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Significance Most nations recently agreed to hold global average temperature rise to well below 2 °C. We examine how much climate mitigation nature can contribute to this goal with a comprehensive analysis of “natural climate solutions” (NCS): 20 conservation, restoration, and/or improved land management actions that increase carbon storage and/or avoid greenhouse gas emissions across global forests, wetlands, grasslands, and agricultural lands. We show that NCS can provide over one-third of the cost-effective climate mitigation needed between now and 2030 to stabilize warming to below 2 °C. Alongside aggressive fossil fuel emissions reductions, NCS offer a powerful set of options for nations to deliver on the Paris Climate Agreement while improving soil productivity, cleaning our air and water, and maintaining biodiversity.
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Natural climate solutions
Bronson W. Griscom
a,b,1
, Justin Adams
a
, Peter W. Ellis
a
, Richard A. Houghton
c
, Guy Lomax
a
, Daniela A. Miteva
d
,
William H. Schlesinger
e,1
, David Shoch
f
, Juha V. Siikamäki
g
, Pete Smith
h
, Peter Woodbury
i
, Chris Zganjar
a
,
Allen Blackman
g
, João Campari
j
, Richard T. Conant
k
, Christopher Delgado
l
, Patricia Elias
a
, Trisha Gopalakrishna
a
,
Marisa R. Hamsik
a
, Mario Herrero
m
, Joseph Kiesecker
a
, Emily Landis
a
, Lars Laestadius
l,n
, Sara M. Leavitt
a
,
Susan Minnemeyer
l
, Stephen Polasky
o
, Peter Potapov
p
, Francis E. Putz
q
, Jonathan Sanderman
c
, Marcel Silvius
r
,
Eva Wollenberg
s
, and Joseph Fargione
a
a
The Nature Conservancy, Arlington, VA 22203;
b
Department of Biology, James Madison University, Harrisonburg, VA 22807;
c
Woods Hole Research Center,
Falmouth, MA 02540;
d
Department of Agricultural, Environmental, and Development Economics, The Ohio State University, Columbus, OH 43210;
e
Cary
Institute of Ecosystem Studies, Millbrook, NY 12545;
f
TerraCarbon LLC, Charlottesville, VA 22903;
g
Resources for the Future, Washington, DC 20036;
h
Institute of Biological and Environmental Sciences, University of Aberdeen, Aberdeen, AB24 3UU, Scotland, United Kingdom;
i
College of Agriculture and
Life Sciences, Cornell University, Ithaca, NY 14853-1901;
j
Ministry of Agriculture, Government of Brazil, Brasilia 70000, Brazil;
k
Natural Resource Ecology
Laboratory & Department of Ecosystem Science and Sustainability, Colorado State University, Fort Collins, CO 80523-1499;
l
World Resources Institute,
Washington, DC 20002;
m
Commonwealth Scientific and Industrial Research Organization, St. Lucia, QLD 4067, Australia;
n
Department of Forest Ecology and
Management, Swedish University of Agricultural Sciences, SE-901 83 Umeå, Sweden;
o
Department of Applied Economics, University of Minnesota, Saint
Paul, MN 55108;
p
Department of Geographical Sciences, University of Maryland, College Park, MD 20742;
q
Department of Biology, University of Florida,
Gainesville, FL 32611-8526;
r
Wetlands International, 6700 AL Wageningen, The Netherlands; and
s
Gund Institute for the Environment, University of
Vermont, Burlington, VT 05405
Contributed by William H. Schlesinger, September 5, 2017 (sent for review June 26, 2017; reviewed by Jason Funk and Will R. Turner)
Better stewardship of land is needed to achieve the Paris Climate
Agreement goal of holding warming to below 2 °C; however, con-
fusion persists about the specific set of land stewardship options
available and their mitigation potential. To address this, we identify
and quantify natural climate solutions(NCS): 20 conservation, res-
toration, and improved land management actions that increase car-
bon storage and/or avoid greenhouse gas emissions across global
forests, wetlands, grasslands, and agricultural lands. We find that
the maximum potential of NCSwhen constrained by food security,
fiber security, and biodiversity conservationis 23.8 petagrams of
CO
2
equivalent (PgCO
2
e) y
1
(95% CI 20.337.4). This is 30% higher
than prior estimates, which did not include the full range of options
and safeguards considered here. About half of this maximum (11.3
PgCO
2
ey
1
) represents cost-effective climate mitigation, assuming
thesocialcostofCO
2
pollution is 100 USD MgCO
2
e
1
by 2030.
Natural climate solutions can provide 37% of cost-effective CO
2
mit-
igation needed through 2030 for a >66% chance of holding warm-
ing to below 2 °C. One-third of this cost-effective NCS mitigation can
be delivered at or below 10 USD MgCO
2
1
. Most NCS actionsif
effectively implementedalso offer water filtration, flood buffer-
ing, soil health, biodiversity habitat, and enhanced climate resilience.
Work remains to better constrain uncertainty of NCS mitigation es-
timates. Nevertheless, existing knowledge reported here provides a
robust basis for immediate global action to improve ecosystem
stewardship as a major solution to climate change.
climate mitigation
|
forests
|
agriculture
|
wetlands
|
ecosystems
The Paris Climate Agreement declared a commitment to hold
the increase in the global average temperature to well below
2 °C above preindustrial levels(1). Most Intergovernmental Panel
on Climate Change (IPCC) scenarios consistent with limiting
warming to below 2 °C assume large-scale use of carbon dioxide
removal methods, in addition to reductions in greenhouse gas
emissions from human activities such as burning fossil fuels and
land use activities (2). The most mature carbon dioxide removal
method is improved land stewardship, yet confusion persists about
the specific set of actions that should be taken to both increase
sinks with improved land stewardship and reduce emissions from
land use activities (3).
The net emission from the land use sector is only 1.5 petagrams
of CO
2
equivalent (PgCO
2
e) y
1
, but this belies much larger gross
emissions and sequestration. Plants and soils in terrestrial eco-
systems currently absorb the equivalent of 20% of anthropo-
genic greenhouse gas emissions measured in CO
2
equivalents
(9.5 PgCO
2
ey
1
) (4). This sink is offset by emissions from land
use change, including forestry (4.9 PgCO
2
ey
1
) and agricultural
activities (6.1 PgCO
2
ey
1
), which generate methane (CH
4
)and
nitrous oxide (N
2
O) in addition to CO
2
(4,5).Thus,ecosystems
have the potential for large additional climate mitigation by com-
bining enhanced land sinks with reduced emissions.
Here we provide a comprehensive analysisof options to mitigate
climate change by increasing carbon sequestration and reducing
emissions of carbon and other greenhouse gases through conser-
vation, restoration, and improved management practices in forest,
wetland, and grassland biomes. This work updates and builds from
work synthesized by IPCC Working Group III (WGIII) (6) for the
greenhouse gas inventory sector referred to as agriculture, forestry,
and other land use (AFOLU). We describe and quantify 20 discrete
Significance
Most nations recently agreed to hold global average tempera-
ture rise to well below 2 °C. We examine how much climate
mitigation nature can contribute to this goal with a compre-
hensive analysis of natural climate solutions(NCS): 20 conser-
vation, restoration, and/or improved land management actions
that increase carbon storage and/or avoid greenhouse gas
emissions across global forests, wetlands, grasslands, and agri-
cultural lands. We show that NCS can provide over one-third of
the cost-effective climate mitigation needed between now and
2030 to stabilize warming to below 2 °C. Alongside aggressive
fossil fuel emissions reductions, NCS offer a powerful set of op-
tions for nations to deliver on the Paris Climate Agreement while
improving soil productivity, cleaning our air and water, and
maintaining biodiversity.
