conference on Desertification (in

Nairobi, 1977), which adopted a set of actions to combat global

desertification, based on the first worldwide map of desertification

(

Middleton and Thomas, 1992

). More than a decade later (in 1990), an important

initiative of the United Nations Environment Programme (UNEP) con-

sisted of sponsoring the first complex global analysis of human-induced

soil degradation, as part of the Global Assessment of Soil Degradation

(GLASOD) project (

Oldeman et al., 1991

). This project was the foun-

dation for preparing the first edition of the World Atlas of Desertification

(WAD 1) (

), which, at the UN Conference

on Environment and Development (UNCED) held in 1992 at Rio de

Janeiro, Brazil (also known as Rio de Janeiro Earth Summit), contrib-

uted to the UN General Assembly decision to establish the UNCCD

(

Middleton and Thomas, 1997

). GLASOD has also provided a major scientific

support for the development of the second edition of the World Atlas of

Desertification (WAD 2), in 1997 (

The UNCCD, which was adopted in 1994, in Paris, France (

UNCCD,

1994

), and entered into force in 1996, is currently ratified by all member

states of the UN, and is therefore the most important global political

vector in fighting land degradation (

). The Convention

played an important role in the implementation and monitoring of

numerous land protection/restoration/rehabilitation actions across the

world, via the fourteen Conferences of the Parties (COP) held between

1997 and 2019 (four annual sessions held between 1997 and 2000, and

ten biannual sessions since 2001). Noteworthy examples of recent/

relatively recent UNCCD initiatives include the preparation of guidelines

for updating WAD at COP 8 (2007, Madrid, Spain), based on which the

third edition of the World Atlas of Desertification (WAD 3) was devel-

oped in 2018 (

), the launch of the Changwon

Initiative at COP 10 (2011, Changwon, South Korea), which enhanced

the interdisciplinary scientific process for supporting the UNCCD

(

UNCCD, 2011

), and the launch of the Ankara Initiative at COP 12

(2015, Ankara, Turkey), which provided practical support for achieving

the Land Degradation Neutrality (LDN) concept (

UNCCD, 2015

).

The LDN tool adopted at COP 12 of the UNCCD currently plays a

major role in fighting degraded lands worldwide (

Akhtar-Schuster et al.,

Metternicht et al., 2019

;

Smetanov

a et al., 2019

). This concept, defined as "a state whereby the

amount and quality of land resources, necessary to support ecosystem

functions and services and enhance food security, remains stable or in-

creases within specified temporal and spatial scales and ecosystems",

essentially entails maintaining a neutral balance (or preferably a posi-

tive one) between non-degraded and degraded lands over a certain time

interval, ideally within the same land cover/use class (

Cherlet et al.,

Fig. 7.

International (scientific and political) initiatives that supported the fight against land degradation over the past decades (left), and concrete control measures

that can currently be applied to ensure land protection/restoration/rehabilitation, according to the specific land degradation processes worldwide (center and right).

Note: the exemplified measures/practices can be used by governments of the world to fulfil the commitments made in international policies (e.g. as part of the

UNCCD) or in their national strategies focused on fighting land degradation; many more specific measures to ensure land protection/restoration/rehabilitation can be

consulted in the World Overview of Conservation Approaches and Technologies (WOCAT) database, which is officially recognized by the UNCCD as a reliable source

for reporting the best practices in worldwide sustainable land management (

Liniger et al., 2019

); the photos in this figure were selected in order to convey a

suggestive visual manifestation of land degradation pathways, and were sourced from various reliable sources, for aridity (

Durant et al., 2012

), biological invasions

(

Roach, 2013

), coastal erosion (

Pontee et al., 2009

), land erosion by water (

Amundson et al., 2015

), land erosion by wind (

), land pollution (

), land subsidence (

), landslides (

), permafrost thawing (

Zimov et al., 2006

), salinization (

), soil acidi-

fication (

AGRA, 2020

), soil biodiversity loss (

Creamer et al., 2017

), soil compaction (Big

Ag, 2017

), soil organic carbon loss (

DPIRD, 2019a

), soil sealing (

RECARE,

), vegetation degradation (

Anderegg et al., 2013

) and waterlogging (

DPIRD, 2019b

); the superscript numbers in each section of the seventeen land degradation

pathways are citations that support the exemplified practical measures that can fight the multiple forms of land degradation worldwide (1

