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Natural climate solutions for the United States


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Limiting climate warming to <2°C requires increased mitigation efforts, including land stewardship, whose potential in the United States is poorly understood. We quantified the potential of natural climate solutions (NCS)—21 conservation, restoration, and improved land management interventions on natural and agricultural lands—to increase carbon storage and avoid greenhouse gas emissions in the United States. We found a maximum potential of 1.2 (0.9 to 1.6) Pg CO 2 e year ⁻¹ , the equivalent of 21% of current net annual emissions of the United States. At current carbon market prices (USD 10 per Mg CO 2 e), 299 Tg CO 2 e year ⁻¹ could be achieved. NCS would also provide air and water filtration, flood control, soil health, wildlife habitat, and climate resilience benefits.
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Fargione et al., Sci. Adv. 2018; 4 : eaat1869 14 November 2018
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Natural climate solutions for the United States
Joseph E. Fargione1*, Steven Bassett2, Timothy Boucher3, Scott D. Bridgham4, Richard T. Conant5,
Susan C. Cook-Patton3,6, Peter W. Ellis3, Alessandra Falcucci7, James W. Fourqurean8,
Trisha Gopalakrishna3, Huan Gu9, Benjamin Henderson10, Matthew D. Hurteau11,
Kevin D. Kroeger12, Timm Kroeger3, Tyler J. Lark13, Sara M. Leavitt3, Guy Lomax14,
Robert I. McDonald3, J. Patrick Megonigal6, Daniela A. Miteva15, Curtis J. Richardson16,
Jonathan Sanderman17, David Shoch18, Seth A. Spawn13, Joseph W. Veldman19,
Christopher A. Williams9, Peter B. Woodbury20, Chris Zganjar3, Marci Baranski21, Patricia Elias3,
Richard A. Houghton17, Emily Landis3, Emily McGlynn22, William H. Schlesinger23,
Juha V. Siikamaki24, Ariana E. Sutton-Grier25,26, Bronson W. Griscom3
Limiting climate warming to <2°C requires increased mitigation efforts, including land stewardship, whose poten-
tial in the United States is poorly understood. We quantified the potential of natural climate solutions (NCS)—21
conservation, restoration, and improved land management interventions on natural and agricultural lands—to
increase carbon storage and avoid greenhouse gas emissions in the United States. We found a maximum potential
of 1.2 (0.9 to 1.6) Pg CO2e year−1, the equivalent of 21% of current net annual emissions of the United States. At
current carbon market prices (USD 10 per Mg CO2e), 299 Tg CO2e year−1 could be achieved. NCS would also pro-
vide air and water filtration, flood control, soil health, wildlife habitat, and climate resilience benefits.
Limiting global warming below the 2°C threshold set by the Paris
Climate Agreement is contingent upon both reducing emissions and
removing greenhouse gases (GHGs) from the atmosphere (1,2).
Natural climate solutions (NCS), a portfolio of discrete land steward-
ship options (3), are the most mature approaches available for car-
bon conservation and uptake compared to nascent carbon capture
technologies (4) and could complement increases in zero-carbon
energy production and energy efficiency to achieve needed climate
change mitigation. Within the United States, the maximum and
economically viable mitigation potentials from NCS are unclear.
Here, we quantify the maximum potential for NCS in the United
States and the portion of this maximum that could be achieved at
several price points. We consider 21 distinct NCS to provide a con-
sistent and comprehensive exploration of the mitigation potential
of conservation, restoration, and improved management in forests,
grasslands, agricultural lands, and wetlands (Fig.1), carefully defined
to avoid double counting (details in the Supplementary Materials).
We estimate the potential for NCS in the year 2025, which is the
target year for the United States’ Nationally Determined Contribution
(NDC) under the Paris Agreement to reduce GHG emissions by 26
to 28% from 2005 levels. Our work refines a coarser-resolution
global analysis (3) and updates and expands the range of options
considered in previous analyses for the United States (58).
For each NCS opportunity (Fig.1 and the Supplementary Materials),
we estimate the maximum mitigation potential of GHGs measured
in CO2 equivalents (CO2e), given the below constraints. We then
estimate the reductions obtainable for less than USD 10, 50, and 100
per Mg CO2e. Current carbon markets pay around USD 10 (9). The
social cost of carbon in 2025 is approximately USD 50, using a 3%
discount rate (10). However, a price of at least USD 100 is thought
to be needed to keep the 100-year average temperature from warm-
ing more than 2.5°C (11), and an even higher price may be needed
to meet the Paris Agreement <2°C target. Many NCS also generate
co-benefits, which, even without a price on carbon, provide incen-
tives to invest in NCS implementation. We identified co-benefits
generated by each NCS in four categories of ecosystem services: air,
biodiversity, water, and soil (Fig.1 and table S2).
To avoid conflicts with other important societal goals for land use,
we constrain our maximum estimate to be compatible with human
needs for food and fiber (Supplementary Materials). Within these
constraints, 5.1 Mha of cropland can be restored to grasslands, for-
ests, and wetlands, equal to the area that has left the Conservation
Reserve Program (CRP) since 2007 (8) and less than half the land
currently dedicated to corn ethanol. We also estimate that 1.3 Mha
of pasture could be reforested without affecting livestock produc-
tion, assuming recent improvements in efficiency continue (see the
Supplementary Materials). We assume that timber production can
temporarily decrease by 10%, which maintains timber production
1The Nature Conservancy, Minneapolis, MN 55415, USA. 2The Nature Conservancy,
Santa Fe, NM 87501, USA. 3The Nature Conservancy, Arlington, VA 22203, USA. 4In-
stitute of Ecology and Evolution, University of Oregon, Eugene, OR 97403, USA.
5Natural Resources Ecology Laboratory, Colorado State University, Fort Collins, CO
80523, USA. 6Smithsonian Environmental Research Center, Edgewater, MD 21037,
USA. 7Food and Agriculture Organization, Rome, Italy. 8Marine Sciences Program,
Florida International University, North Miami, FL 33181, USA. 9Graduate School of
Geography, Clark University, Worcester, MA 01610, USA. 10Trade and Agriculture
Directorate, Organization for Economic Cooperation and Development, Paris 75016,
France. 11Department of Biology, University of New Mexico, Albuquerque, NM 87131,
USA. 12Woods Hole Coastal and Marine Science Center, United States Geological
Survey, Woods Hole, MA 02543, USA. 13Center for Sustainability and the Global En-
vironment, University of Wisconsin-Madison, Madison, WI 53726, USA. 14The Nature
Conservancy, Oxford OX1 1HU, UK. 15Department of Agricultural, Environmental
and Development Economics, Ohio State University, Columbus, OH 43210, USA.