Author contributions: B.W.G., J.A., P.W.E., R.A.H., G.L., D.A.M., W.H.S., D.S., J.V.S., P.S., P.W.,
C.Z., A.B., J.C., R.T.C., C.D., M.R.H., J.K., E.L., S.P., F.E.P., J.S., M.S., E.W., and J. Fargione designed
research; B.W.G., P.W.E., R.A.H., G.L., D.A.M., W.H.S.,D.S., J.V.S., P.W., C.Z., R.T.C., P.E., J.K., E.L.,
and J. Fargione performed research; L.L., S.M., and P.P. contributed new reagents/analytic
tools; B.W.G., P.W.E., R.A.H., G.L., D.A.M., D.S., J.V.S., P.W., C.Z., T.G., M.H., S.M.L., and
J. Fargione analyzed data; and B.W.G., J.A., P.W.E., G.L., D.A.M., W.H.S, D.S., P.S., P.W.,
C.Z., S.M.L., and J. Fargione wrote the paper.
Reviewers: J. Funk, Center for Carbon Removal; and W.R.T., Conservation International.
The authors declare no conflict of interest.
Freely available online through the PNAS open access option.
Data deposition: A global spatial dataset of reforestation opportunities has been depos-
ited on Zenodo (https://zenodo.org/record/883444).
1
To whom correspondence may be addressed. Email: bgriscom@tnc.org or schlesingerw@
caryinstitute.org.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.
1073/pnas.1710465114/-/DCSupplemental.
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mitigation options (referred to hereafter as pathways) within the
AFOLU sector. The pathways we report disaggregate eight options
reported by the IPCC WGIII and fill gaps by including activities
such as coastal wetland restoration and protection and avoided
emissions from savanna fires. We also apply constraints to safe-
guard the production of food and fiber and habitat for biological
diversity. We refer to these terrestrial conservation, restoration,
and improved practices pathways, which include safeguards for
food, fiber, and habitat, as natural climate solutions(NCS).
For each pathway, we estimate the maximum additional mitiga-
tion potential as a starting point for estimating mitigation potential
at or below two price thresholds: 100 and 10 USD MgCO
2
e
1
.The
100 USD level represents the maximum cost of emissions reduc-
tions to limit warming to below 2 °C (7), while 10 USD MgCO
2
e
1
approximates existing carbon prices (8). We aggregate mitigation
opportunities at the 100 USD threshold to estimate the overall
cost-effective contribution of NCS to limiting global warming to
below 2 °C. For 10 of the most promising pathways, we provide
global maps of mitigation potential. Most notably, we provide a
global spatial dataset of reforestation opportunities (https://zenodo.
org/record/883444) constrained by food security and biodiversity
safeguards. We also review noncarbon ecosystem services associ-
ated with each pathway.
These findings are intended to help translate climate commit-
ments into specific NCS actions that can be taken by government,
private sector, and local stakeholders. We also conduct a com-
prehensive assessment of overall and pathway-specific uncertainty
for our maximum estimates to expose the implications of variable
data quality and to help prioritize research needs.
Results and Discussion
Maximum Mitigation Potential of NCS with Safeguards. We find that
the maximum additional mitigation potential of all natural path-
ways is 23.8 PgCO
2
ey
1
(95% CI 20.337.4) at a 2030 reference
year (Fig. 1 and SI Appendix, Table S1). This amount is not
constrained by costs, but it is constrained by a global land cover
scenario with safeguards for meeting increasing human needs for
food and fiber. We allow no reduction in existing cropland area,
but we assume grazing lands in forested ecoregions can be refor-
ested, consistent with agricultural intensification and diet change
scenarios (9, 10). This maximum value is also constrained by ex-
cluding activities that would either negatively impact biodiversity
(e.g., replacing native nonforest ecosystems with forests) (11) or
have carbon benefits that are offset by net biophysical warming
(e.g., albedo effects from expansion of boreal forests) (12). We
avoid double-counting among pathways (SI Appendix,TableS2).
We report uncertainty estimated empirically where possible (12
pathways) or from results of an expert elicitation (8 pathways). See
Fig. 1 for synthesis of pathway results.
Our estimate of maximum potential NCS mitigation with safe-
guards is 30% higher than prior constrained and unconstrained
maximum estimates (5, 9, 1316). Our estimate is higher, despite
our food, fiber, and biodiversity safeguards, because we include a
larger number of natural pathways. Other estimates do not include
all wetland pathways (5, 9, 1316), agricultural pathways (1316),
or temperate and boreal ecosystems (13, 14). The next highest
estimate (14) (18.3 PgCO
2
y
1
) was confined to tropical forests,
but did not include a food production safeguard and was higher
than our estimate for tropical forest elements of our pathways
(12.6, 6.618.6 PgCO
2
y
1
). Similarly, our estimates for specific
pathways are lower than other studies for biochar (17), conser-
vation agriculture (15), and avoided coastal wetland impacts (18).
We account for new research questioning the magnitude of po-
tential for soil carbon sequestration through no-till agriculture
(19) and grazing land management (20), among other refinements
to pathways discussed below. Our estimate for avoided forest
conversion falls between prior studies on deforestation emissions
(2124). Our spatially explicit estimate for reforestation was
slightly higher compared with a prior nonspatially explicit estimate
*
*
012 3410
Reforestation
Avoided Forest Conv.
Natural Forest Mgmt.
Improved Plantations
Avoided Woodfuel
Fire Mgmt.
Biochar
Trees in Croplands
Nutrient Mgmt.
Grazing - Feed
Conservation Ag.
Improved Rice
Grazing - Animal Mgmt.
Grazing - Optimal Int.
Grazing - Legumes
Avoided Grassland Conv.
Coastal Restoration
Peat Restoration
Avoided Peat Impacts
Avoided Coastal Impacts
Forests
Wetlands
A
g. & Grasslands
Climate mitigation potential in 2030 (PgCO
2
e yr
-1
)
<2°C ambition
low cost portion
of <2°C ambition
climate mitigation
biodiversity
water
soil
air
other benefits
maximum with safeguards
Fig. 1. Climate mitigation potential of 20 natural pathways. We estimate maximum climate mitigation potential with safeguards for reference year 2030.
Light gray portions of bars represent cost-effective mitigation levels assuming a global ambition to hold warming to <C(<100 USD MgCO
2
e
1
y
1
). Dark
gray portions of bars indicate low cost (<10 USD MgCO
2
e
1
y
1
) portions of <2 °C levels. Wider error bars indicate empirical estimates of 95% confidence
intervals, while narrower error bars indicate estimates derived from expert elicitation. Ecosystem service benefits linked with each pathway are indicated by
colored bars for biodiversity, water (filtration and flood control), soil (enrichment), and air (filtration). Asterisks indicate truncated error bars. See SI Appendix,
Tables S1, S2, S4, and S5 for detailed findings and sources.
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www.pnas.org/cgi/doi/10.1073/pnas.1710465114 Griscom et al.
(9). Natural pathway opportunities differ considerably among
countries and regions (SI Appendix, Figs S1S3 and Table S3).
Cost-Effective and Low-Cost NCS. We explore the proportion of
maximum NCS mitigation potential that offers a cost-effective
contribution to meeting the Paris Climate Agreement goal of lim-
iting warming to below 2 °C. We define a <Ccost-effective
level of mitigation as a marginal abatement cost not greater than
100 USD MgCO
2
1
as of 2030. This value is consistent with
estimates for the avoided cost to society from holding warming to
below 2 °C (7, 25). We find that about half (11.3 PgCO
2
ey
1
)of
the maximum NCS potential meets this cost-effective threshold.
