Page 1
The role of soil carbon in natural climate solutions
1
Authors:
D. A. Bossio
1*
,
S. C. Cook-Patton
1
,
P. W. Ellis
1
, J. Fargione
1
, J. Sanderman
2
, P.
2
Smith
3
, S. Wood
1,4
, R. J. Zomer
5
, M. von Unger
6
, I. M. Emmer
6
, and B. W. Griscom
7
3
Affiliations
4
1
The Nature Conservancy, 4245 N Fairfax Dr, Suite 100, Arlington, VA 22203, USA
5
2
Woods Hole Research Center, 149 Woods Hole Rd, Falmouth, MA 02540, USA
6
3
Institute of Biological and Environmental Sciences, University of Aberdeen, 23 St Machar
7
Drive, Aberdeen, AB24 3UU, UK
8
4
Yale School of Forestry and Environmental Studies, New Haven, CT 06511, USA
9
5
Center for Mountain Ecosystem Studies Kunming Institute of Botany, Chinese Academy of
10
Sciences, Kunming, China
11
6
Silvestrum Climate Associates LLC, San Francisco, CA 94102, USA
12
7
Conservation International, 2011 Crystal Dr #600, Arlington, VA 22202, USA
13
14
*Corresponding Author.
All correspondence and requests should be addressed to Deborah
15
Bossio, deborah.bossio@tnc.org.
16
17
18
19
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Summary
20
Mitigating climate change requires clean energy and removing atmospheric carbon. Building
21
soil carbon is an appealing way to increase carbon sinks and reduce emissions due to the
22
associated benefits to agriculture. However, practical implementation of soil carbon climate
23
strategies lag behind the potential, partly because we lack clarity around the magnitude of
24
opportunity and how to capitalize on it. Here we quantify the role of soil carbon in natural (land-
25
based) climate solutions (NCS), and review some of the project design mechanisms available to
26
tap into the potential. We show that soil carbon represents 25% of the 23.8 GtCO
2
eyr
-1
NCS
27
potential of which 40% is protection of existing soil carbon and 60% is rebuilding depleted
28
stocks. Soil carbon comprises 9% of the mitigation potential of forests, 72% for wetlands, and
29
47% for agriculture and grasslands. Soil carbon is important to land-based efforts to prevent
30
carbon emissions, remove atmospheric carbon dioxide and deliver ecosystem services in addition
31
to climate mitigation.
32
33
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Protecting and restoring soil organic matter delivers many benefits to people and
34
nature
1,2
. Globally, soils hold three times more carbon than the atmosphere
3
, and the role of soil
35
organic matter as a regulator of climate has been recognized by scientists for decades
4
. Recent
36
work has highlighted the historical loss of carbon from this pool
3
, and the threat of future
37
accelerated loss under warming scenarios
4,5
. Soil organic carbon as a natural climate solution
38
(NCS) thus has a role both through restoring a carbon sink and protecting against further CO
2
39
emissions in response to predicted land use change and climate change.
40
This dual role for soil in the global carbon budget suggests climate benefits can be
41
achieved through strategies that both conserve existing soil organic carbon stocks (avoid loss),
42
and restore stocks in carbon-depleted soils
6
. There are important additional benefits. Protecting
43
and increasing soil carbon storage can (i) protect or increase soil fertility, (ii) maintain or
44
increase resilience to climate change, (iii) reduce soil erosion, and where implemented through
45
conservation of natural ecosystems iv) reduce habitat conversion, all in line with the United
46
Nations Sustainable Development Goals (SDG’s)
7
, the goals of the United Nationals Framework
47
Convention on Climate Change (UNFCCC) and the United Nations Convention on Combating
48
Desertification (UNCCD). As such, soil carbon is promoted as a common denominator amongst
49
a variety of global and national initiatives
7
. Although recent academic comment and perspective
50
pieces point the way towards accelerated action on soils
8,9
, there remains much uncertainty
51
around actionable pathways for achieving the global opportunity. Here we examine the scientific
52
and policy context surrounding soil carbon projects, to aid prioritization and decision making.
53
54
55
56
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Status of soil carbon as a climate solution
57
Despite scientific consensus around its potential and multiple benefits, deployment of soil
58
carbon storage and sequestration for climate mitigation remains limited in practice. There is
59
growing interest in soil in international climate mitigation conversations, with the recognition of
60
‘wetland drainage and rewetting’ (WDR) as an accounting option under the Kyoto Protocol
61
(formalized in 2011), the launch of the ‘4 per 1000’ Initiative in Paris in 2015 and formal
62
recognition of soil carbon sequestration in the UNFCCC process in 2017 (COP23 decision
63
4/CP.23)
.
To date there are only a few dozen projects that address soil organic carbon in
64
registered compliance or voluntary carbon markets. Fewer than 60 projects (half of them in
65
Australia) provided under 50 thousand tonnes of CO
2
-equivalent removals by soil in agriculture
66
and grassland projects per year
10
. This is less than 0.0001% of the estimated mitigation
67
potential
11
. As a comparison there are 1,500 carbon projects covering 12 Mha of land in the
68
forest sector
12
. The small soil carbon numbers are due in part to the sector’s near exclusion from
69
early carbon market mechanisms, notably the Kyoto Protocol’s Clean Development Mechanism
70
(CDM) which limited potential soil carbon mitigation to afforestation and reforestation projects.