16Duke University Wetland Center, Nicholas School of the Environment, Durham, NC
27708, USA. 17Woods Hole Research Center, Falmouth, MA 02540, USA. 18TerraCarbon
LLC, Charlottesville, VA 22903, USA. 19Department of Ecosystem Science and Manage-
ment, Texas A&M University, College Station, TX 77843, USA. 20College of Agricul-
ture and Life Sciences, Cornell University, Ithaca, NY 14853, USA. 21U.S. Department
of Agriculture, Washington, DC 20250, USA. 22Department of Agriculture and Re-
source Economics, University of California, Davis, Davis, CA 95616, USA. 23Cary
Institute of Ecosystem Studies, Millbrook, NY 12545, USA. 24International Union for
Conservation of Nature, Washington, DC 20009, USA. 25The Nature Conservancy,
Bethesda, MD 20814, USA. 26Earth System Science Interdisciplinary Center, Univer-
sity of Maryland, College Park, MD 20740, USA.
*Corresponding author. Email:
Copyright © 2018
The Authors, some
rights reserved;
exclusive licensee
American Association
for the Advancement
of Science. No claim to
original U.S. Government
Works. Distributed
under a Creative
Commons Attribution
License 4.0 (CC BY).
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Fargione et al., Sci. Adv. 2018; 4 : eaat1869 14 November 2018
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levels within the historic range of variation and enables managed
forests and plantations to transition to longer harvest rotations (see
the Supplementary Materials). We assume that extensive natural
forests on private lands can all undergo harvest extension, with
the temporary loss of timber supply replaced by reforestation and
thinning for fire risk reduction (12) or with thinning or select har-
vest practices that still provide timber but maintain carbon levels
(Supplementary Materials) (13,14). We further constrain our analy-
sis to avoid impacts on biodiversity. This biodiversity constraint pre-
cludes both the conversion of natural habitat to energy crops and
the afforestation of native grasslands.
We find a maximum additional NCS mitigation potential of 1.2 Pg
CO2e year−1 [95% confidence interval (CI), 0.9 to 1.6 Pg CO2e year−1]
in the year 2025 (Fig.1 and table S1). This is 21% of the 5794.5 Tg
CO2e of net emissions in 2016 (15). The majority (63%) of this po-
tential comes from increased carbon sequestration in plant bio-
mass, with 29% coming from increased carbon sequestration in soil
and 7% coming from avoided emissions of CH4 and N2O. At the
USD 10, 50, and 100 price points, 25, 76, and 91%, respectively, of
the maximum mitigation would be achieved. This means that 1.1 Pg
CO2e year−1 are available at USD 100 per Mg CO2e, which equals
the emission reductions needed to meet the U.S. NDC under the
Paris Agreement (see the Supplementary Materials). If NCS were
pursued in combination with additional mitigation in the energy
sector, then it would therefore enable the United States to exceed its
current NDC ambition. This is important because, globally, current
NDCs (7 to 9 Pg CO2e year−1) would need to be dramatically in-
creased (by an additional 10 to 16 Pg CO2e year−1) to limit warming
below 2°C (16).
This estimate of maximum NCS potential is similar to or higher
than several previous syntheses of mitigation opportunities in the
land sector. For example, the United States Mid-Century Strategy
for Deep Decarbonization estimated a potential land sink of 912 Tg
CO2e year−1, 30% lower than our estimate (5). While other efforts
have focused on the forest sector (7) or the agricultural sector (6),
this analysis presents a comprehensive and up-to-date synthesis of
NCS opportunities in the United States. For example, this analysis
considers potential additional mitigation from tidal wetlands and
seagrass (“blue carbon”), which has been comprehensively analyzed
for its current status in the United States (17), but not its potential
for additional mitigation.
Fig. 1. Climate mitigation potential of 21 NCS in the United States. Black lines indicate the 95% CI or reported range (see table S1). Ecosystem service benefits linked
with each NCS are indicated by colored bars for air (filtration), biodiversity (habitat protection or restoration), soil (enrichment), and water (filtration and flood control).
See the Supplementary Materials for detailed findings and sources.
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Reforestation has the single largest maximum mitigation poten-
tial (307 Tg CO2e year−1). The majority of this potential occurs in
the northeast (35%) and south central (31%) areas of the United States
(fig. S1). This mitigation potential increases to 381 Tg CO2e year−1
if all pastures in historically forested areas are reforested. Previous
estimates of reforestation potential range widely from 208 to 1290
Tg CO2e year−1 (7). Higher estimates than ours can be obtained by
reforesting or afforesting areas that we excluded (e.g., productive
crop and pasture lands and natural grasslands) and/or by using rates
of carbon sequestration from plantation systems rather than from
natural regenerating forests [e.g., (7)].
Natural forest management of privately held forests has the sec-
ond largest maximum mitigation potential (267 Tg CO2e year−1).
This maximum mitigation is achieved by extending harvest cycles.
Mitigation can also be achieved through forest management prac-
tices such as reduced impact logging and improved silvicultural prac-
tices that release suppressed forest growth (1820), although often
at lower sequestration rates than extending harvest cycles. These
management practices can be implemented at low or no net cost
(21,22) and do not require a change in business-as-usual (BAU)
land use or ownership rights.
Another promising opportunity associated with forests is fire
management (18 Tg CO2e year−1; fig. S6). Fire management entails
restoring frequent, low-intensity, understory fires in fire-prone forest
ecosystems to reduce the potential for high-severity wildfires (23).
The primary carbon benefit from fire management is avoiding de-
creased net ecosystem production from tree-killing wildfire. In the
absence of improved fire management, climate change is expected
to continue to increase the frequency of high-severity fires and com-
promise the ability of forests to regenerate following these fires (24).
The high uncertainty associated with the climate mitigation bene-
fits of fire management would be reduced by additional research to
quantify the carbon storage benefits of prescribed fire across a di-
versity of forest types, including the length of time that prescribed
fire reduces the risk of subsequent high-severity fires.
Avoided conversion protects carbon stored in extant forests and
grasslands from ongoing losses. More than two-thirds of the avoided
forest conversion potential (38 Tg CO2e year−1) occurs in the Southern
and Pacific Northwest regions (table S14 and fig. S9). Many of the
most intensive areas of rapid forest conversion were located near
urban zones, with additional hot spots in recent agricultural expan-
sion zones (such as California’s Central Valley) and semi-arid re-
gions of the West. Avoided conversion of grassland to cropland
prevents emissions from soils and root biomass (107 Tg CO2e year−1;
fig. S12). The emissions from grassland conversion exceed the emis-
sions from forest conversion because both the rate of conversion
and the per hectare emissions are higher (table S1). Cropland ex-
pansion is a major cause of conversion that affects grasslands much
more than forests (25). The higher rate of emissions occurs because
the conversion of grasslands to croplands results in a 28% loss of
soil carbon from the top meter of soil (26). This generates 125 Mg
CO2e ha−1 in emissions, comprising 81% of the emissions from grass-
land conversion (see the Supplementary Materials). Because research
shows conflicting conclusions regarding the impact of forest con-
version in the United States on soil carbon, we do not include the
soil carbon pool in our estimate of emissions from forest conversion
(see the Supplementary Materials).