To estimate the portion of NCS that are cost effective for holding
warming to below 2 °C, we estimated the fraction of the maximum
potential of each natural pathway (high =90%, medium =60%,
or low =30%) that could be achieved without exceeding costs of
100 USD MgCO
2
1
, informed by published marginal abatement
cost curves. Our assignment of these indicative high, medium, and
low cost-effective mitigation levels reflects the coarse resolution of
knowledge on global marginal abatement costs for NCS. These
default levels structured our collective judgment where cost curve
data were incomplete (SI Appendix,TableS4). Using parallel
methods, we find that more than one-third of the <2 °C cost
effectivelevels for natural pathways are low cost (<10 USD
MgCO
2
1
; 4.1 PgCO
2
ey
1
; Fig. 1 and SI Appendix, Table S4).
The low-costand cost-effective NCS carbon sequestration
opportunities compare favorably with cost estimates for emerging
technologies, most notably bioenergy with carbon capture and
storage (BECCS)which range from 40 USD MgCO
2
1
to over
1,000 USD MgCO
2
1
. Furthermore, large-scale BECCS is un-
tested and likely to have significant impacts on water use, bio-
diversity, and other ecosystem services (2, 26).
Our 100 USD constrained estimate (11.3 PgCO
2
ey
1
)isconsid-
erably higher than prior central estimates (6, 14, 27, 28), and it is
somewhat higher than the upper-end estimate from the IPCC Fifth
Assessment Report (AR5) (10.6 PgCO
2
ey
1
). Aside from our in-
clusion of previously ignored pathways as discussed above, this
aggregate difference belies larger individual pathway differences
between our estimates and those reported in the IPCC AR5. We find
a greater share of cost-constrained potential through reforestation,
forestry, wetland protection, and trees in croplands than the IPCC
AR5, despite our stronger constraints on land availability, biodiversity
conservation, and biophysical suitability for forests (14, 29).
NCS Contribution to a <2 °C Pathway. To what extent can NCS
contribute to carbon neutrality by helping achieve net emission
targets during our transition to a decarbonized energy sector?
Warming will likely be held to below 2 °C if natural pathways are
implemented at cost-effective levels indicated in Fig. 1, and if we
avoid increases in fossil fuel emissions for 10 y and then drive them
downto7%ofcurrentlevelsby2050andthentozeroby2095(Fig.
2). This scenario (14) assumes a 10-y linear increase of NCS to the
cost-effective mitigation levels, and a >66% likelihood of holding
warming to below 2 °C following a model by Meinshausen et al.
(30). Under this scenario, NCS provide 37% of the necessary CO
2
e
mitigation between now and 2030 and 20% between now and 2050.
Thereafter, the proportion of total mitigation provided by NCS
further declines as the proportion of necessary avoided fossil fuel
emissions increases and as some NCS pathways saturate. Natural
climate solutions are thus particularly important in the near term
for our transition to a carbon neutral economy by the middle of this
century. Given the magnitude of fossil fuel emissions reductions
required under any <2 °C scenario, and the risk of relying heavily
on negative emissions technologies (NETs) that remain decades
from maturity (3), immediate action on NCS should not delay
action on fossil fuel emissions reductions or investments in NETs.
Half of this cost-effective NCS mitigation is due to additional
carbon sequestration of 5.6 PgCO
2
ey
1
by nine of the pathways,
while the remainder is from pathways that avoid further emissions
of CO
2
,CH
4
,andN
2
O(SI Appendix, Fig. S4 and Table S1). Ag-
gregate sequestration levels begin to taper off around 2060, al-
though most pathways can maintain the 2030 mitigation levels we
report for more than 50 years (Fig. 2 and pathway-specific satu-
ration periods in SI Appendix, Table S1). The NCS scenario il-
lustrated in Fig. 2 will require substantial near-term ratcheting up
of both fossil fuel and NCS mitigation targets by countries to
achieve the Paris Climate Agreement goal to hold warming to
below 2 °C. Countries provided nationally determined contri-
butions (NDCs) with 2025 or 2030 emissions targets as a part of
the Paris Climate Agreement. While most NDCs indicate inclusion
of land sector mitigation, only 38 specify land sector mitigation
contributions, of 160 NDCs assessed (31). Despite these limitations,
analyses indicate that if NDCs were fully implemented, NCS would
contribute about 20% of climate mitigation (31) and about 2
PgCO
2
ey
1
mitigation by 2030 (31, 32). As such, a small portion of
the 11.3 PgCO
2
ey
1
NCS opportunity we report here has been
included in existing NDCs. Across all sectors, the NDCs fall short by
1114 PgCO
2
ey
1
of mitigation needed to keep 2030 emissions in
line with cost-optimal 2 °C scenarios(33).Hence,NCScould
contributealargeportionabout 9 PgCO
2
ey
1
of the increased
ambition needed by NDCs to achieve the Paris Climate Agreement.
Our assessment of the potential contribution of NCS to meeting
the Paris Agreement is conservative in three ways. First, payments for
ecosystem services other than carbon sequestration are not consid-
ered here and could spur cost-effective implementation of NCS be-
yond the levels we identified. Natural climate solutions enhance
biodiversity habitat, water filtration, flood control, air filtration, and
soil quality (Fig. 1) among other services, some of which have high
monetary values (3436) (see SI Appendix,TableS5for details).
Improved human health from dietary shifts toward plant-based foods
reduce healthcare expenses and further offset NCS costs (37).
Second, our findings are conservative because we only include
activities and greenhouse gas fluxes where data were sufficiently
robust for global extrapolation. For example, we exclude no-till
agriculture (Conservation Agriculture pathway), we exclude im-
proved manure management in concentrated animal feed opera-
tions (Nutrient Management pathway), we exclude adaptive
multipaddock grazing (Grazing pathways), and we exclude soil
Fig. 2. Contribution of natural climate solutions (NCS) to stabilizing warming
to below 2 °C. Historical anthropogenic CO
2
emissions before 2016 (gray line)
prelude either business-as-usual (representative concentration pathway, sce-
nario 8.5, black line) or a net emissions trajectory needed for >66% likelihood of
holding global warming to below 2 °C (green line). The green area shows cost-
effective NCS (aggregate of 20 pathways), offering 37% of needed mitigation
through 2030, 29% at year 2030, 20% through 2050, and 9% through 2100. This
scenario assumes that NCS are ramped up linearly over the next decade to <C
levels indicated in Fig. 1 and held at that level (=10.4 PgCO
2
y
1
, not including
other greenhouse gases). It is assumed that fossil fuel emissions are held level
over the next decade then decline linearly to reach 7% of current levels by 2050.
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carbon emissions that may occur with conversion of forests to
pasture (Avoided Forest Conversion pathway). Future research
may reveal a robust empirical basis for including such activities
and fluxes within these pathways.
Third, the Paris Agreement states goals of limiting warming to
well below 2 °Cand pursuing efforts to limit the temperature
increase to 1.5 °C.Ouranalysisspecifiesa>66% chance of holding
warming to just below 2 °C (30). Additional investment in all miti-
gation efforts (i.e., beyond 100 USD MgCO
2
1
), including NCS,
would be warranted to keep warming to well below 2 °C, or to 1.5 °C,
particularly if a very likely (90%) chance of success is desired.