71
Nevertheless, the past two decades have witnessed the emergence of a variety of robust
72
methodological approaches for the calculation of mitigation benefits and the issuance of ‘carbon
73
credits’ in a wide range of project categories covering croplands, grasslands, savannahs, as well
74
as peatlands and coastal wetlands. While still occupying no more than a niche in the toolbox for
75
international climate action, there is experience on soil carbon projects to provide confidence and
76
to support development of mitigation plans at larger scales
10
.
77
Experience with implementation has not yet caught up with aspirations in the political
78
arena. While soil targets for mitigation are included in only eight Nationally Determined
79
Page 5
Contributions (NDCs) to the UNFCCC
9
, the UNFCCC is now exploring agriculture and soils –
80
including with respect to “[improved] soil carbon, soil health and soil fertility under grassland
81
and cropland as well as integrated systems, including water management” as a more explicit part
82
of their agenda
13
. At the same time, nations are moving forward to invest in solutions and set
83
targets that address the food security and land use commitments of the United Nations
84
Sustainable Development Goals (SDGs). Beyond governments, a growing number of companies
85
are including soil organic carbon within their set of options to build the resilience and long-term
86
profitability of agricultural value chains
9
. The enthusiasm arises because, in general, soil carbon
87
enhancement practices are considered to have positive co-benefits, do not require additional land
88
area, have minimal water footprint, and are readily deployable considering that they do not
89
require changes in land use
11,14
.
90
The science supporting the global technical potential of soil carbon mitigation is
91
relatively well established, even though measuring changes in soil carbon is more difficult than
92
for plant biomass. Recent estimates of global soil carbon sequestration technical potential, i.e.
93
the level of mitigation that could be achieved when accounting only for biophysical constraints,
94
if there were no economic, social, institutional or other barriers to implementation, align around
95
2-5 Gt CO
2
per year
11, 14-18
albeit many of these estimates rely on the same underlying data.
96
Counter to this relative certainty, recent scholarly debates focused primarily on debunking claims
97
that soil carbon sequestration could fully offset current increases in atmospheric CO
2
19-21
have
98
created confusion for practitioners. Yet, even these debates do not call into question the
99
significance of the global potential, and the multiple benefits of increasing global soil carbon
100
stocks.
101
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Caveats surrounding soil carbon sequestration such as sink saturation and non-
102
permanence risk (reversibility) have also been well explored in the soil science literature. Soil
103
carbon saturation refers to a maximum capacity of the soil to retain organic carbon
15
, meaning
104
that soil organic carbon does not increase indefinitely with the exception of some wetland
105
systems
16
. For most improved carbon management practices the rate at which soils will store
106
additional carbon therefore begins to decline after some decades, and eventually will reach a new
107
steady state when a higher carbon stock is achieved. The time period before a new steady state is
108
reached will vary greatly depending on soil type, management intervention, climate regime, and
109
pre-existing SOC depletion
15
, but is generally on the order of decades
22
. This aligns with the
110
need to reduce peak atmospheric CO
2
levels and mitigate peak warming. With respect to non-
111
permanence, maintaining high SOC stocks, such as with cover cropping and manuring in
112
croplands, requires some form of maintenance (continuation of improved soil carbon
113
management practices), even after a new steady-state is reached and no further mitigation
114
benefits accrue
14
. In other cases, i.e. when there is protection of existing soil carbon stocks, such
115
as avoided grassland conversion, it is likely that SOC levels are at steady-state, and the
116
management activity (in this case protection) also needs to be maintained to maintain those SOC
117
stocks
23
. Nevertheless, SOC may be more resilient to fire, pests, and wind than carbon in
118
aboveground biomass in many environments
17
, and some forms of soil carbon, such as biochar,
119
can persist for millennia
18
.
120
Meanwhile, outside of soil science, carbon project design approaches have moved
121
forward to deal with heterogeneity, uncertainty, additionality, and non-permanence in particular
122
which are challenges for the entire Agriculture, Forestry and Land Use (AFOLU) sector. Soil
123
does not differ substantially from forestry in this regard, and because this has been a topic for
124
Page 7
decades, significant experience exists in managing these risks as part of project and policy
125
design
24
. Some methods to account for and resolve these issues in soil carbon project design are
126
reviewed by
25
. The CDM issues temporary credits that are continuously renewed as long as the
127
removal benefit persists. If a reversal event occurs, renewal of the temporary credit concerned is
128
no longer possible (Decision 5/CMP.1; Decision 14/CMP.1).
129
An alternative approach to non-permanence of soil carbon sequestration is based on the
130
installation of portfolio-wide “buffer” reserves – each project contributes with a share of the
131
credits achieved – that works as an insurance scheme. For any event of intentional (subsequent
132
land degradation, land conversion) or unintentional (usually force majeure events such as
133
extreme weather events, storms, flooding, fire, and so on) that causes sink reversals or carbon
134
stock losses reversal risks, credits held in the buffer account will be released (in an amount
135
equivalent to the reversal or stock loss amount) and permanently canceled
26
. Most voluntary
136
carbon market standards operate with a buffer reserve
27
based on some standardized or project-
137
specific risk assessment. In Australian carbon farming associated with the government’s land
138
based strategies for climate mitigation follow a mixed approach that combines buffer reserves
139
with discount elements: farmers that would receive a certain amount of credits in a 100-year
140
permanence scenario (with maintenance obligations being transferred to subsequent landowners
141
within the 100-year window) will receive 20% less credits if they commit to 25-year stable
142
conditions only
10
; the discount comes on top of the general 5% buffer amount. No case is known
143
in which a buffer reserve was ever depleted, which suggests that, while important, permanence is
144
a manageable issue. As a caveat, this experience arises primarily from the forest sector, and
145
given that most soil carbon projects in the agriculture sector are relatively new, there has been
146
little time for permanence issues to appear. Soil carbon sequestration ambitions can benefit from
147
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this experience in the markets and the accepted protocols that now exist for most types of soil
148
carbon sequestration project types including for grasslands, peatlands and croplands
10
.