Carbon sequestration opportunities in croplands include the use of
cover crops and improved cropland nutrient management. Cover
crops, grown when fields are normally bare, provide additional car-
bon inputs to soils. Growing cover crops on the 88 Mha of the five
primary crops in United States not already using cover crops presents
a substantial opportunity for mitigation (103 Tg CO2e year−1). Cover
crops are increasingly used by U.S. farmers to improve soil health,
yields, and yield consistency (27). Improved management of nitro-
gen fertilizers reduces N2O emissions and avoids fossil fuel emis-
sions associated with fertilizer production (52 Tg CO2e year−1).
Fertilizer rates can be reduced while maintaining yields by using
precision agriculture to apply only the amount required in each part
of the field and by splitting fertilizer applications to match the timing
and supply of fertilizer with crop demand (see the Supplementary
Materials). Emissions can also be reduced by switching from anhy-
drous fertilizer to urea, which has lower N2O emission (6).
The agronomic practices of biochar incorporation (95 Tg CO2e
year−1) and alley cropping (planting widely spaced trees interspersed
with a row crop; 82 Tg CO2e year−1) also have high maximum poten-
tial. However, current adoption is negligible due to a variety of cul-
tural, technological, and cost barriers that would need to be overcome
if these practices were to achieve their mitigation potential (28,29).
Tidal wetland restoration is the largest wetland NCS (12 Tg CO2e
year−1). Roughly 27% of U.S. salt marshes are disconnected from the
ocean and subject to freshwater inundation. This results in a large
increase in CH4 emissions from these “freshened” salt marshes. Re-
connecting salt marshes with the ocean, such as via culverts under
roads or other barriers, can avoid these CH4 emissions (30).
The 10 opportunities described above account for 90% (1082 Tg
CO2e year−1) of the maximum NCS mitigation potential across all
21 opportunities. An additional 11 opportunities, which sum to 122
Tg CO2e year−1, account for just 10% of the maximum potential.
However, these NCS may offer optimal ecological and economic
opportunities at local scales (Fig.1 and Supplementary Materials).
For example, peatland restoration offers a high per hectare mitiga-
tion benefit, especially in regions of the United States with warm
temperate climates (8.2 Mg CO2e ha−1 year−1).
Lower-cost opportunities represent particularly promising areas
for increased near-term investment. We identified 299 Tg CO2e year−1
of NCS opportunities that could be realized for USD 10 Mg CO2e−1
or less (table S1), a price that is in line with many current carbon
markets (9). The two largest lower-cost opportunities are improved
management practices: cover crops (100 Tg CO2e year−1) and im-
proved natural forest management (64 Tg CO2e year−1). Both of
these practices, along with planting windbreaks (5 Tg CO2e year−1)
and legumes in pastures (3 Tg CO2e year−1), have the potential to
increase yields (21,22,27) and therefore to generate additional rev-
enue for landowners. Improved manure management can also provide
low-cost mitigation (12 Tg CO2e year−1) (8). In addition, lower- cost
NCS include increased efficiencies (cropland nutrient management,
28 Tg CO2e year−1; grazing optimization, 6 Tg CO2e year−1) and
avoided conversion (avoided forest conversion, 37 Tg CO2e year−1;
avoided grassland conversion, 24 Tg CO2e year−1).
By itself, the marginal abatement cost gives an incomplete pic-
ture of the potential for implementation of NCS, in part because NCS
provide a variety of co-benefits (Fig.1 and table S2). The values of
these co-benefits are not captured in our marginal abatement costs
yet may drive NCS implementation. For example, investments in fire
management are needed to avoid impacts on air quality and drink-
ing water provision; urban forestry provides human health, aesthetic,
and direct temperature reduction benefits; nutrient management is
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needed to improve water quality and avoid toxic algal blooms (table S2).
Further, NCS can help provide resilience to climate change impacts
on nature and people. For example, building soil carbon increases
the resilience of cropland (31); protecting coastal wetlands can pro-
vide coastal defense against storms (32); and fire management can
help avoid damaging wildfires (23).
We have restricted our analysis to those opportunities where the
literature conclusively demonstrates the potential for mitigation.
This suggests that new research may reveal additional opportunities
for NCS, which would increase the potential identified here. At the
same time, substantial uncertainties exist in some NCS opportuni-
ties (Fig.1 and table S1), highlighting the need for implementation
to be coupled with monitoring and assessment of NCS.
The United States is the largest cumulative emitter of carbon dioxide
from fossil fuels (33). Despite the immense size of U.S. GHG emissions
from fossil fuel use, we find that NCS have the potential to generate
mitigation equivalent to 21% of net annual emissions. This reveals
the important contribution to climate mitigation that the land sec-
tor can make, even in developed countries such as the United States.
Globally, current NCS efforts receive only 0.8% of public and
private climate financing (34), despite offering roughly 37% of po-
tential mitigation needed through 2030 (3). One concern that may
have limited the adoption of NCS to date includes competition with
other land uses such as food and bioenergy production. A growing
body of literature suggests that future global food demand can be
met via investments in yield increases, closing yield gaps, diet shifts,
aquaculture, and biofuel policy, without the need to further expand
cropland into natural areas (35,36). In the United States, marginal
cropland, much of which is unprofitable (37), could be restored to
grassland or forests with net societal benefits (38). Similarly, NCS
may compete with bioenergy production. However, this conflict can
be reduced or avoided depending on the form of bioenergy produc-
tion or NCS. Some forms of biomass production, such as residues
and wastes, or high-yielding methods, such as algae, do not require
productive land (39). Our grassland restoration pathway could pro-
duce a limited amount of additional biomass while maintaining carbon
sequestration in soils if low-productivity croplands are converted to
perennial energy grasses (40). Further, NCS based on improved
management of existing land uses do not create land use conflict and
can even increase productivity within that land use (e.g., fire manage-
ment or cover crops). However, aggressive expansion of dedicated
bioenergy crops, given the large land requirement of both first- and
second-generation bioenergy crops (41), would be likely to reduce the
mitigation potential available through NCS, notably via reforesta-
tion, avoided grassland conversion, and natural forest management.
A second concern is that ecosystems have a limited ability to store
additional carbon. For each pathway, we quantified the duration of
time for which mitigation is expected to occur at the rates we esti-
mate, before saturation effects decrease this rate (table S1). We note
that carbon can continue to accumulate in forests for hundreds
of years and in soils for centuries or millennia (table S1 and the
Supplementary Materials). Further, four of our NCS opportunities
(cropland nutrient management, tidal wetland restoration, manure
management, and improved rice management) are based on avoided
emissions of CH4 and N2O, which are benefits that do not saturate.