Specific Pathway Contributions. Forest pathways offer over two-
thirds of cost-effective NCS mitigation needed to hold warming
to below 2 °C and about half of low-cost mitigation opportunities
(SI Appendix, Table S4). Reforestation is the largest natural
pathway and deserves more attention to identify low-cost miti-
gation opportunities. Reforestation may involve trade-offs with
alternative land uses, can incur high costs of establishment, and
is more expensive than Avoided Forest Conversion (38). How-
ever, this conclusion from available marginal abatement cost
curves ignores opportunities to reduce costs, such as involving
the private sector in reforestation activities by establishing
plantations for an initial commercial harvest to facilitate natural
and assisted forest regeneration (39). The high uncertainty of
maximum reforestation mitigation potential with safeguards
(95% CI 2.717.9 PgCO
2
ey
1
) is due to the large range in
existing constrained estimates of potential reforestation extent
(3451,779 Mha) (14, 16, 4042). As with most forest pathways,
reforestation has well-demonstrated cobenefits, including bio-
diversity habitat, air filtration, water filtration, flood control, and
enhanced soil fertility (34). See SI Appendix, Table S5 for de-
tailed review of ecosystem services across all pathways.
Our maximum reforestation mitigation potential estimate is
somewhat sensitive to our assumption that all grazing land in
forested ecoregions is reforested. If we assume that 25%, 50%,
or 75% of forest ecoregion grazing lands were not reforested, it
would result in 10%, 21%, and 31% reductions, respectively, in
our estimate of reforestation maximum mitigation potential. While
42% of reforestation opportunities we identify are located on
lands now used for grazing within forest ecoregions, at our <C
ambition mitigation level this would displace only 4% of global
grazing lands, many of which do not occur in forested ecoregions
(20). Grazing lands can be released by shifting diets and/or
implementing Grazing-Feed and Grazing-Animal Management
pathways, which reduce the demand for grazing lands without
reducing meat and milk supply (43).
Avoided Forest Conversion offers the second largest maxi-
mum and cost-effective mitigation potential. However, imple-
mentation costs may be secondary to public policy challenges in
frontier landscapes lacking clear land tenure. The relative suc-
cess of Brazils efforts to slow deforestation through a strong
regulatory framework, accurate and transparent federal moni-
toring, and supply chain interventions provides a promising
model (44), despite recent setbacks (45). We find relatively low
uncertainty for Avoided Forest Conversion (±17%), reflecting
considerable global forest monitoring research in the last decade
stimulated by interest in reducing emissions from deforestation
and forest degradation (REDD) (46).
Improved forest management (i.e., Natural Forest Management
and Improved Plantations pathways) offers large and cost-effective
mitigation opportunities, many of which could be implemented
rapidly without changes in land use or tenure. While some activities
can be implemented without reducing wood yield (e.g., reduced-
impact logging), other activities (e.g., extended harvest cycles)
would result in reduced near-term yields. This shortfall can be
met by implementing the Reforestation pathway, which includes
new commercial plantations. The Improved Plantations pathway
ultimately increases wood yields by extending rotation lengths from
the optimum for economic profits to the optimum for wood yield.
Grassland and agriculture pathways offer one-fifth of the total
NCS mitigation needed to hold warming below 2 °C, while main-
taining or increasing food production and soil fertility. Collectively,
the grassland and agriculture pathways offer one-quarter of low-cost
NCS mitigation opportunities. Cropland Nutrient Management is
the largest cost-effective agricultural pathway, followed by Trees in
Croplands and Conservation Agriculture. Nutrient Management
and Trees in Croplands also improve air quality, water quality, and
provide habitat for biodiversity (SI Appendix,TableS5). Our analysis
of nutrient management improves upon that presented by the IPCC
AR5 in that we use more recent data for fertilizer use and we project
future use of fertilizers under both a business as usualand a best
management practicescenario. Future remote sensing analyses to
improve detection of low-density trees in croplands (47) will constrain
our uncertainty about the extent of this climate mitigation opportu-
nity. The addition of biochar to soil offers the largest maximum
mitigation potential among agricultural pathways, but unlike most
other NCS options, it has not been well demonstrated beyond re-
search settings. Hence trade-offs, cost, and feasibility of large scale
implementation of biochar are poorly understood. From the livestock
sector, two improved grazing pathways (Optimal Intensity and Le-
gumes) increase soil carbon, while two others (Improved Feed and
Animal Management) reduce methane emission.
Wetland pathways offer 14% of NCS mitigation opportunities
needed to hold warming to <2 °C, and 19% of low-cost NCS
mitigation. Wetlands are less extensive than forests and grass-
lands, yet per unit area they hold the highest carbon stocks and
the highest delivery of hydrologic ecosystem services, including
climate resilience (47). Avoiding the loss of wetlandsan urgent
concern in developing countriestends to be less expensive than
wetland restoration (49). Improved mapping of global wetlands
particularly peatlandsis a priority for both reducing our reported
uncertainty and for their conservation and restoration.
Challenges. Despite the large potential of NCS, land-based se-
questration efforts receive only about 2.5% of climate mitigation
dollars (50). Reasons may include not only uncertainties about
the potential and cost of NCS that we discuss above, but also
concerns about the permanence of natural carbon storage and
social and political barriers to implementation. A major concern
is the potential for Reforestation, Avoided Forest Conversion,
and Wetland/Peatland pathways to compete with the need to
increase food production. Reforestation and Avoided Forest
Conversion remain the largest mitigation opportunities despite
avoiding reforestation of mapped croplands and constraints we
placed on avoiding forest conversion driven by subsistence ag-
riculture (SI Appendix, Table S1). A large portion (42%) of our
maximum reforestation mitigation potential depends on reduced
need for pasture accomplished via increased efficiency of beef
production and/or dietary shifts to reduce beef consumption. On
the other hand, only a 4% reduction in global grazing lands is
needed to achieve <2 °C ambition reforestation mitigation levels,
and reduced beef consumption can have large health benefits (51).
A portion of wetland pathways would involve limited displacement
of food production; however, the extremely high carbon density
of wetlands and the valuable ecosystem services they provide
suggest that protecting them offers a net societal benefit (52).
Feedbacks from climate change on terrestrial carbon stocks
are uncertain. Increases in temperature, drought, fire, and pest
outbreaks could negatively impact photosynthesis and carbon
storage, while CO
2
fertilization has positive effects (53). Unchecked
climate change could reverse terrestrial carbon sinks by midcentury
and erode the long-term climate benefits of NCS (54). Thus, cli-
mate change puts terrestrial carbon stocks (2.3 exagrams) (55) at
risk. Cost-effective implementation of NCS, by increasing terrestrial
carbon stocks, would slightly increase (by 4%) the stocks at risk by
4of6
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www.pnas.org/cgi/doi/10.1073/pnas.1710465114 Griscom et al.
2050. However, the risk of net emissions from terrestrial carbon
stocks is less likely under a <2 °C scenario. As such, NCS slightly
increase the total risk exposure, yet will be a large component of any
successful effort to mitigate climate change and thus help mitigate
this risk. Further, most natural pathways can increase resilience to
climate impacts. Rewetting wetlands reduces risk of peat fires (56).
Reforestation that connects fragmented forests reduces exposure to
forest edge disturbances (57). Fire management increases resilience
to catastrophic fire (58). On the other hand, some of our pathways
assume intensification of food and wood yieldsand some con-
ventional forms of intensification can reduce resilience to climate
change (59). All of these challenges underscore the urgency of
aggressive, simultaneous implementation of mitigation from both
NCS and fossil fuel emissions reductions, as well as the importance
of implementing NCS and land use intensification in locally appro-
priate ways with best practices that maximize resilience.