149
Practical solutions aside, the relevance of the non-permanence issue is also fading
28
.
150
While of great importance in the context of project-level offsetting, the non-permanence risk of
151
mitigation action within wider jurisdictional or national schemes is less a concern of
152
environmental integrity but of legal responsibility (liability). Within the Paris Agreement, in
153
particular, nations are expected “to include all categories of anthropogenic emissions or removals
154
in their nationally determined contributions and, once a source, sink or activity is included,
155
continue to include it” (Decision 1/CP.21, paragraph 31.c). Once soil carbon emissions are thus
156
covered under a target, the non-permanence issue in specific measures is solved at the higher-
157
level accounting framework: Any reversal events will translate into a fresh obligation (a priori
158
for the government) to reduce or avoid emissions. As with permanence, issues of additionality
159
and leakage require strong safeguards and binding agreements. Australia’s direct action subsidy
160
approach may fund non-additional projects and therefore deliver less abatement than expected
29
.
161
There are several other challenges to implementation of soil as a climate mitigation
162
strategy. Historically, there have been limited finance and policy options. The Kyoto
163
mechanisms failed to address soil carbon interventions. Then, carbon prices (the price paid per
164
tCO
2
e) collapsed following the 2008 global economic recession and the Copenhagen summit in
165
2009
10
failed to generate a new agreement. Further, carbon pricing currently covers only about
166
20% of global emissions. However, there are some signs that viability of climate financing for
167
soil is improving. There is increased action on agriculture under the Paris agreement. The Green
168
Climate Fund has established a funding window targeting land-use and agriculture. There are a
169
range of fresh private-sector initiatives on soil carbon that promise sufficient funding and
170
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transformational change
30,31
, and impact investors focusing on landscape, soil resources, and
171
payments for ecosystem services schemes
10
.
172
173
Soil contribution to Natural Climate Solution Pathways
174
Experience and trends in the AFOLU market sector, emerging finance opportunities for
175
climate positive agriculture, and earlier global potential analyses provide the framework for
176
actions on soil carbon. Here we extend the analysis of Griscom et al.
32
to offer improved
177
guidance on the set of actions available for realizing the soil carbon climate mitigation
178
opportunity. The recent study by Griscom et al.
32
provides a framework for an integrated
179
assessment of the overall global mitigation potential of “natural climate solutions” (NCS). In the
180
Griscom et al.
32
study the potential of 20 conservation, restoration and improved land
181
management actions, including reforestation, planting trees in croplands, grazing land
182
management, peatland protection and others, to increase carbon storage and/or avoid greenhouse
183
gas emissions across global forests, wetlands, grasslands, and agricultural lands was determined
184
to be 23.8 Gt CO
2
e yr
-1
. This analysis estimated mitigation potentials constrained by a
185
requirement for additionality and by food security and biodiversity safeguards. A benefit of this
186
analysis is that researchers, policy makers and practitioners can prioritize across various sectors
187
of potential activity. An additional benefit is that by using a common framework, the analysis
188
avoids double counting across the various mitigation options, referred to as “pathways” - an
189
important consideration for national accounting with NDC commitments. While soil-related
190
ecosystem services are identified as a co-benefit in 16 of the 20 pathways, the specific
191
contribution of soil carbon storage (avoided losses and enhanced sinks) to each of these
192
pathways, and overall, was accounted for but not reported as a component distinct from biomass
193
Page 10
carbon. Here we elaborate on Griscom et al.
32
by incorporating findings from a few key papers
194
published since 2017 and by separating out the contribution of soils to each pathway (see
195
methods). Table 1 describes the soil carbon protection and sequestration pathways, the annual
196
mitigation potential and benefits for sustainability.
197
Our results (Figure 1) show the global additional mitigation potential of protecting and
198
rebuilding soil carbon to be 5.5 Gt CO
2
e yr
-1
, representing 25% of the total mitigation potential
199
of the 20 NCS pathways. Of this, 4.3 Gt CO
2
e yr
-1
comes from non-forest pathways, thus soil
200
carbon represents more than half of the 7.6 Gt CO
2
e yr
-1
NCS potential of non-forested lands,
201
with safeguards for food security, fiber security and biodiversity conservation. Avoidable losses
202
represent 2.2 Gt CO
2
e yr
-1
, or 40%, of the total soil carbon mitigation potential of all NCS
203
pathways. Protection is important not only because the potential is large, but also because soil
204
carbon is lost more quickly than it can be gained
33
, and in many cases it is not possible to restore
205
to soil organic carbon to original levels on climate-relevant timescales
3,34
. These estimates do not
206
include land or agricultural management practices that reduce non-CO
2
GHG emissions (i.e. N
2
O
207
and CH
4
) without protecting or enhancing soil carbon sinks, for example improved rice, nutrient
208
and livestock management strategies, which together constitute an additional 1.85 Gt CO
2
e yr
-1
209
32
.