The mitigation potential of avoided conversion of habitat is limited
by the total carbon contained in the habitat. Our analysis assumes
that rates of conversion persist at current levels in a BAU scenario,
which would represent a continuing source of emissions for at least
67 years for each habitat considered here before reaching “saturation”
when the total area has been lost. However, the long-term benefit of
avoided conversion depends on assumed future BAU conversion rates.
The permanence of the ~2270 Pg C currently stored globally in
biomass (42) and soils to 1 m (26) is a significant concern, because
unmitigated climate change is likely to cause feedbacks that may
increase disturbances such as fire or pest outbreaks (43) or limit net
ecosystem productivity or forest regeneration (24). While NCS would
marginally increase this large carbon pool, putting some additional
carbon at risk, rapid and widespread implementation of NCS would
reduce the overall risk of impermanence to the terrestrial biosphere
that unmitigated climate change is likely to cause.
Another challenge is that avoiding conversion in one area can
cause conversion to shift to other areas, often referred to as “leakage.”
Large-scale sectoral and landscape approaches to land use planning
and policies will be needed to realize the NCS opportunities identi-
fied here. These approaches can and should be designed to buffer
risks of leakage associated with individual projects (44).
Reducing carbon-intensive energy consumption is necessary but
insufficient to meet the ambitious goals of the Paris Agreement.
Comprehensive mitigation efforts that include fossil fuel emission
reductions coupled with NCS hold promise for keeping warming
below 2°C. Beyond providing meaningful climate mitigation, NCS
investment can increase other important ecosystem services. The
conservation, restoration, and improved management of lands in
the United States represent a necessary and urgent component of
efforts to stabilize the climate.
Below, we provide a brief overview of methods for each of the 21 NCS
that we quantified. Full methodological details are provided in the
Supplementary Materials.
Reforestation: Additional carbon sequestration in above- and
belowground biomass and soils gained by converting nonforest (<25%
tree cover) to forest [>25% tree cover (45)] in areas of the contermi-
nous United States where forests are the native cover type. We ex-
cluded areas with intensive human development, including all major
roads (46), impervious surfaces (47), and urban areas (48). To elimi-
nate double counting with the peatland restoration pathway, we re-
moved Histosol soils (49). To safeguard food production, we removed
most cropland and pasture. We discounted the carbon sequestra-
tion mitigation benefit in conifer-dominated forests to account for
albedo effects.
Natural forest management: Additional carbon sequestration in
above- and belowground biomass gained through improved manage-
ment in forests on private lands under nonintensive timber manage-
ment. The maximum mitigation potential was quantified on the basis
of a “harvest hiatus” scenario starting in 2025, in which natural for-
ests are shifted to longer harvest rotations. This could be accom-
plished with less than 10% reduction in timber supply with new
timber supply from thinning treatments for fuel risk reduction until
new timber from reforestation is available in 2030.
Fire management: Use of prescribed fire to reduce the risk of
high-intensity wildfire. We considered fire-prone forests in the west-
ern United States. We assume that treatment eliminates the risk of
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subsequent wildfire for 20 years, but only on the land that was di-
rectly treated. We assume that 5% of lands are treated each year, and we
calculated the benefits that accrue over 20 years, finding that the ini-
tial increase in emissions associated with prescribed fire treatment
is more than offset over time by the avoided impacts of wildfires. We
report the average annual benefit across these 20 years. The impact
of wildfires includes both direct emissions from combustion and
suppression of net ecosystem productivity following wildfires.
Avoided forest conversion: Emissions of CO2 avoided by avoiding
anthropogenic forest conversion. Most forest clearing is followed by
forest regeneration rather than conversion to another land use. To
estimate the rate of persistent conversion (i.e., to another land use),
we first calculated forest clearing in the conterminous United States
from 2000 to 2010 and then used the proportion of forest clearing
that historically was converted to another land use to estimate con-
version rates in 2000 to 2010. We used estimates of avoided carbon
emissions from above- and belowground biomass that are specific
to each region and forest type. We did not count forest loss due to
fire to avoid double counting with the improved fire management
opportunity. We did not count forest loss due to pests because it
is unclear whether this loss can be avoided. We reduced the benefit
of avoided conversion in conifer-dominated forests to account for
their albedo effects.
Urban reforestation: Additional carbon sequestration in above-
and belowground biomass gained by increasing urban tree cover.
We considered the potential to increase urban tree cover in 3535
cities in the conterminous United States. We considered the poten-
tial for additional street trees, and for those cities not in deserts, we
also considered the potential for park and yard tree plantings. The
potential percent increase in tree cover was estimated on the basis of
high-resolution analysis of 27 cities, which excluded sports fields,
golf courses, and lawns (50).
Improved plantations: Additional carbon sequestration gained in
above- and belowground tree biomass by extending rotation lengths
for a limited time in even-aged, intensively managed wood production
forests. Rotation lengths were extended from current economic opti-
mal rotation length to a biological optimal rotation length in which
harvest occurs when stands reach their maximum annual growth.
Cover crops: Additional soil carbon sequestration gained by
growing a cover crop in the fallow season between main crops. We
quantified the benefit of using cover crops on all of the five major
crops in the United States (corn, soy, wheat, rice, and cotton) that
are not already growing cover crops (27), using the mean sequestra-
tion rate quantified in a recent meta-analysis (51).
Avoided conversion of grassland: Emissions of CO2 avoided by
avoiding conversion of grassland and shrubland to cropland. We
quantified avoided emissions from soil and roots (for shrubs, we also
considered aboveground biomass) based on the spatial pattern of
conversion from 2008 to 2012. We used spatial information on lo-
cation of recent conversion and variation in soil carbon and root
biomass to estimate mean annual emission rate from historic con-
version. We estimated a 28% loss of soil carbon down to 1 m (26).
We modeled spatial variation in root biomass based on mean annual
temperature and mean annual precipitation using data from (52).
Biochar: Increased soil carbon sequestration by amending agri-
cultural soils with biochar, which converts nonrecalcitrant carbon
(crop residue biomass) to recalcitrant carbon (charcoal) through
pyrolysis. We limited the source of biochar production to crop resi-
due that can be sustainably harvested. We assumed that 79.6% of
biochar carbon persists on a time scale of >100 years (53,54) and that
there are no effects of biochar on emissions of N2O or CH4 (55,56).
Alley cropping: Additional carbon sequestration gained by plant-
ing wide rows of trees with a companion crop grown in the alley-
ways between the rows. We estimated a maximum potential of alley
cropping on 10% of U.S. cropland (15.4 Mha) (57).
Cropland nutrient management: Avoided N2O emissions due to
more efficient use of nitrogen fertilizers and avoided upstream emis-
sions from fertilizer manufacture. We considered four improved
management practices: (i) reduced whole-field application rate, (ii)
switching from anhydrous ammonia to urea, (iii) improved timing
of fertilizer application, and (iv) variable application rate within
field. We projected a 4.6% BAU growth in fertilizer use in the United
States by 2025. On the basis of these four practices, we found a maxi-
mum potential of 22% reduction in nitrogen use, which leads to
a 33% reduction in field emissions and a 29% reduction including
upstream emissions.