While the extent of changes needed in global land stewardship is
large (SI Appendix, Tables S1 and S4), we find that the environ-
mental ambition reflected in eight recent multilateral announce-
mentsiswellalignedwithour<2 °C NCS mitigation levels.
However, only four of these announcements are specific enough for
quantitative comparison: The New York Declaration on Forests,
the Bonn Challenge, the World Business Council on Sustainable
Development Vision 2050, and the 4 pour 1000initiative (SI
Appendix,TableS6). The first three of these have quantitative
targets that are somewhat more ambitious than our <2 °C mitigation
levels for some pathways, while the 4 pour 1000 initiative is con-
siderably more ambitious for soil carbon storage. More explicit and
comprehensive policy targets for all biomes and natural pathways are
needed to clarify the role of NCS in holding warming to below 2 °C.
Next Steps. Considerable scientific work remains to refine and
reduce the uncertainty of NCS mitigation estimates. Work also
remains to refine methods for implementing pathways in socially
and culturally responsible ways while enhancing resilience and
improving food security for a growing human population (60).
Nevertheless, our existing knowledge reported here provides a
solid basis for immediately prioritizing NCS as a cost-effective way
to provide 11 PgCO
2
ey
1
of climate mitigation within the next
decadea terrestrial ecosystem opportunity not fully recognized
by prior roadmaps for decarbonization (15, 61). Delaying imple-
mentation of the 20 natural pathways presented here would in-
crease the costs to society for both mitigation and adaptation,
while degrading the capacity of natural systems to mitigate climate
change and provide other ecosystem services (62). Regreening the
planet through conservation, restoration, and improved land
management is a necessary step for our transition to a carbon
neutral global economy and a stable climate.
Methods
Estimating Maximum Mitigation Potential with Safeguards. We estimate the
maximum additional annual mitigation potential above a business-as-usual
baseline at a 2030 reference year, with constraints for food, fiber, and bio-
diversity safeguards (SI Appendix,TablesS1andS2). For food, we allow no re-
duction in existing cropland area, but do allow the potential to reforest all grazing
lands in forested ecoregions, consistent with agricultural intensification scenarios
(9) and potential for dietary changes in meat consumption (10). For fiber, we as-
sume that any reduced timber production associated with implementing our
Natural Forest Management pathway is made up by additional wood production
associated with Improved Plantations and/or Reforestation pathways. We also
avoid activities within pathways that would negatively impact biodiversity, such as
establishing forests where they are not the native cover type (11).
For most pathways, we generated estimates of the maximum mitigation
potential (M
x
) informed by a review of publications on the potential extent (A
x
)
and intensity of flux (F
x
), where M
x
=A
x
×F
x
. Our estimates for the reforestation
pathway involved geospatial analyses. For most pathways the applicable extent
was measured in terms of area (hectares); however, for five of the pathways
(Biochar, Cropland Nutrient Management, GrazingImproved Feed, Grazing
Animal Management, and Avoided Woodfue l Harvest) othe r units of extent
were used (SI Appendix,TableS1). For five pathways (Avoided Woodfuel
Harvest; GrazingOptimal Intensity, Legumes, and Feed; and Conservation
Agriculture) estimates were derived directly from an existing published esti-
mate. An overview of pathway definitions, pathway-specific methods, and
adjustments made to avoid double counting are provided in SI Appendix,
Table S2.SeeSI Appendix,pp3679 for methods details.
Uncertainty Estimates. We estimated uncertainty for maximum mitigation
estimates of each pathway using methods consistent with IPCC good practice
guidance (63) for the 12 pathways where empirical uncertainty estimation
was possible. For the remaining eight pathways (indicated in Fig. 1), we used
the Delphi method of expert elicitation (64) following best practices outline
by Mach et al. (65) where applicable and feasible. The Delphi method in-
volved two rounds of explicit questions about expert opinion on the potential
extent (A
x
) and intensity of flux (F
x
) posed to 20 pathway experts, half of
whom were not coauthors (see SI Appendix,pp3839 for names). We com-
bined A
x
and F
x
uncertainties using IPCC Approach 2 (Monte Carlo simulation).
Assigning Cost-Constrained Mitigation Levels. We assumed that a maximum
marginal cost of 100 US dollars MgCO
2
e
1
y
1
in 2030 would be required
across all mitigation options (including fossil fuel emissions reductions and
NCS) to hold warming to below 2 °C (7). This assumption is consistent with
the values used in other modeling studies (16, 66) and was informed by a
social cost of carbon in 2030 estimated to be 82260 USD MgCO
2
e
1
to meet
the 1.52 °C climate target (7).
To calibrate individual NCS pathways with a goal of holding warming to
below 2 °C, we assessed which of three default mitigation levels30%, 60%,
or 90% of maximumcaptures mitigation costs up to but not more than
100 USD MgCO
2
e
1
, informed by marginal abatement cost (MAC) curve
literature. Our assignment of these default levels reflects that the MAC lit-
erature does not yet enable a precise understanding of the complex and
geographically variable range of costs and benefits associated with our
20 natural pathways. We also assessed the proportion of NCS mitigation that
could be achieved at low cost. For this we used a marginal cost threshold of
10 USD MgCO
2
e
1
, which is consistent with the current cost of emission
reduction efforts underway and current prices on existing carbon markets.
For references and details see SI Appendix.
Projecting NCS Contribution to Climate Mitigation. We projected the potential
contributions of NCS to overall CO
2
e mitigation action needed for a likely
(greater than 66%) chance of holding warming to below 2 °C between
2016 and 2100. We compared this NCS scenario to a baseline scenario in which
NCS are not implemented. In our NCS scenario, we assumed a linear ramp-up
period between 2016 and 2025 to our <2 °C ambition mitigation levels
reported in SI Appendix, Table S4. During this period, we assumed fossil fuel
emissions were also held constant, after which they would decline. We as-
sumed a maintenance of <2 °C ambition NCS mitigation levels through 2060,
allowing for gradual pathway saturation represented as a linear decline of
natural pathway mitigation from 2060 to 2090. We consider this a conserva-
tive assumption about overall NCS saturation, given the time periods we es-
timate before saturation reported in SI Appendix,TableS1. This scenario and
the associated action on fossil fuel emissions reductions needed are repre-
sented in Fig. 2 through 2050. Scenario construction builds from ref. 14, with
model parameters from Meinshausen et al. (30). The proportion of CO
2
miti-
gation provided by NCS according to the scenario described above is adjusted
to a proportion of CO
2
e with the assumption that non-CO
2
greenhouse gases
are reduced at the same rate as CO
2
for NCS and other sectors.
Characterizing Activities and Cobenefits. We identified mitigation activities and
noncarbon ecosystem services associated with each of the 20natural pathways
(SI Appendix,TablesS5andS7). We used a taxonomy of conservation actions
developed by the International Union for Conservation of Nature (IUCN) and
the Conservation Measures Partnership (67) to link pathways with a known set
of conservation activities. The IUCN taxonomy does not identify activities that
are specific to many of our pathways, so we list examples of more specific
activities associated with each pathway (SI Appendix, Table S7). We identify
four generalized types of ecosystem services (biodiversity, water, soil, and air)
that may be enhanced by implementation of activities within eac h natural
pathwaybut only where one or more peer-reviewed publication confirms
thelink(Fig.1andSI Appendix,TableS5).