2

; 3

; 4

; 5

; 6

–

Men

endez et al., 2020

; 7

–

van Rijn, 2011

; 8

Prosdocimi et al., 2016

; 9

–

Chen

et al., 2017

; 10

–

Wang et al., 2010

; 11

Hugenholtz et al., 2012

; 12

Wang et al., 2015

; 13

–

Paleari, 2017

; 14

–

; 15

Temmerman and Kirwan,

2015

; 16

Gariano and Guzzetti, 2016

Giosan et al., 2014

; 17

–

; 18

Piciullo et al., 2018

; 19

Zimov et al., 2006

; 20

–

; 21

22

–

Qadir et al., 2014

; 23

–

Wang et al., 2016

; 24

–

Dai et al., 2017

; 25

–

Veresoglou et al., 2015

; 26

–

Rodríguez-Rojas et al., 2018

; 27

–

; 28

; 29

–

Poeplau and Don, 2013

; 30

–

Villarino et al., 2017

; 31

–

Bradford et al., 2019

; 32

–

Gardi et al., 2015

; 33

–

; 34

–

; 35

–

Trumbore et al., 2015

; 36

–

Singh, 2019

; 37

–

Valipour, 2014

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Dallimer and Stringer, 2018

). This unique approach of UNCCD allows for a concrete counter-

balance of anticipated losses in productive lands with measures to

achieve equivalent gains in land productivity, through land restoration

and rehabilitation. Monitoring LDN achievement is possible by using

environmental indicators, based on which the net balance between the

area of gains and area of losses in land productivity is quantified within

individual land types (

Cowie et al., 2018

;

;

Easdale et al., 2019

;

Gilbey et al., 2019

Sciortino et al., 2020

). So far,

more than 120 countries of the world have committed to setting LDN

targets (

UNCCD, 2020

), and, in coming years, other states are expected

to join this international mechanism, which has proven to be efficient in

stabilizing the complex land degradation process.

Other UN policies have indirectly contributed to fighting land

degradation over the past decades. The UN Framework Convention on

Climate Change (UNFCCC) and the Convention on Biological Diversity

(CBD), the other two 1992 UNCED conventions, also played an impor-

tant role in protecting lands worldwide, through sustained efforts to

fight climate change and protect biodiversity. For instance, with the

"Reducing emissions from deforestation and forest degradation and the

role of conservation, sustainable management of forests and enhance-

ment of forest carbon stocks in developing" program (REDD

+

), launched

in 2008 by the UNFCCC, this convention aims to reduce carbon emis-

sions in developing countries, through deforestation reduction, forest

protection, sustainable forestlands management and carbon bio-

sequestration (

Romijn et al., 2012

Streck, 2021

). It is therefore apparent

that through the objectives aimed at protecting forest ecosystems,

components of lands, the convention also contributes to the protection

of land itself. Another example consists of the 2015 Paris Agreement,

through which UNFCCC

’

s effort to stabilize global warming under 2

◦

C

by 2100, relative to preindustrial levels (

Fawcett et al., 2015

), will

generate multiple positive effects, including in terms of land protection

(e.g. by reducing climate warming, global aridity and, therefore, by

mitigating the decline of water resources and desertification conditions)

by the end of this century. CBD could also play an important role in land

ecosystems stability through the Nagoya protocol (2010, Nagoya, Aichi

Prefecture, Japan), Aichi Biodiversity Target 15, which aims to restore

at least 15% of the world degraded ecosystems (

Navarro et al., 2017

), or

even through Target 11, which aims to ensure the formal protection of at

least 17% of the global terrestrial area by 2020 (

Joppa et al., 2013

Montanarella and Panagos,

In addition to these conventions, the UN has also developed other

actions that converge towards land protection. With the New York

Declaration on Forests (2014, New York, United States), inspired by the

Bonn Challenge (2011, Bonn, Germany), the UN aims to restore 150

million ha of degraded forestlands and other landscapes by 2020, but

also an additional 200 million ha (i.e. 350 million hectares in total) by

2030 (

Suding et al., 2015

). The 2030 Agenda for Sustainable Develop-

ment of the UN, adopted in 2015, is another international policy

framework with direct relevance for land degradation, through SDG 15

("Protect, restore and promote sustainable use of terrestrial ecosystems,

sustainably manage forests, combat desertification, and halt and reverse

land degradation and halt biodiversity loss"), Target 15.3 ("By 2030,

combat desertification, restore degraded land and soil, including land

affected by desertification, drought and floods, and strive to achieve a

land degradation-neutral world") (

United Nations, 2020

). Another

important vector of UN

’

s influence on fighting land degradation consists

of the Food and Agriculture Organization (FAO), which, through the

2011 Global Soil Partnership initiative, addresses the various issues and

conservation solutions that pertain to global soils (

Montanarella and

Vargas, 2012

;

Koch et al., 2013

).