Improved manure management: Avoided CH4 emissions from
dairy and hog manure. We estimated the potential for emission
reductions from improved manure management on dairy farms
with over 300 cows and hog farms with over 825 hogs. Our calcu-
lations are based on improved management practices described
by Pape etal. (8).
Windbreaks: Additional sequestration in above- and belowground
biomass and soils from planting windbreaks adjacent to croplands
that would benefit from reduced wind erosion. We estimated that
windbreaks could be planted on 0.88 Mha, based on an estimated
17.6 Mha that would benefit from windbreaks, and that windbreaks
would be planted on ~5% of that cropland (8).
Grazing optimization: Additional soil carbon sequestration due
to grazing optimization on rangeland and planted pastures, derived
directly from a recent study by Henderson etal. (58). Grazing opti-
mization prescribes a decrease in stocking rates in areas that are
overgrazed and an increase in stocking rates in areas that are under-
grazed, but with the net result of increased forage offtake and live-
stock production.
Grassland restoration: Additional carbon sequestration in soils
and root biomass gained by restoring 2.1 Mha of cropland to grass-
land, equivalent to returning to the 2007 peak in CRP enrollment.
Grassland restoration does not include restoration of shrubland.
Legumes in pastures: Additional soil carbon sequestration due
to sowing legumes in planted pastures, derived directly from a re-
cent global study by Henderson etal. (58). Restricted to planted
pastures and to where sowing legumes would result in net seques-
tration after taking into account potential increases in N2O emis-
sions from the planted legumes.
Improved rice management: Avoided emissions of CH4 and
N2O through improved practices in flooded rice cultivation. Prac-
tices including mid-season drainage, alternate wetting and drying,
and residue removal can reduce these emissions. We used a U.S. En-
vironmental Protection Agency (EPA) analysis that projects the po-
tential for improvement across U.S. rice fields, in comparison with
current agricultural practices (59).
Tidal wetland restoration: In the United States, 27% of tidal wet-
lands (salt marshes and mangroves) have limited tidal connection
with the sea, causing their salinity to decline to the point where CH4
emissions increase (30). We estimated the potential for reconnect-
ing these tidal wetlands to the ocean to increase salinity and reduce
CH4 emissions.
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Peatland restoration: Avoided carbon emissions from rewetting
and restoring drained peatlands. To estimate the extent of restor-
able peatlands, we quantified the difference between historic peat-
land extent [based on the extent of Histosols in soil maps (60)] and
current peatland extent. Our estimate of mitigation potential ac-
counted for changes in soil carbon, biomass, and CH4 emissions,
considering regional differences, the type of land use of the converted
peatland, and whether the peatland was originally forested.
Avoided seagrass loss: Avoided CO2 emissions from avoiding sea-
grass loss. An estimated 1.5% of seagrass extent is lost every year (61).
We assumed that half of the carbon contained in biomass and sedi-
ment from disappearing seagrass beds is lost to the atmosphere (62).
Seagrass restoration: Increased sequestration from restoring the
estimated 29 to 52% of historic seagrass extent that has been lost
and could be restored (61). We estimated the average carbon se-
questration rate in the sediment of seagrass restorations based on
data from six seagrass restoration sites in the United States (63).
Supplementary material for this article is available at
Supplementary Materials and Methods
Fig. S1. Mapped reforestation opportunity areas in the lower 48 states.
Fig. S2. Conceptual framework for improved forest management carbon accounting.
Fig. S3. MAC for carbon sequestration through forest management and aging, after
Golub et al. (99).
Fig. S4. MAC for natural forest management after Latta et al. (98) and best-fit functions.
Fig. S5. MAC curves for improved plantations.
Fig. S6. Fire management analysis area.
Fig. S7. Regions used for reporting avoided forest conversion results.
Fig. S8. Forest conversion from 1986 to 2000.
Fig. S9. Potential carbon emissions from areas at high risk of forest conversion.
Fig. S10. Cities included in the urban reforestation analysis.
Fig. S11. Calibration of remote sensing data for forest cover estimation in urban areas.
Fig. S12. Avoided grassland conversion map.
Fig. S13. MAC curve for avoided grassland conversion.
Fig. S14. Nitrogen fertilizer use in the United States.
Fig. S15. Marginal abatement cost curve for reducing N fertilizer rate.
Fig. S16. Marginal abatement cost curve for applying variable rate technology fertilizer
Fig. S17. Grazing optimization map.
Fig. S18. Legumes in pastures map.
Fig. S19. Grassland restoration map.
Fig. S20. MAC curve for grassland restoration.
Fig. S21. Break-even prices for GHG abatement from rice production.
Fig. S22. MAC curve for salt marsh restoration.
Fig. S23. MAC of avoided GHG emissions from seagrass.
Table S1. Mitigation potential of NCS in 2025.
Table S2. Co-benefits of NCS.
Table S3. Literature MAC estimates for reforestation of agricultural lands.
Table S4. Literature estimates of reforestation costs used to estimate MAC of reforesting
natural ecosystems.
Table S5. Estimated marginal abatement cost of fire management by major forest region.
Table S6. Forest disturbance rates by source.
Table S7. Mean annual forest hectares cleared per year from 1986 to 2000.
Table S8. Mean annual forest hectares cleared per year from 2001 to 2010.
Table S9. Mean annual forest hectares converted per year from 1986 to 2000.
Table S10. Proportion of areas cleared from 1986 to 2000 that had not regenerated to forest
by 2010.
Table S11. Mean predisturbance dry biomass (kg m−2) in forest areas converted from 1986
to 2000.
Table S12. Mean predisturbance dry biomass (kg m−2) in forest areas converted from 2001
to 2010.
Table S13. Carbon emissions (Mg C year−1) from estimated forest conversion from 2001
to 2010.
Table S14. Albedo-adjusted carbon emissions equivalent (Mg Ce year−1) from estimated forest
conversion from 2001 to 2010.
Table S15. Urban reforestation maximum potential annual net C sequestration in 2025.
Table S16. Uncertainty in urban reforestation average annual abatement (Tg CO2) by 2025 at a
cost of USD 100 per Mg CO2.
Table S17. Profitability impacts of cover crops for selected crops.
Table S18. Marginal abatement costs of cover crops in the five primary crops.
Table S19. Maximum feasible N2O reduction for multiple nitrogen fertilizer practices.
Table S20. Results from the literature of the potential for reducing N fertilizer rate using
within-field management.
Table S21. Current and projected GHG emissions from nitrogen fertilizer manufacturing in the
United States.
Table S22. Mitigation potential for grazing optimization and legumes in pasture NCS at
different marginal abatement costs.