ACKNOWLEDGMENTS. We thank L. Almond, A. Baccini, A. Bowman, S. Cook-
Patton, J. Evans, K. Holl, R. Lalasz, A. Nassikas, M. Spalding, and M. Wolosin for
inputs, and expert elicitation respondents. We also thank members of the
Matthew Hansen laboratory for the development of datasets and the National
Evolutionary Synthesis Center grasslands working group, which includes
Griscom et al. PNAS Early Edition
|
5of6
EARTH, ATMOSPHERIC,
AND PLANETARY SCIENCES
SUSTAINABILITY
SCIENCE
C. Le hm ann , D. G rif fi th, T. M. A nders on, D. J. Beerling, W. Bond, E. Denton,
E. Edwards, E. Forrestel, D. Fox, W. Hoffmann, R. Hyde, T. Kluyver, L. Mucina,
B. Passey, S. Pau, J. Ratnam, N. Salamin, B. Santini, K. Simpson, M. Smith,
B. Spriggs, C. Still, C. Strömberg, and C. P. Osborne. This study was made
possible by funding from the Doris Duke Charitable Foundation. P.W. was
supported in part by US Department of AgricultureNational Institute of
Food and Agriculture Project 2011-67003-30205. E.W.s contribution was in
association with the CGIAR Research Program on Climate Change,
Agriculture and Food Security, carried out with support from CGIAR
Fund Donors and through bilateral funding agreements.
1. United Nations Framework Convention on Climate Change (2015) COP21ClimateAgree-
ment (UNFCCC, Paris) Available at unfccc.int/resource/docs/2015/cop21/eng/l09r01.pdf Ac-
cessed June 20, 2017.
2. Smith P, et al. (2016) Biophysical and economic limits to negative CO2 emissions. Nat
Clim Chang 6:4250.
3. Field CB, Mach KJ (2017) Rightsizing carbon dioxide removal. Science 356:706707.
4. Le Quéré C, et al. (2015) Global carbon budget 2014. Earth Syst Sci Data 7:4785.
5. Intergovernmental Panel on Climate Change (2014) Climate Change 2014: Mitigation
of Climate Change.Contribution of Working Group III to the Fifth Assessment Report
of the Intergovernmental Panel on Climate Change, eds Edenhofer O, et al. (Cam-
bridge Univ Press, Cambridge, UK).
6. Smith P, et al. (2014) Agriculture, forestry and other land use (AFOLU). Climate
Change 2014: Mitigation of Climate Change. Contribution of Working Group III to
the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, eds
Edenhofer O, et al. (Cambridge Univ Press, Cambridge, UK), p 179.
7. Dietz S, Stern N (2015) Endogenous growth, convexity of damage and climate risk: How
Nordhausframework supports deep cuts in carbon emissions. Econ J (Oxf) 125:574620.
8. World Bank Ecofys (2016) State and Trends of Carbon Pricing 2016 (The World Bank,
Washington, DC).
9. Smith P, et al. (2013) How much land-based greenhouse gas mitigation can be achieved
without compromising food security and environmental goals? Glob Change Biol 19:
22852302.
10. Stehfest E, et al. (2009) Climate benefits of changing diet. Clim Change 95:83102.
11. Veldman JW, et al. (2015) Tyranny of trees in grassy biomes. Science 347:484485.
12. Li Y, et al. (2015) Local cooling and warming effects of forests based on satellite
observations. Nat Commun 6:6603.
13. Houghton RA (2013) The emissions of carbon from deforestation and degradation in
the tropics: Past trends and future potential. Carbon Manag 4:539546.
14. Houghton RA, Byers B, Nassikas AA (2015) A role for tropical forests in stabilizing
atmospheric CO2. Nat Clim Chang 5:10221023.
15. Pacala S, Socolow R (2004) Stabilization wedges: Solving the climate problem for the
next 50 years with current technologies. Science 305:968972.
16. Canadell JG, Raupach MR (2008) Managing forests for climate change mitigation.
Science 320:14561457.
17. Woolf D, Amonette JE, Street-Perrott FA, Lehmann J, Joseph S (2010) Sustainable
biochar to mitigate global climate change. Nat Commun 1:56.
18. Pendleton L, et al. (2012) Estimating global blue carbonemissions from conversion
and degradation of vegetated coastal ecosystems. PLoS One 7:e43542.
19. Powlson DS, Stirling CM, Jat ML (2014) Limited potential of no-till agriculture for
climate change mitigation. Nat Clim Chang 4:678683.
20. Henderson BB, et al. (2015) Greenhouse gas mitigation potential of the worlds
grazing lands: Modeling soil carbon and nitrogen fluxes of mitigation practices. Agric
Ecosyst Environ 207:91100.
21. Baccini A, et al. (2012) Estimated carbon dioxide emissions from tropical deforestation
improved by carbon-density maps. Nat Clim Chang 2:182185.
22. Tyukavina A, et al. (2015) Aboveground carbon loss in natural and managed tropical
forests from 2000 to 2012. Environ Res Lett 10:114.
23. Harris NL, et al. (2012) Baseline map of carbon emissions from deforestation in
tropical regions. Science 336:15731576.
24. Zarin DJ, et al. (2015) Can carbon emissions from tropical deforestation drop by 50%
in five years? Glob Chang Biol 22:13361347.
25. Nordhaus W (2014) Estimates of the social cost of carbon: Concepts and results from the
DICE-2013R model and alternative approaches. J Assoc Environ Resour Econ 1:273312.
26. Santangeli A , et al. (2016) Global change synergies and trade-offs between renew-
able energy and biodiversity. Glob Change Biol Bioenergy 8:941951.
27. Houghton RA (2013) Role of forests and impact of deforestation in the global carbon
cycle. Global Forest Monitoring from Earth Observation, eds Achard F, Hansen MC
(CRC Press, Boca Raton, Florida), pp 1538.
28. Global Commission on the Economy and Climate (2015) Emission reduction potential.
Available at http://newclimateeconomy.report/workingpapers/wp-content/uploads/
sites/5/2016/0 4/NCE-technical -note-emission- reduction-poten tial_final.pdf. Ac cessed
February 23, 2017.
29. Sohngen B, Sedjo R (2006) Carbon sequestration in global forests under different
carbon price regimes. Energy J 27:109126.
30. Meinshausen M, et al. (2009) Greenhouse-gas emission targets for limiting global
warming to 2 degrees C. Nature 458:11581162.
31. Forsell N, et al. (2016) Assessing the INDCsland use, land use change, and forest
emission projections. Carbon Balance Manag 11:26.
32. Grassi G, Dentener F (2015) Quantifying the contribution of the Land Use sector to the
Paris Climate Agreement. Available at http://publications.jrc.ec.europa.eu/repository/
bitstream/JRC98451/jrc%20lulucf-indc%20report.pdf. Accessed December 15, 2016.
33. Rogelj J, et al. (2016) Paris agreement climate proposals need a boost to keep
warming well below 2 °C. Nature 534:631639.
34. Millennium Ecosystem Assessment (2005) Ecosystems and Human Well-being: Synthesis
(Island Press, Washington, DC).
35. Bendor TK, Livengood A, Lester TW, Davis A, Yonavjak L (2015) Defining and eval-
uating the ecological restoration economy. Restor Ecol 23:209219.
36. Paustian K, et al. (2016) Climate-smart soils. Nature 532:4957.
37. Springmann M, Godfray HCJ, Rayner M, Scarborough P (2016) Analysis and valuation
of the health and climate change cobenefits of dietary change. Proc Natl Acad Sci
USA 113:41464151.
38. Strengers BJ, Van Minnen JG, Eickhout B (2008) The role of carbon plantations in
mitigating climate change: Potentials and costs. Clim Change 88:343366.