In addition to these aforementioned UN-driven international policies

(which are the most relevant, considering that the UN is the global au-

thority in the management of land degradation and other worldwide

environmental issues), regional policies can also be crucial. The policies

of the European Union (EU) are a relevant example in this respect,

considering that, for many years, they have been paying special atten-

tion to controlling numerous forms of land degradation explored in this

paper. One of the most important policies aimed at fighting land

degradation within European borders is the EU Soil Thematic Strategy,

which addresses the main threats to soil health on the continent, namely

soil erosion, soil salinization, soil organic matter (carbon) loss, soil

contamination, soil biodiversity loss, soil sealing, soil compaction, soil

acidification, landslides and desertification (

Montanarella and Panagos,

). In a broader context of EU policies, these land degradation pro-

cesses are considered directly or indirectly in the European Green Deal

major policy, which has the ambitious goal of making the EU the world

’

s

first climate-neutral continent by 2050 (

Abbas, A., Khan, S., Hussain, N., Hanjra, M.A., Akbar, S., 2013. Characterizing soil

The national policy actions to fight land degradation are also of

major importance for land protection around the world. In order to fulfil

the commitments made as part of the above international initiatives or

of their own national policies, world governments have at their disposal

a wide range of measures for addressing the multiple forms of land

degradation present within national boundaries (

Fig. 7

). If appropriately

implemented locally or regionally, based on sound scientific grounds,

these measures can efficiently fight the various pathways of land

degradation (

Fig. 7

) and can ensure the sustainable protection/resto-

ration/rehabilitation of lands across the planet.

6. Conclusions

Through the multiple ecosystem functions and services they provide,

land systems (soils, vegetation and inland water resources) are crucial

for the survival and prosperity of humanity, life support and the Earth

systems

’

stability. In terms of human wellbeing, these terrestrial systems

are vital given the wide range of eco-services that fundamentally in-

fluence the quality of human life across the planet. Lands are also a vital

foundation for terrestrial biodiversity, supporting various biotic habitats

and gene reserves for fauna, microorganisms and flora from all over the

world. Land systems are at the same time an essential pillar for the

stability of the global climatic system, especially by controlling carbon

fluxes between terrestrial systems and the atmosphere, and by storing

huge carbon amounts in soil and vegetation biomass pools.

All these physical and ecological attributes of land systems are

constantly eroded by the multitude of land degradation pathways/pro-

cesses that occur on various spatial scales throughout the world. The

findings of this study, based on the analysis of over 500 international

scientific papers, show that the spectrum of global land degradation

pathways goes well beyond traditional knowledge and approaches,

which usually focus on several conventional degradation processes, such

as water erosion, wind erosion or soil salinization. Starting from the

official definitions of land degradation and upon analysis of the most

relevant papers in the field, it was found that there currently are 17 land

degradation processes around the globe, a surprisingly high number that

confirms the extremely high complexity of land degradation globally

and draws attention to the multiple ecological and socio-economic

threats that are associated with the global land multi-degradation.

Essentially, identifying the 17 land degradation pathways

throughout the world confirms that land multi-degradation is un-

doubtedly one of the most critical global environmental issues humanity

is currently facing. Therefore, this review brings forward the fact that

land degradation should be deemed a major environmental issue in in-

ternational policies, which must be urgently addressed by UNCCD and

governments worldwide, by applying a series of measures (in a rapid,

efficient and coordinated manner) which can efficiently control the

global land degradation processes. The results of this review should

firstly draw the attention of the UN, which is the international authority

(through UNCCD) in the worldwide management of land degradation.

Such urgent measures are not only necessary for the five major

degradation processes prioritized in the present study (aridity, land

erosion by water, salinization, soil organic carbon loss and vegetation

degradation). While, in this paper, the five land degradation pathways

were considered major and analysed extensively, based on the five

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aforementioned criteria, it is possible that other land degradation di-

mensions may actually be major globally-reaching threats too. In fact,

other land degradative processes can already also be considered major

across the planet (e.g. wind erosion, land pollution, soil compaction or

soil acidification), but the global data needed to conduct detailed

planetary-scale analyses is currently not available for many such facets

of land degradation.

Depending on the availability and accessibility of such additional

global data in the coming years, future studies should also approach

other forms of land degradation, as major land degradation pathways on

global scale. These future analyses can be of crucial importance for

gaining a more detailed understanding of the multidimensional pattern

of land degradation. Such a deep understanding is vital for enhancing

the scientific support used in the implementation of international and

national policies designed to protect/restore/rehabilitate degraded

lands across the globe.

Declaration of Competing Interest

The authors declare that they have no known competing financial

interests or personal relationships that could have appeared to influence

the work reported in this paper.

Acknowledgements

The article has enjoyed the support of the CLIMFOREST project (No.

20/2020) financed by UEFISCDI program, Romania. Also, the article

was supported by the Romanian Young Academy, which is funded by

Stiftung Mercator and the Alexander von Humboldt Foundation for the

period 2020

–

2022. The author would like to thank the anonymous re-

viewers for their highly constructive comments and suggestions that

helped improve this paper.

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