210
The predominance of SOC protection and sequestration within the overall contribution of
211
NCS differs among different biomes (Figure 2). Across forest pathways, the SOC mitigation
212
potential of 1.2 Gt CO
2
e yr
-1
is a small portion (9%) of the total and is split almost equally
213
between increased sequestration from reforestation and avoidable emissions through prevented
214
conversion. In grasslands and agriculture, 47% of the total potential mitigation (2.3 Gt CO
2
e yr
-1
)
215
arises from soil carbon protection and sequestration, while 20% involves other greenhouse gases
216
Page 11
involved with improved soil management practices. In wetland pathways soil carbon is estimated
217
to comprise 2.0 Gt CO
2
e yr
-1
, 72% of the total mitigation potential of wetland pathways. For
218
forest pathways, soil carbon can bring an additional component to mitigation accounting which is
219
largely dominated by the above ground tree biomass, while in wetland pathways soil carbon is
220
the main vehicle through which climate mitigation can be achieved (Table 1). In agriculture and
221
grassland pathways overall, soil carbon is approximately half of the abatement potential, and
222
importantly accounting for soil carbon can bring large areas of grasslands and croplands under
223
the Paris Agreement.
224
About half of the soil carbon mitigation potential, 2.8 Gt CO
2
e yr
-1
, is considered cost-effective
225
at $100/tCO
2
(based on Griscom et al.
32
methodology) which is one estimate of the amount that
226
society is expected to have to pay to mitigate climate change
35
. About one quarter, 1.2 Gt CO
2
e
227
yr
-1
, is considered to be low cost at $10/tCO
2
. Low-cost and cost-effective removal is, therefore,
228
equivalent to about 3% and 7%, respectively, of recent annual anthropogenic emissions of
229
carbon dioxide to the atmosphere. In other studies, negative costs have been estimated for soil
230
carbon sequestration, based on the co-benefits such as increased productivity and resilience of
231
soils
36
, and have suggested that many soil-based NCS are cost-effective even without supportive
232
climate policy. The IPCC recently concluded that the cost for soil carbon mitigation is below
233
$100/tCO
2
37
. Despite the relatively low or negative costs, soil carbon actions are not yet
234
implemented due to other economic, social, institutional or other barriers as noted and
235
highlighted above.
236
237
Soil science knowledge gaps
238
Page 12
Given the availability of project design mechanisms to realize the potential for soil
239
carbon mitigation actions (see Table 2 for example actions for each pathway), soil management
240
planning and prioritization at various scales would benefit from increasingly more accurate
241
system and practice-specific estimates of climate impacts. For agriculture and grassland
242
pathways, future work should disaggregate mitigation accounting to specific activities each with
243
their own mitigation estimates, trade-offs, and co-benefits. Tillage, cover cropping, enhanced
244
crop rotations and grazing management are in fact broad sets of activities, each with potentially
245
very different impacts on soil organic carbon
8
, different N
2
O emissions, and different
246
feasibilities. An activity that builds organic carbon on one soil type might be ineffective on a
247
different soil
38
. In wetland pathways, more research should focus on accurately predicting the
248
magnitude of increasing CH
4
emissions when soil organic carbon is restored in wetland
249
environments, and improving estimates of the potential and existing carbon storage in peatland
250
soils.
251
Our estimates are lower overall than Fuss et al.
11
for the sequestration pathways, and
252
lower for agriculture than Zomer et al.
39
, which used unconstrained cropland area availability.
253
We provide conservative estimates because we exclude interventions for which there is less
254
consensus on the impact, such as no-till
40
and we use conservative estimates for pathways with a
255
large range in published numbers, such as biochar
41,42
and optimal grazing
43
. Thus, agricultural
256
pathways in our analysis encompass only the best understood options for incremental change to
257
existing farming practices. Opportunities for greater innovation may result in higher per hectare
258
mitigation rates than those reflected here, but data are lacking to make robust global estimates of
259
their potential. Regenerative agriculture, organic farming, agroecology, silvo-pasture, climate
260
smart agriculture, agroforestry, and permaculture are all complex and not mutually exclusive
261
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agricultural systems that can have significant positive impacts on soil organic carbon in specific
262
geographies, according to a recent literature review by Toensmeier
44
. Other, less well-
263
established, opportunities for SOC management take advantage of the potential to build organic
264
matter into deeper soil layers through deep rooted grasses and new crop varieties
45
, and deep
265
inversion techniques
46
. Organic biosolids from cities are a large pool of organic material that are
266
often a pollution and waste disposal problem
47
, that could provide substrate to build soil health
267
and sequester carbon in soils. Exogenous organic matter additions can stimulate rangeland
268
productivity and sequester endogenous organic matter beyond the actual tonnage of
269
compost/biosolids applied
48
, but may pose a risk to native plant biodiversity
49
and more research
270
is needed (and is underway) to understand how universal these findings are. Early research from
271
row-crop systems suggests endogenous vs. exogenous organic matter have similar effects
50
.
272
Soil carbon fluxes associated with forest pathways are often ignored, given the more
273
obvious changes observed in woody biomass, even though the contribution from forest pathways
274
to soil carbon sequestration is substantial (Fig 1). Conversion of forests to permanent croplands
275
and pastures often generates soil carbon emissions and forest restoration is expected to increase
276
soil carbon
34
. Recent estimates for the extent of potential reforestation vary widely
51,52
, and our
277
estimate is based on an intermediate spatial extent of potential reforestation (6.8 M km
2
), and
278
includes food security and biodiversity safeguards
32
. However, the potential for additional soil
279
carbon storage from improved management practices on natural and plantation forests are much
280
more complex and more research is needed to include the potential soil carbon benefits in this
281
NCS framework.
