Natural climate solutions

Abstract

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.

Full text

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 NCS—when constrained by food security,
fiber security, and biodiversity conservation—is 23.8 petagrams of
CO
2
equivalent (PgCO
2
e) y
−1
(95% CI 20.3–37.4). 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 actions—if
effectively implemented—also 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 20.3–37.4) 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, 13–16). 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, 13–16), agricultural pathways (13–16),
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, 6.6–18.6 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
(21–24). 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 <2°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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(9). Natural pathway opportunities differ considerably among
countries and regions (SI Appendix, Figs S1–S3 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 <2°C“cost-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
effective”levels 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-cost”and 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
11–14 PgCO
2
ey
−1
of mitigation needed to keep 2030 emissions in
line with cost-optimal 2 °C scenarios(33).Hence,NCScould
contributealargeportion—about 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 (34–36) (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 <2°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 °C”and 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 2.7–17.9 PgCO
2
ey
−1
) is due to the large range in
existing constrained estimates of potential reforestation extent
(345–1,779 Mha) (14, 16, 40–42). 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 <2°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 Brazil’s 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 usual”and a “best
management practice”scenario. 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 wetlands—an urgent
concern in developing countries—tends to be less expensive than
wetland restoration (49). Improved mapping of global wetlands—
particularly peatlands—is 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
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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 yields—and 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 1000”initiative (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
decade—a 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, Grazing—Improved 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; Grazing—Optimal 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,pp36–79 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,pp38–39 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 82–260 USD MgCO
2
e
−1
to meet
the 1.5–2 °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 levels—30%, 60%,
or 90% of maximum—captures 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
pathway—but 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
|
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AND PLANETARY SCIENCES
SUSTAINABILITY
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