Table S23. Areas and carbon fluxes for Histosols in the conterminous United States.
Table S24. Peatland restoration mitigation calculations for climate zones within the United
Table S25. 95% CIs for Histosol calculations.
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Funding: This study was made possible by funding from the Doris Duke Charitable
Foundation. C.A.W. and H.G. acknowledge financial support from NASA’s Carbon Monitoring
System program (NNH14ZDA001N-CMS) under award NNX14AR39G. S.D.B. acknowledges
support from the DOE’s Office of Biological and Environmental Research Program under the
award DE-SC0014416. J.W.F. acknowledges financial support from the Florida Coastal
Everglades Long-Term Ecological Research program under National Science Foundation grant
no. DEB-1237517. Any use of trade, firm, or product names is for descriptive purposes only and
does not imply endorsement by the U.S. Government. The findings and conclusions in this
publication have not been formally disseminated by the U.S. Department of Agriculture and
should not be construed to represent any agency determination or policy. We thank L. Hansen
(USDA Economic Research Service) for providing a shapefile of county-level wetland
restoration cost estimates. Author contributions: S.B., T.B., S.D.B., R.T.C., S.C.C.-P., P.W.E., A.F.,
J.E.F., J.W.F., T.G., B.W.G., H.G., B.H., M.D.H., K.D.K., T.K., T.J.L., S.M.L., G.L., R.I.M., J.P.M., D.A.M.,
C.J.R., J.S., D.S., S.A.S, J.W.V., C.A.W., P.B.W., and C.Z. developed individual NCS opportunities.
J.E.F. drafted the manuscript. All authors discussed the results and edited and commented on
the manuscript. Competing interests: G.L. has been a consultant for Virgin Management Ltd.
advising on land-based carbon sequestration strategies. The authors declare no other
competing interests. Data and materials availability: All data needed to evaluate the
conclusions in the paper are present in the paper and/or the Supplementary Materials.
Additional data related to this paper may be requested from the authors.
Submitted 1 February 2018
Accepted 12 October 2018
Published 14 November 2018
Citation: J. E. Fargione, S. Bassett, T. Boucher, S. D. Bridgham, R. T. Conant, S. C. Cook-Patton,
P. W. Ellis, A. Falcucci, J. W. Fourqurean, T. Gopalakrishna, H. Gu, B. Henderson, M. D. Hurteau,
K. D. Kroeger, T. Kroeger, T. J. Lark, S. M. Leavitt, G. Lomax, R. I. McDonald, J. P. Megonigal,
D. A. Miteva, C. J. Richardson, J. Sanderman, D. Shoch, S. A. Spawn, J. W. Veldman, C. A. Williams,
P. B. Woodbury, C. Zganjar, M. Baranski, P. Elias, R. A. Houghton, E. Landis, E. McGlynn, W. H. Schlesinger,
J. V. Siikamaki, A. E. Sutton-Grier, B. W. Griscom, Natural climate solutions for the United States.
Sci. Adv. 4, eaat1869 (2018).
on November 14, 2018 from
Natural climate solutions for the United States
Emily McGlynn, William H. Schlesinger, Juha V. Siikamaki, Ariana E. Sutton-Grier and Bronson W. Griscom
Christopher A. Williams, Peter B. Woodbury, Chris Zganjar, Marci Baranski, Patricia Elias, Richard A. Houghton, Emily Landis,
Megonigal, Daniela A. Miteva, Curtis J. Richardson, Jonathan Sanderman, David Shoch, Seth A. Spawn, Joseph W. Veldman,
Hurteau, Kevin D. Kroeger, Timm Kroeger, Tyler J. Lark, Sara M. Leavitt, Guy Lomax, Robert I. McDonald, J. Patrick
W. Ellis, Alessandra Falcucci, James W. Fourqurean, Trisha Gopalakrishna, Huan Gu, Benjamin Henderson, Matthew D.
Joseph E. Fargione, Steven Bassett, Timothy Boucher, Scott D. Bridgham, Richard T. Conant, Susan C. Cook-Patton, Peter
DOI: 10.1126/sciadv.aat1869
(11), eaat1869.4Sci Adv
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... Particularly missing from current discussions of land-based mitigation scenarios are quantitative assessments of potential solutions that include both nature-based (Fargione et al., 2018) and cellulosic bioenergy (Field et al., 2020) solutions. ...
... Additionally, such estimates typically consider only a subset of available land-based strategies, with an emphasis on BECCS (e.g., Calvin et al., 2019). Bottom-up efforts, on the other hand, effectively identify specific practices with substantial mitigation potentials, whether carbon capture or emissions avoidance, but struggle to capture the spatial resolution needed to avoid double-counting activities with competing land needs (NASEM, 2019), or promote one set of practices (such as reforestation) to the exclusion of others (such as bioenergy) (Fargione et al., 2018). And no efforts to derive land-based estimates capture the combined uncertainties of local practice outcomes, available land base, likely adoption rates, and the durations of different carbon sink strengths. ...
... Recent estimates of U.S. land-based sequestration potentials suggest a maximum sequestration capacity of 1.0-2.4 Gt of CO 2 equivalents (CO 2 e) per year at mid-century (NASEM, 2019), and a recent spatial analysis of potential nature-based solutions (Fargione et al., 2018) suggests an end-of-century capacity for ~74 Gt CO 2 e by 2100. This estimate excludes bioenergy, however, an especially important opportunity in the United States and other countries where an available land base allows capacity to scale appreciably (Hilaire et al., 2019). ...
Full-text available
Meeting end-of-century global warming targets requires aggressive action on multiple fronts. Recent reports note the futility of addressing mitigation goals without fully engaging the agricultural sector, yet no available assessments combine both nature-based solutions (reforestation, grassland and wetland protection, and agricultural practice change) and cellulosic bioenergy for a single geographic region. Collectively, these solutions might offer a suite of climate, biodiversity, and other benefits greater than either alone. Nature-based solutions are largely constrained by the duration of carbon accrual in soils and forest biomass; each of these carbon pools will eventually saturate. Bioenergy solutions can last indefinitely but carry significant environmental risk if carelessly deployed. We detail a simplified scenario for the United States that illustrates the benefits of combining approaches. We assign a portion of non-forested former cropland to bioenergy sufficient to meet projected mid-century transportation needs, with the remainder assigned to nature-based solutions such as reforestation. Bottom-up mitigation potentials for the aggregate contributions of crop, grazing, forest , and bioenergy lands are assessed by including in a Monte Carlo model conservative ranges for cost-effective local mitigation capacities, together with ranges for (a) areal extents that avoid double counting and include realistic adoption rates and (b) the projected duration of different carbon sinks. The projected duration illustrates the net effect of eventually saturating soil carbon pools in the case of most strategies, and additionally saturating biomass carbon pools in the case of forest management. Results show a conservative end-of-century mitigation capacity of 110 (57-178) Gt CO 2 e for the U.S., ~50% higher than existing estimates that prioritize nature-based or bioenergy solutions separately. Further research is needed to shrink uncertainties, but there is sufficient confidence in the general magnitude and direction of a combined approach to plan for deployment now.