282
283
Looking forward
284
Page 14
As the urgency to harness all available opportunities to mitigate catastrophic climate
285
change grows
53,54
, we emphasize that if we are to limit warming well below 2°C as called for by
286
the Paris Agreement, soil carbon can be an important way to increase carbon sinks and reduce
287
emissions. Soil carbon sequestration is not an alternative to emission reductions in other sectors,
288
but rather an additional opportunity for increasing currently insufficient ambition in existing
289
nationally determined contributions (NDCs) to the Paris Agreement. This opportunity should
290
neither be dismissed nor exaggerated. Our analysis disaggregates this opportunity across all land
291
sectors in a way that is relevant to target setting and prioritization efforts at scales from NDCs to
292
sub-national programs.
293
A strong benefit of soil carbon mitigation action is that it can positively engage rural
294
landowners and the agricultural sector as beneficiaries of mitigation incentives that are likely to
295
be produced by successful climate negotiations. Further, the majority of soil carbon pathways are
296
“no regrets” opportunities for climate mitigation, by delivering improved soil fertility, climate
297
resilience and other ecosystem services in addition to climate mitigation. As such, soil carbon
298
aligns targets across different international conventions (SDG, UNFCCC, UNCCD) and agendas
299
by providing measurable benefits towards diverse goals with a common metric. Prospects for soil
300
carbon sequestration action are promising because project design tools are sufficient to address
301
accounting challenges, and climate financing seems to be growing for the sector; notably,
302
because enhancing soil carbon brings multiple benefits, there are opportunities to incentivize
303
action beyond formal carbon markets. Policies in both the climate sector with a focus on
304
mitigation and the agriculture sector with a focus on soil health are needed to achieve significant,
305
cost effective soil carbon protection and enhancement to meet climate targets and improve
306
resilience.
307
Page 15
308
Methods
309
Estimating soil carbon mitigation potential in NCS pathways
310
Griscom et al.
32
identified 20 pathways by which natural systems could contribute to
311
mitigation of greenhouse gases. For these pathways, an analysis of over 300 publications was
312
conducted in concert with expert elicitation to define the maximum areal extent, the amount of
313
avoided emissions or sequestration rate (“flux”) and time until a new steady-state, and the
314
amount of total mitigation attainable at different costs informed by marginal abatement curves.
315
For complete sources see Griscom et al.
32
supplementary information. Pathways were
316
constructed carefully to estimate additional annual mitigation potential above a business-as-usual
317
baseline, to avoid double counting and to safeguard biodiversity and human needs for food, fiber
318
and fuel. The analysis also included estimates of uncertainty around extent, flux and mitigation
319
for each pathway, and propagated across all pathways. In this current work, we have separated
320
out the soil contribution of each pathway as briefly described below; full details of pathway
321
methods are found in Griscom et al.
32
:
322
•
Avoided conversion of forested ecosystems (>25% tree cover) where they are threatened
323
by agriculture preventing the loss of soil carbon. Don et al
55
estimated that 17.4 Mg C
324
ha
-1
are lost when forests are converted to various commercial agricultural uses. Powers
325
et al.
56
further found that conversion of forests for shifting cultivation results in a slightly
326
lower impact to soil carbon stocks (14.5 Mg C ha
-1
). These avoided emission values were
327
then applied to the 5.93 Mha of tropical forest that are lost annually with the assumption
328
that 54% of the loss goes to commercial agriculture and the remainder to shifting
329
cultivation. Most temperate and all boreal regions excluded due to lack of spatial data
330
Page 16
and/or albedo considerations. Forested wetlands excluded to avoid double-counting with
331
wetland pathways.
332
•
Soil carbon sequestration arising through reforestation, including silvopastoral practices.
333
The reforestation pathway quantifies potential conversion from non-forest (< 25% tree
334
cover) to forest (> 25% tree cover) in areas that historically supported forests. This
335
pathway excludes afforestation of grass-dominated ecosystems to avoid negative
336
biodiversity impacts on grassland ecosystem
57,58
and croplands for food security reasons.
337
The pathway does allow for reforestation of potentially forested grazing lands based on
338
recent analyses that show the potential to shrink the footprint of livestock production
339
through improved efficiencies in production and/or shifts towards a more plant-based
340
diet
59,60
, but to avoid double counting, the mitigation potential from grazing pathways
341
was deducted from the mitigation potential for reforestation. To further avoid double
342
counting, the area of reforestation opportunity excluded wetland areas. Finally, the
343
reforestation pathway did not include opportunity assessments in boreal zones, since
344
changes in albedo can offset the climate benefits of carbon capture
61
, and excluded
345
opportunity within denser human settlements where widespread tree cover expansion is
346
constrained. The original NCS assessment included an average soil carbon accumulation
347
rate of 0.4 Mg C ha
-1
yr
-1
for tropical and subtropical reforestation from Powers et al
56
,
348
which we disaggregated here. We then further quantified the soil carbon accumulation for
349
temperate forests using a more recent study by Nave et al
34
. This analysis estimated that
350
reforesting stands accumulated between 0.11 to 0.34 Mg C ha
-1
yr
-1
in the topsoil. We
351
therefore used the midpoint of this range (0.23 Mg C ha
-1
yr
-1
) to estimate potential soil
352
accumulation in temperate biomes.
353
Page 17
•
Biochar amendment to increase the soil carbon pool of agricultural soils is a soil-only
354
pathway in Griscom et al
.