39. Ashton MS, et al. (2014) Restoration of rain forest beneath pine plantations: A relay flo-
ristic model with special application to tropical South Asia. For Ecol Manage 329:351359.
40. Zomer RJ, Trabucco A, Bossio DA, Verchot LV (2008) Climate change mitigation: A
spatial analysis of global land suitability for clean development mechanism affores-
tation and reforestation. Agric Ecosyst Environ 126:6780.
41. Nilsson S, Schopfhauser W (1995) The carbon-sequestration potential of a global af-
forestation program. Clim Change 30:267293.
42. Minnemeyer S, Laestadius L, Potapov P, Sizer N, Saint-Laurent C (2014) Atlas of Forest
Landscape Restoration Opportunities (World Resour Inst itute, Washington, DC).
Available at www.wri.org/resources/maps/atlas-forest-and-lands cape-restoration-
opportunities. Accessed May 30, 2017.
43. Havlík P, et al. (2014) Climate change mitigation through livestock system transitions.
Proc Natl Acad Sci USA 111:37093714.
44. Nepstad D, et al. (2014) Slowing Amazon deforestation through public policy and
interventions in beef and soy supply chains. Science 344:11181123.
45. Instituto Nacional de Pesquisas Espaciais (INPE) (2016) INPE Noticias: PRODES estima
7.989 km2 de desmatamento por corte raso na Amazônia em 2016. Available at www.
inpe.br/noticias/noticia.php?Cod_Noticia=4344. Accessed March 1, 2017.
46. Goetz SJ, et al. (2015) Measurement and monitoring needs, capabilities and potential
for addressing reduced emissions from deforestation and forest degradation under
REDD+.Environ Res Lett 10:123001.
47. Zomer RJ, et al. (2016)Global Tree Cover and Biomass Carbon on Agricultural Land: The
contribution of agroforestry to global and national carbon budgets. Sci Rep 6:29987.
48. Barbier EB, et al. (2011) The value of estuarine and coastal ecosystem services. Ecol
Monogr 81:169193.
49. Bayraktarov E, et al. (2016) The cost and feasibility of marine coastal restoration. Ecol
Appl 26:10551074.
50. Buchner BK, et al. (2015) Global landscape of climate finance 2015. Available at
https://climatepolicyinitiative.org/publication/global-landscape-of-climate-finance-2015/.
Accessed September 22, 2017.
51. Tilman D, Clark M (2014) Global diets link environmental sustainability and human
health. Nature 515:518522.
52. Alongi DM (2002) Present state and future of the worlds mangrove forests. Environ
Conserv 29:331349.
53. Cox PM, et al. (2013) Sensitivity of tropical carbon to climate change constrained by
carbon dioxide variability. Nature 494:341344.
54. Friedlingstein P (2015) Carbon cycle feedbacks and future climate change. Philos
Trans R Soc A Math Phys Eng Sci 373:20140421.
55. House JI, Prentice IC, Le Quéré CC (2002) Maximum impacts of future reforestation or
deforestation on atmospheric CO2. Glob Change Biol 8:10471052.
56. Page S, et al. (2009) Restoration ecology of lowland tropical peatlands in Southeast
Asia: Current knowledge and future research directions. Ecosystems (N Y) 12:888905.
57. Pütz S, et al. (2014) Long-term carbon loss in fragmented Neotropical forests. Nat
Commun 5:5037.
58. Wiedinmyer C, Hurteau MD (2010) Prescribed fire as a means of reducing forest
carbon emissions in the western United States. Environ Sci Technol 44:19261932.
59. Smith P, et al. (2007) Agriculture. Climate Change 2007: Mitigation. Contribution of
Working Group III to the Fourth Assessment Report of the Intergovernmental Panel
on Climate Change, eds Metz B, Davidson OR, Bosch PR, Dave R, Meyer LA (Cam-
bridge Univ Press, Cambridge, MA).
60. World Resources Institute (2013) Creating a Sustainable Food Future : A menu of solutions
to sustainably feed more than 9 billion people by 2050. World Resour Rev 2013-14:130.
61. Rockström J, et al. (2017) A roadmap for rapid decarbonization. Science 355:12691271.
62. Rogelj J, McCollum DL, Reisinger A, Meinshausen M, Riahi K (2013) Probabilistic cost
estimates for climate change mitigation. Nature 493:7983.
63. Eggleston S, Buendia L, Miwa K, Ngara T, Tanabe K, eds (2006) 2006 IPCC guidelines
for national greenhouse gas inventories. Available at http://www.ipcc-nggip.iges.or.
jp/public/2006gl/vol4.html. Accessed March 14, 2016.
64. Groves C, Game ET (2015) Conservation Planning: Informed Decisions for a Healthier
Planet (W. H. Freeman, New York, NY), 1st Ed.
65. Mach KJ, Mastrandrea MD, Freeman PT, Field CB (2017) Unleashing expert judgment
in assessment. Glob Environ Change 44:114.
66. Kindermann G, et al. (2008) Global cost estimates of reducing carbon emissions
through avoided deforestation. Proc Natl Acad Sci USA 105:1030210307.
67. International Union for Conservation and Nature, Conservation Measure Partnership
(2006) Unified Classification of Conservation Actions, Version 1.0. Available at http://
www.iucn.org/themes/ssc/sis/classification.htm. Accessed June 14, 2017.
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... Enthusiasm for NCS is often grounded in their potential to advance development, conservation and sustainability goals through the co-occurrence of biodiversity and human well-being benefits (that is, 'NCS co-benefits', or any combination of one or more NCS pathway and a human well-being or biodiversity co-benefit). Indeed, co-benefits are a consistent theme motivating NCS in scientific studies 1,[4][5][6] , policy reports and government documents [7][8][9] , as well as broad appeals to accelerate NCS implementation [10][11][12] . Assuming NCS yield co-benefits, they also bridge the Sustainable Development Goals 13 , Paris Climate Accord 14 and Global Biodiversity Framework 15 through synergies between biodiversity conservation and climate change mitigation via NCS 16,17 . ...
... We are unaware of any data-driven taxonomy that could identify which articles and topics map to NCS pathways prior to our study. Our unsupervised large language topic model permitted us to discover a categorization of literature to NCS pathways and co-impacts using established frameworks 1,28 . Before recent watershed advances in language modelling, processing the relatively nuanced differences between scientific abstracts would have been difficult, if not impossible. ...
... We performed 242 queries to Web of Science and Scopus in August 2022 resulting in 2.28 million unique citations, or abstracts with metadata. The search strings were informed by International Union for Conservation of Nature definitions of biomes 66 and definitions of NCS pathways from Griscom et al. 1 . We adopt the concept of NCS because we use the Griscom et al. 1 definition and framework for NCS pathways, as well as the NCS hierarchy from Cook-Patton et al. 5 when presenting results. ...
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Natural climate solutions (NCS) play a critical role in climate change mitigation. NCS can generate win–win co-benefits for biodiversity and human well-being, but they can also involve trade-offs (co-impacts). However, the massive evidence base on NCS co-benefits and possible trade-offs is poorly understood. We employ large language models to assess over 2 million published journal articles, primarily written in English, finding 257,266 relevant studies on NCS co-impacts. Using machine learning methods to extract data (for example, study location, species and other key variables), we create a global evidence map on NCS co-impacts. We find that global evidence on NCS co-impacts has grown approximately tenfold in three decades, and some of the most abundant evidence relates to NCS that have lower mitigation potential. Studies often examine multiple NCS, indicating some natural complementarities. Finally, we identify countries with high carbon mitigation potential but a relatively weak body of evidence on NCS co-impacts. Through effective methods and systematic and representative data on NCS co-impacts, we provide timely insights to inform NCS-related research and action globally.