... Impact from land-use competitions, especially those between climate change mitigation and securing food provision (e.g., cropland), has been identified as one of the key issues in climate change mitigation studies (IPCC, 2019;Meyfroidt et al., 2022). However, in previous estimation of NCS mitigation potential, impact of land-use competition (e.g., incurred by cropland expansion) has not been taken into consideration as they overlooked the inevitable cropland expansion in the next few decades (Fargione et al., 2018;Griscom et al., 2017). Understanding how NCS potential is limited by future land-use competition is important as it provides us with a more realistic and equitable estimate. ...
... It has also been reported that using a 10-km data would miss out at least 60 % of small-scale land-use change that could be identified in 1-km resolution data . Another consideration is that previous estimation of NCS potential is based on a non-mitigation condition (i.e., baseline scenarios without applying additional mitigation efforts (Fargione et al., 2018;Griscom et al., 2017). Thus, to make consistent analysis on the impact of land-use competition on NCS potential, the land-use projection data should also be under non-mitigation scenarios. ...
... This scenario assumed the total amount of cropland areas maintained the same as baseline level of 2010 from 2010 to 2100 (Fig. 1). This additional scenario was originated from the key assumption embedded in previous NCS mitigation potential estimations: "current extent of cropland land can effectively feed projected future populations as future global food demand can be met via an ideal yield increases" (Fargione et al., 2018;Griscom et al., 2017). When calculating and projecting NCS mitigation potential, previous studies do not take the land-use competition between future cropland expansion and natural areas needed for implementing NCS into consideration. ...
Full-text available
Natural climate solutions (NCS) are an essential complement to climate mitigation and have been increasingly incorporated into international mitigation strategies. Yet, with the ongoing population growth, allocating natural areas for NCS may compete with other socioeconomic priorities, especially urban development and food security. Here, we projected the impacts of land-use competition incurred by cropland and urban expansion on the climate mitigation potential of NCS. We mapped the areas available for implementing 9 key NCS strategies and estimated their climate change mitigation potential. Then, we overlaid these areas with future cropland and urban expansion maps projected under three Shared Socioeconomic Pathway (SSP) scenarios (2020-2100) and calculated the resulting mitigation potential loss of each selected NCS strategy. Our results estimate a substantial reduction, 0.3-2.8 GtCO 2 yr −1 or 4-39 %, in NCS mitigation potential, of which cropland expansion for fulfilling future food demand is the primary cause. This impact is particularly severe in the tropics where NCS hold the most abundant mitigation potential. Our findings highlight immediate actions prioritized to tropical areas are important to best realize NCS and are key to developing realistic and sustainable climate policies.
... As shown by land surface modeling, global warming, drought stress, and deforestation in the past decades have greatly affected the structure and function of forests through change in biomass accumulation, thus altering this crucial natural resource for human well-being (Piao et al., 2009). Therefore, a better understanding of how biomass changes in response to the environment, especially along elevation and climatic gradients in mountainous areas, is vital to inform sustainable forest management (Grassi et al., 2017;Liang et al., 2016) and strengthen natural climate solutions (Fargione et al., 2018). ...
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Forest biomass is an important component of terrestrial carbon pools. However, how climate, biodiversity, and structural attributes co-determine spatiotemporal variation in forest biomass remains not well known. We aimed to shed light on these drivers of forest biomass by measuring diversity and structural attributes of tree species in 400-m 2 plots located every 100 m along a 4200-m elevational gradient in the eastern Himalayas. We applied structural equation models to test how climate, species richness, structural attributes, and their interactions influence forest biomass. Importantly, species richness was a stronger driver of biomass than environmental and structural attributes such as annual air temperature or stem density. Integrating the availability of energy and the demand for water, potential evapotranspiration was more strongly correlated with biomass than water availability, likely due to the strong influence of the Indian summer monsoon. Thus, interactions between climate and tree community composition ultimately control how much carbon is stored in woody biomass across bioclimatic gradients. This fundamental understanding will support predictive efforts of the forest carbon sink in this
... To mitigate this risk of slow or absent tree regeneration and kickstart carbon sequestration after fire, actions such as planting trees are increasingly used to mitigate the carbon lost to fire (Povak et al., 2020). These nature-based solutions to climate change are immediate actions to prevent worst case climate forecasts (Fargione et al., 2018;Drever et al., 2021), but also require ongoing estimation of the carbon consequences of these actions (i.e., Graves et al., 2020). Tree planting after fire may coincide with additional treatments that occur prior to planting. ...
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Wildfire is a natural disturbance in many forested biomes, with the loss of carbon to the atmosphere and mortality of trees actively sequestering carbon of global concern as a contribution to climate change. Natural regeneration is often successful at reestablishing a forest in ecosystems adapted to fire, but there is increasing concern that the changing size, frequency and severity of wildfire is causing regeneration failures or inadequate densities of trees that sequester and store carbon following these disturbances. It remains unclear whether the action of planting trees accelerates carbon storage following fire compared to forests established through natural regeneration. The central interior of British Columbia recently experienced multiple years of record-breaking fire activity. Rehabilitation planting focused on reestablishing trees in the managed forest but was also prescribed in previously unmanaged forests to initiate carbon sequestration. Planting is often accompanied by other stand treatments such as salvage harvesting or snag removal and debris clearing to ensure planter safety. Here, we determine carbon recovery and stores in 21 wildfires across a chronosequence from the early 1960s to 2015. We measured above and belowground carbon pools to determine the effect of time since fire and planting treatments on carbon. Tree planting did not increase total ecosystem carbon over time, but rather decreased carbon through the loss of dead wood from site preparation. All carbon pools were affected by time since fire except the mineral soil pool, which was best predicted by soil clay content and coarse fragments positive effects. Live tree carbon increased over time, with more stored in planted stands over 60 years compared to stands that were not planted. Projecting growth to 100 years since fire suggests we may see increasing divergence in carbon stores in planted stands over a full fire-return interval, but these differences remain relatively small [mean (sd): 140.8 (19.6) Mg⋅ha–1 in planted compared to 136.9 (27.5) Mg⋅ha–1 in not-planted stands], with 1.4 Mg⋅ha–1 year–1 sequestered in not-planted compared to 1.5 Mg⋅ha–1 year–1 in planted stands. To meet carbon objectives, replanting trees on average sites in burned forests of BC’s central interior would require preserving the carbon legacy of fire, including dead wood.
... On a per-area basis, forests have been reported to rank highest among all ecosystems in the Midwest for potential for carbon sequestration and help mitigate the effects of increasing CO 2 (Fargione et al. 2018). With forest cover currently only at 32% of the baseline at the time of Euro-American settlement, there are opportunities to plan reforestations to consolidate forest and woodland fragments. ...