32
and remains unchanged in this analysis. Increased soil
355
carbon pool results from conversion of non-recalcitrant carbon (crop residue biomass) to
356
recalcitrant carbon (charcoal) through pyrolysis. Biochar carbon mitigation was estimated
357
using a mid-range estimate of available crop residues and multiplying this value by the
358
amount of persistent biochar assuming 79% is recalcitrant, a 50% conversion efficiency
359
during pyrolysis and a carbon content of crop residues of 45% of available crop residues.
360
•
Cover cropping is a soil-only pathway in Griscom et al.
32
, and remains unchanged in this
361
analysis. We assumed that 50% of the 800 Mha of cropped land were amenable to cover
362
cropping. To this area we applied a mean sequestration rate of 0.32 Mg C ha
-1
yr
-1 62
.
363
Effects of no-till and other potential conservation agriculture practices were not included
364
to avoid double-counting with cover crops, and unresolved questions about long-term
365
efficacy.
366
•
The trees in annual croplands pathway entails the expansion of three agroforestry
367
practices into annual croplands that currently have low (<10% tree cover). These include
368
expansion of farmer-managed natural regeneration across dry croplands in Africa (150
369
Mha), windbreaks over 50% of non-African croplands (318 Mha), and alley cropping
370
across 22% of non-African croplands (140 Mha). Note that windbreaks and alley
371
cropping were applied to non-African croplands to avoid double counting with farmer-
372
managed natural regeneration. Estimates of soil carbon accumulation derive from a
373
literature review around the soil benefits of windbreaks, or shelterbelts based
63-65
and
374
alley cropping
66-68
. We estimate that windbreaks capture an additional 0.69 Mg C ha
-1
yr
-
375
1
, whereas alley cropping capture an additional 0.59 Mg C ha
-1
yr
-1
. Because we could not
376
Page 18
find independent estimates of soil carbon accumulation for farmer managed natural
377
regeneration, we assumed 25% of the mitigation potential was attributable to soil
378
accumulation, averaging together the proportion of mitigation potential for alley cropping
379
and windbreaks. Silvopastoral systems were not included here to avoid double counting
380
with the reforestation pathway.
381
•
Avoided grassland conversion refers to avoided soil carbon loss by protecting grasslands
382
from conversion to croplands in areas where grasslands are threatened. For this pathway,
383
we updated the initial NCS analysis of Griscom et al.
32
by allowing 28% of soil carbon
384
to be lost down to 1 m in the soil based upon the findings of Sanderman et al
3
; and the
385
new soil carbon modeling for temperate and tropical grasslands based on ISRIC
386
database
3
.
Thus, we applied this soil carbon loss to the estimated 155 tC ha
-1
in temperate
387
grasslands, and 122 tC ha
-1
in tropical grasslands over 0.7 Mha and 1.0 Mha respectively
388
for temperate and tropical grasslands converted annually
69
.
389
•
Grazing – optimal intensity is a soil only pathway in Griscom et al.
32
and remains
390
unchanged in this analysis representing changes in grazing intensity that optimize forage
391
removal and increase soil carbon on both rangeland and planted pasture. We assumed
392
additional sequestration potential of 0.06 MgC ha
-1
yr
-1
over 712 Mha of land. This
393
includes global rangelands and planted pastures. There is some spatial overlap with
394
Reforestation and Grazing – Legumes, therefore the mitigation potential of this pathway
395
was subtracted from Reforestation mitigation potential to avoid double-counting.
396
Accounting with Grazing – Legumes is additive, so no double-counting concerns.
397
•
Grazing – legumes, sowing leguminous crops on planted pastures to increase soil carbon,
398
is a soil only pathway in Griscom et al
.
32
, and remains unchanged in this analysis. The
399
Page 19
pathway quantifies the net increase in soil carbon (after accounting for increases in N
2
O
400
emissions) in planted pastures due to the fertilizing effect of increased nitrogen fixation.
401
We estimate an additional sequestration potential of 0.56 MgC ha
-1
yr
-1
over 72Mha of
402
land. This was restricted to global planted pastures. Spatial overlap with Reforestation
403
and Grazing – Optimal Intensity. Mitigation potential of this pathway was subtracted
404
from Reforestation mitigation potential to avoid double-counting. Accounting with
405
Grazing – Optimal Intensity is additive, so no double-counting concerns.
406
•
Peatland restoration includes restoration of global non-tidal freshwater forested and non-
407
forested wetlands. The restoration opportunity across tropical, temperate and boreal
408
peatlands estimated at 46 Mha was not changed
32
. Avoidable soil carbon losses of 5.44
409
tC ha
-1
yr
-1
for tropical peatlands, 3.55 tC ha
-1
yr
-1
for temperate peatlands, and 1.42 tC ha
-
410
1
yr
-1
for boreal peatlands were estimated by assuming an avoided loss of 50% of the
411
original soil carbon
70-72
occurring over a 20-year period. Due to the strong likelihood of
412
near-term increased CH
4
emissions arising from increased soil organic carbon in
413
peatlands
73
, we do not include increased soil carbon sinks in freshwater peatlands upon
414
rewetting for restoration. In other words, we assumed that any possible enhanced carbon
415
sink was at risk of being offset by increased CH
4
emissions
32
. Recent work shows that
416
this problem may be greater than expected also in coastal wetlands
74
.
417
•
Avoided peat impacts refers to avoided soil carbon loss by protecting threatened tropical,
418
temperate and boreal peatlands. It includes all threatened non-tidal freshwater forested
419
and non-forested wetlands estimated to cover 0.78 M ha yr
-1
72
. Avoidable soil carbon
420
fluxes were estimated to be 217 tC ha
-1
for tropical peatlands
70,72
, 142 tC ha
-1
for
421
Page 20
temperate peatlands
71,72
and 57 tC ha
-1
for boreal peatlands
71,72
. Forested wetlands were
422
excluded from Avoided Forest Conversion pathway to avoid double-counting.