... Studies have also highlighted the range of everyday climate adaptation strategies in agriculture. First, tree management not only acts as a natural carbon sink (Griscom et al. 2017) but also enhances dietary diversity through increased availability of fruits and nuts (Powell et al. 2013), supports animal husbandry (Franzel et al. 2014), and provides essential fuel for 2.4 billion people (Wan et al. 2011), contributing to household income in tropical regions (Angelsen et al. 2014). Moreover, trees enhance food system resilience to climate change, especially in arid regions and during lean seasons in Africa (Koffi et al. 2020). ...
... production systems (Foley et al., 2011;Griscom et al., 2017;Poore & Nemecek, 2018). ...
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Cover crops in organic cotton systems can offset the carbon loss typically observed in conventional systems. However, their effects on greenhouse gas (GHG) emissions and soil microclimate are poorly understood. Our objective was to investigate the effects of cover crops on soil carbon dioxide (CO2), nitrous oxide (N2O), and methane (CH4) emissions and soil moisture and temperature dynamics in organic cotton systems. To achieve this, we used static chamber techniques with soil sensors in a field study near College Station, TX, from 2020 to 2022. Cover crops tested were oat (Avena sativa L.), Austrian winter pea (Pisum sativum L.) (AWP), turnip (Brassica rapa subsp. rapa), a mixture of all three, and a fallow control. In the first year of organic transition (2020), mixed species treatment enhanced CO2 emission by 39.6%, 34.4%, and 40% than AWP, turnip, and control, respectively. Compared to the control, N2O emissions were lower in AWP, turnip, and oat treatments by 77%, 57.2%, and 53% in 2020. Weed pressure and drought in 2021 and 2022 neutralized cover crops’ effect on soil GHG emissions. Soils generally acted as net CH4 sinks, but the uptake did not differ among the treatments. Cover crops depleted soil moisture during their growing period, but surface residues helped retain more moisture during the cotton season. Compared to fallow, mixed species and AWP were observed to reduce soil temperature fluctuations. Therefore, in transitioning, organic systems effects of cover crops on soil GHG emissions can vary depending on weather, weed management, and the cover crop types.
... Together, these direct and indirect effects can have lasting impacts on processes such as tree growth, which, in turn, have major implications for global carbon dynamics (Anderegg et al., 2015;McDowell et al., 2020). However, many of Earth's forests are actively managed by humans (Grantham et al., 2020;Lesiv et al., 2022), and effective management may help mitigate the impacts of drought on forested ecosystems (Griscom et al., 2017). Thus, understanding the effects of management activities on drought resistance is critical to ensure that such actions promote forest ecosystem integrity in a warming world. ...
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The frequency and severity of drought events are predicted to increase due to anthropogenic climate change, with cascading effects across forested ecosystems. Management activities such as forest thinning and prescribed burning, which are often intended to mitigate fire hazard and restore ecosystem processes, may also help promote tree resistance to drought. However, it is unclear whether these treatments remain effective during the most severe drought conditions or whether their impacts differ across environmental gradients. We used tree‐ring data from a system of replicated, long‐term (>20 years) experiments in the southwestern United States to evaluate the effects of forest restoration treatments (i.e., evidence‐based thinning and burning) on annual growth rates (i.e., basal area increment; BAI) of ponderosa pine (Pinus ponderosa), a broadly distributed and heavily managed species in western North America. The study sites were established at the onset of the most extreme drought event in at least 1200 years and span much of the climatic niche of Rocky Mountain ponderosa pine. Across sites, tree‐level BAI increased due to treatment, where trees in treated units grew 133.1% faster than trees in paired, untreated units. Likewise, trees in treated units grew an average of 85.6% faster than their pre‐treatment baseline levels (1985 to ca. 2000), despite warm, dry conditions in the post‐treatment period (ca. 2000–2018). Variation in the local competitive environment promoted variation in BAI, and larger trees were the fastest‐growing individuals, irrespective of treatment. Tree thinning and prescribed fire altered the climatic constraints on growth, decreasing the effects of belowground moisture availability and increasing the effects of atmospheric evaporative demand over multi‐year timescales. Our results illustrate that restoration treatments can enhance tree‐level growth across sites spanning ponderosa pine's climatic niche, even during recent, extreme drought events. However, shifting climatic constraints, combined with predicted increases in evaporative demand in the southwestern United States, suggest that the beneficial effects of such treatments on tree growth may wane over the upcoming decades.
... Climate change may further exacerbate these ongoing shifts from significant carbon sinks to sources of GHGs (Dadap et al., 2022;Leng et al., 2019). Their conservation and restoration are an important component of natural climate solutions for large countries such as Indonesia, Brazil, and the Republic of the Congo (Griscom et al., 2017;Lestari et al., 2023;Novita et al., 2022). ...
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Tropical peat swamp forests provide many important ecosystem services, especially their function as global carbon sinks. These carbon‐rich wetlands are widespread in South America, yet few studies have examined carbon stocks or losses due to land use change. In the lower Amazon, they are being converted to pastures largely utilized by domestic water buffalo (Bubalus bubalis). We quantified carbon stocks in intact peat forests and recently converted pastures (<10 years) at the Lago Piratuba Biosphere Reserve (LPBR) in the lower Amazon of Brazil. The soils of intact forests were typified by shallow organic (peat) horizons at the soil surface. The mean total ecosystem carbon stock (TECS) in intact forests was 354 ± 28 Mg C ha⁻¹. In contrast, the TECS of disturbed sites was significantly lower (p = 0.02) with a mean of 248 ± 17 Mg C ha⁻¹. We estimated greenhouse gas (GHG) emissions from water buffalo (due to enteric fermentation and manure deposition) to be 7.5 Mg CO2e ha⁻¹ year⁻¹. Considering GHG emissions from this land use, the social carbon costs (SCCs) arising from the degradation of coastal Amazon peatlands are as high as US2742ha1year1.TheSCCofmeatproducedfromthislanduseisashighasUS2742 ha⁻¹ year⁻¹. The SCC of meat produced from this land use is as high as US100/kg of meat produced, which far exceeds the economic returns from livestock. Based on the estimated numbers of water buffalo for the southern portion of the LPBR and the time since initial disturbance, the annual GHG emissions from this land use are estimated to be 602,846 Mg CO2e year⁻¹ with an SCC as high as US$111,526,524 million year⁻¹. This land use also eliminates opportunity values and services of carbon storage and biodiversity that would be possible from a regenerating biosphere reserve.
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China’s commitment to carbon neutrality by 2060 relies on the Land Use, Land-Use Change, and Forestry (LULUCF) sector, with forestation targets designed to enhance carbon removal. However, the exact sequestration potential of these initiatives remains uncertain due to differing accounting conventions between national inventories and scientific assessments. Here, we reconcile both estimates and reassess LULUCF carbon fluxes up to 2100, using a spatially explicit bookkeeping model, state-of-the-art historical data, and national forestation targets. We simulate a carbon sink of −0.24 ± 0.03 Gt C yr⁻¹ over 1994–2018 from past forestation efforts, aligned well with the national inventory. Should the official forestation targets be followed and extended, this could reach −0.35 ± 0.04 Gt C yr⁻¹ in 2060, offsetting 43 ± 4% of anticipated residual fossil CO2 emissions. Our findings confirm the key role of LULUCF in carbon sequestration, but its potential will decline if forestation efforts cease, highlighting the necessity for emission reductions in other sectors to achieve carbon neutrality.
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