423
•
Restoration of coastal blue carbon ecosystems (mangroves, salt marshes and seagrass
424
meadows) typically leads to significant soil carbon accumulation. Mean literature
425
estimates of carbon sequestration rates during ecosystem restoration were applied to the
426
historic area lost of each of these ecosystems, 11 Mha, 2 Mha and 17 Mha respectively
427
for mangrove, salt marsh and seagrass and was not changed from Griscom et al.
32
. Here
428
both avoided losses of soil carbon and enhanced sequestration are included, and were
429
estimated based on addition sequestration at an average rate of 1.7 tC ha
-1
yr
-1
75,76
, and
430
avoided fluxes averaging 3.4 tC ha
-1
yr
-1
estimated by assuming a potential 50% loss of
431
the original soil carbon
77,78
occurring over a 20-year period.
432
•
Avoided coastal impacts refer to the avoided soil carbon emissions by protecting
433
threatened blue carbon ecosystems (mangroves, salt marshes and seagrass meadows).
434
This pathway was updated from Griscom et al
.
32
by using more recent lower estimates of
435
ongoing mangrove loss rates
79,80
. The soil portion was calculated based on estimates of
436
soil carbon stocks to 1m and expected losses resulting in avoidable fluxes of 197.47 tC
437
ha
-1
, 133.78 tC ha
-1
and 77.43 tC ha
-1
respectively over 0.05 Mha yr
-1
of mangroves, 0.08
438
Mha yr
-1
of salt marshes and 0.45 Mha yr
-1
of seagrass meadows
77,78,80
. Mangroves were
439
excluded from Avoided Forest Conversion pathway to avoid double-counting.
440
441
Uncertainty estimates
442
Uncertainty for maximum mitigation estimates of each pathway can be found in Griscom
443
et al.
32
In brief, methods consistent with IPCC good practice guidance were used when empirical
444
Page 21
uncertainty estimation was possible. For other pathways, the Delphi method of expert elicitation
445
involving two rounds of explicit questions about expert opinion on the potential extent and
446
intensity of flux were combined.
447
448
Data Deposition
449
A global spatial dataset of reforestation opportunities is available on Zenodo
450
(
https://zenodo.org/record/883444
). Figures 1 and 2 have associated raw data that can be made
451
available upon request.
452
453
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Acknowledgements
. This study was made possible by funding from the Craig and Susan
667
McCaw Foundation.
668
Author contributions.
DB and BG designed the study; BG, SC, PW, JF, JS provided data
669
analysis; DB, BG, SC, PW, JF, JS, PS, SW, RZ, MvU, and IE interpreted the data and wrote the
670
paper.
671
672
673
674
675
676
677
678
679
680
Page 27
Figure Legends
681
Figure 1. Additional soil carbon storage potential for 12 natural pathways to climate mitigation.
682
We estimate annual maximum climate mitigation potential with safeguards for the reference year
683
2030. Light gray portions of bars represent cost-effective mitigation levels assuming a global
684
ambition to hold warming below 2°C (<100 USD MgCO
2
e
-1
y
-1
). Dark grey portions of bars
685
indicate low cost (<10 USD MgCO
2
e
-1
y
-1
) portions. Ecosystem service benefits linked with each
686
pathway are indicated by colored bars for biodiversity, water (filtration and flood control), food
687
and air filtration. Most pathways also contribute biomass carbon, (see Figure 2), with the
688
exception of pathways that are entirely soil carbon: biochar, cover cropping, both grazing
689
options, and avoided grassland conversion. More than half of the pathways (reforestation, cover
690
cropping, biochar, trees in croplands, grazing, improved pasture options and coastal wetland
691
restoration), represent enhanced soil carbon sinks, while the others are avoided soil carbon
692
losses. The remaining 8 of the 20 pathways from Griscom et al.
32
are not expected to have an
693
impact on soil organic carbon, and therefore have not been included in this figure.
694
695
Figure 2. Maximum climate mitigation potential of soil in 2030 across forest, agriculture and
696
grassland, and wetland biome pathways with safeguards. Bars to the left indicate the magnitude
697
of potential sinks, whereas the bars to the right indicate magnitude of avoided emissions. Dark
698
portions of bars represent soil carbon; white portions of bars represent vegetative biomass; and
699
dotted portion of bar is avoided CH
4
and N
2
O through improved nutrient, rice, and animal
700
management. Note that due to the strong likelihood of near-term increased CH
4
emissions arising
701
from increased soil organic carbon in peatlands
73
, we do not included increased soil carbon sinks
702
in freshwater peatlands upon rewetting for restoration.
703
Page 28
Tables
704
Table 1. Summary of soil carbon elements of natural climate solutions (NCS): the role of soil
705
and co-benefits for sustainable development. Cells in green indicate Forest pathways, in yellow
706
indicate grassland/agricultural pathways, and in blue indicate wetland pathways.
Adapted from
707
Table S2 and S5 in Griscom et al.
32
.
708
NCS Pathway
Contribution of soil carbon
Co-benefits for Sustainable development
Avoided Forest
Conversion
1.2 Gt CO
2
e yr
-1
for soil protection and
carbon sequestration is about 9% of the
mitigation benefit from these two forest
pathways.
Water retention and flow regulation. Biodiversity benefits. Maintains soil
biological and physical properties ensuring health and productivity of
forests.
Reforestation
Measured increase in soil fauna in reforested sites. Drought resilience.
Water retention and flow regulation.
Biochar
1.1 Gt CO
2
e yr
-1
biochar direct mitigation
potential.
Soil quality and fertility enhancement in temperate regions.
Cover cropping
0.41 Gt CO
2
e yr
-1
is entirely soil carbon.
Soil quality and fertility enhancement. Reduced agricultural water demands
with appropriate cover crops. Reduced soil erosion and redistribution
maintaining soil depth and water retention.
Trees in Croplands
0.28 Gt CO
2
e yr
-1
in soil carbon is 40%
of the total mitigation potential.
Biodiversity, habitat connectivity, erosion control, water recharge, and
reduced soil erosion. Tree planting helps capture airborne particles and
pollutant gasses.
Avoided Grassland
Conversion
0.23 Gt CO
2
e yr
-1
is entirely soil carbon.
Permanent grasslands provide "biological flood control" and maintain
ecosystem water balance assuring adequate water resources. Important
habitat for nesting and foraging birds.
Grazing - Optimal
Intensity
0.15 Gt CO
2
e yr
-1
is entirely soil carbon.
Reduces disturbance to plant-insect interactions. Reduce water use on
managed pastures, increase the soils ability to trap contaminants.
Grazing - Legumes
in Pastures
0.15 Gt CO
2
e yr
-1
is entirely soil carbon.
Higher insect diversity, biological nitrogen fixation, improved soil structure,
erosion protection and greater biological diversity.
Peatland
Restoration
0.65 Gt CO
2
e yr
-1
in soil carbon is 80%
of the total mitigation potential.
Restoring reestablishes diverse communities and increases faunal species
that help develop soil structure and fertility. Waste water treatment and
storm water remediation. Flood attenuation. Reduced fire risk lessening
exposure to pollutants associated with lung and pulmonary disorders.
Avoided Peatland
Impacts
0.54 Gt CO
2
e yr
-1
in soil carbon is 72%
of the total mitigation potential.
Coastal Wetland
Restoration
0.52 Gt CO
2
e yr
-1
in soil carbon is 62%
of the total mitigation potential.
Maintains the provision of structure, nutrients and primary productivity and
nurseries for commercially important fish and shrimp. High economic value
for water treatment. Benefits of cross-system nutrient transfer to coral reefs,
coastal protection, and water quality regulation.
Avoided Coastal
Wetland Impacts
0.24 Gt CO
2
e yr
-1
in soil carbon is 79%
of the total mitigation potential.
709
Page 29
Table 2. Example activities to achieve mitigation potentials of soil carbon sequestration
710
pathways. Cells in green indicate Forest pathways, in yellow indicate grassland/agricultural
711
pathways, and in blue indicate wetland pathways.
Adapted from Table S7 in Griscom et al.
32
and
712
Griscom et al.
81
.
713
714
NCS Pathway
General Activities
Specific Activities
Avoided Forest Conversion
PROTECTION
Protected areas establishment
and improved enforcement,
improved land tenure,
indigenous community
management
Improved citing of non-forest land use; forest certification; zero
deforestation commitments; sustainable intensification of agriculture;
diet shifts; avoided loss of high carbon forests.
Avoided Grassland Conversion
Prevent conversion of grasslands to tilled croplands; intensification of
existing croplands.
Avoided Peatland Impacts
No-net-loss mitigation regulations; re- siting of oil palm plantation
permits to non-peat locations.
Avoided Coastal Wetland Impacts
No-net-loss mitigation regulations; avoided harvest of mangroves for
charcoal; avoided consumption of food products with acute impacts on
coastal wetlands (e.g. mangrove replacing shrimp farms).
Biochar
MANAGEMENT
Realignment of agriculture
support programs, ecosystem
services payments,
certification schemes,
improved land tenure,
mitigation programs and
markets
Extension programs to build capacity on biochar management.
Cover cropping
Cultivation of additional cover crops in fallow periods; shift to reduced-
tillage or zero-tillage systems and other conservation agriculture
practices may enhance soil carbon benefits of cover crops.
Trees in Croplands
Regulations and certification programs that promote windbreaks
(shelterbelts), alley cropping, agroforestry systems and farmer managed
natural regeneration (FMNR).
Grazing - Optimal Intensity
Maintaining forage consumption rates that enable maximum forage
production.
Grazing - Legumes in Pastures
Sowing legumes in existing planted pastures.
Reforestation
RESTORATION
Certification and mitigation
programs, indigenous
community management
Regulations that advance minimum forest cover requirements;
integration of trees into grazing lands (i.e. silvopastoral systems); diet
shifts.
Peatland Restoration
Re-wetting and re-planting with native freshwater wetlands species.
Coastal Wetland Restoration
Re-wetting and re-planting with native salt-water wetlands.
715
716
Page 30
Figure 1
717
718
719
Figure 2
720
721
722
723
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Avoided Coastal Impacts
Coastal restoration
Avoided Peat Impacts
Peatland Restoration
Grazing - legumes
Grazing - optimal intensity
Avoided Grassland Conversion
Trees in annual croplands
Cover Cropping
Biochar - crop residue
Avoided Forest Conversion
Reforestation
Soil Carbon Mitigation Potential in 2030 (Pg CO
2e
yr
-1
)
low-cost
cost-effective
maximum with safeguards
climate mitigation
biodiversity
water
other benefits
air
food