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There have been dramatic changes to forest lands since the end of the last ice age, about 14,000 years before present, when boreal ecosystems were eventually replaced by deciduous forest and grassland. In Illinois at the time of Euro-American Settlement (circa 1820), forest lands, including fire-maintained woodlands and savannas, comprised about 42% of the land area. Habitat destruction, fire absence, livestock grazing, and infestations of non-native species have altered forests since the 1800s. Currently, forest land cover statewide is about 13.5%, mostly (83%) in private ownership and predomi-nately (68%) classified as oak-hickory cover type. Further modifications can be expected due to climate change, predicted for Illinois over the next 100 years to include warmer winter temperatures, warmer and wetter springs, and hotter, drier summers. Models predicting potential futures for 113 tree species as a response to climate change over the next 100 years were generated for ten primary Illinois ecoregions. Results indicate that there are likely to be increases in habitat suitability and capability for some species and decreased habitat suitability and capability for others with variability across ecoregions. Many species demonstrate differential responses to changing climate from north to south in the state. The dominant species in the oak-hickory cover type generally are projected to have fair to good capabilities, with some notable exceptions; however , Acer saccharum, a competitor in many oak-hickory stands, also is projected to have fair to good capability. Dominant species in mesic upland and bottomland forests include a rich variety of species about evenly split between those with fair-to-good capabilities and those expected to have poor capability. Potential 'New Habitat' and 'Migrate' species also are identified. New Habitat species are those that have potential habitat appearing in the state within 100 years; Migrate species have some potential for natural distribution to the state within 100 years and could be considered as candidates for assisted migration northward. Considerations for conservation and management of forest lands are discussed.
Coastal marshes and seagrass beds store millions of tons of carbon in their sediments and sequester carbon at higher per-area rates than most terrestrial ecosystems. There is substantial interest in this “blue carbon” as a carbon mitigation strategy, despite the major threat that sea level rise (SLR) poses to these habitats. Many projections of habitat and carbon change with SLR emphasize the potential for inland marsh migration and increased rates of marsh carbon sequestration, but do not consider carbon fluxes associated with habitat conversion. We integrated existing data and models to develop a spatial model for predicting habitat and carbon changes due to SLR in six mid-Atlantic U.S. states likely to face coastal habitat loss over the next century due to low tidal ranges and sediment supply. Our primary model projection, using an intermediate SLR scenario (1.2 m SLR by 2104), predicts loss of 83% of existing coastal marshes and 26% of existing seagrasses in the study area. In addition, 270,000 hectares of forest and forested wetlands in low-lying coastal areas will convert to coastal marshes. These SLR-driven habitat changes cause the study area to shift from a carbon sink to a source in our primary model projection. Given the many uncertainties about the habitat and carbon changes represented in our model, we also identified the parameters and assumptions that most strongly affected the model results to inform future research needs. These included: land availability for inland marsh migration, the baseline extent and location of coastal marshes, proportion of stored carbon emitted from lost habitats (coastal marsh sediments or terrestrial biomass carbon), and methane emissions from freshwater habitats. The study area switched from a net carbon sink to a net carbon source under SLR for all but three model runs; in those runs, net carbon sequestration declined by 57–99%.
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Background The natural removal of carbon dioxide (CO2) from the atmosphere through land conservation, restoration, and management is receiving increasing attention as a scalable approach for climate change mitigation. However, different land-use sectors compete for resources and incentives within and across geopolitical regions, resulting in divergent goals and inefficient prioritization of CO2 removal efforts. Thus, a unifying framework is needed to accelerate basic research and coordinated interventions to accelerate climate change mitigation. Scope We propose a generalizable framework for Enhanced Natural Climate Solutions (NCS +), which we define as activities that can be coordinated to increase carbon drawdown and permanence on land while improving livelihoods and the provision of natural resources in vulnerable communities and ecosystems. The framework builds on interdisciplinary scientific convergence, including critical socioecological interactions, to inform both top-down policy incentives and bottom-up adoption by industries and managers. To achieve this goal, we suggest a multi-tiered approach for the prioritization of projects at local to regional scales that would simultaneously accelerate scientific discovery and broad implementation of CO2 removal projects. Conclusions Our vision leverages input from hundreds of researchers and land managers, including social and environmental scientists as well as representatives from tribal governments, state, and federal agencies in the Pacific Northwest of the USA, as a model system. Five guiding principles orient the framework which would be applicable in any region. As evidence of feasibility, we provide a synthesis of interdisciplinary studies that illustrate how coordinated action, with explicit consideration of system-specific technical and socioecological limitations, can lead to scalable projects with multiple co-benefits. Using theory as a linchpin for innovation, we propose that NCS + could better align climate change mitigation, adaptation, and justice goals at multiple scales.
On the Ground •Natural solutions, such as “avoided conversion of grasslands,” offer agricultural land managers a way to mitigate climate change while monetizing climate benefits. •Managers who avoid converting grasslands to other uses, such as row crops, can quantify the amount of stored carbon and sell credits, but high costs of developing carbon credit projects price many landowners out of the carbon market. •Aggregation can create economies of scale, which lower barriers of entry and allow more landowners to participate in the market. •Given the current low prices in the carbon market, aggregation is not a panacea and aggregated projects are not financially viable for many landowners. •As the demand for carbon credits continues to grow, land managers can position themselves to take advantage of carbon market opportunities should prices increase, and projects become financially viable.
Privately-owned forests in the Pacific Northwest (PNW) are important potential carbon sinks and play a large role in carbon sequestration and storage. Non-industrial private forest (NIPF) owners constitute a substantial portion of overall forest landownership in productive regions of the PNW; however, little is known about their preferences for non-market incentive programs aimed at increased carbon storage and sequestration, specifically by limiting timber harvest, and how those preferences might impact the outcome of forest carbon programs. We simulated landscape-scale outcomes of hypothetical forest carbon incentive programs in western Oregon (USA) by combining empirical models of NIPF owners' participation with spatially explicit forest carbon storage and sequestration data. We surveyed landowners to determine their willingness to enroll in various hypothetical forest management incentive programs that varied in terms of harvest restrictions, contract length, annual payment and incentive payment amounts, and cost-share percentages, as well as the program framing (e.g., carbon versus forest health). We used multinomial logistic regression to model whether landowners might enroll based on program attributes, landowners' attitudes toward climate change and forest management, past and planned future forest harvest activities, and socio-demographics. We found that 36% of respondents stated that they would probably or definitely enroll in at least one of the hypothetical programs they were shown while 21% of respondents refused all programs that they were offered. Our final model of landowner willingness to enroll indicated that higher annual and higher cost-share payments were the strongest positive predictors of whether landowners would enroll vs. not enroll. Landowners' willingness to enroll was not influenced by program framing as either a “forest carbon” or a “forest health”; however, landowner attitudes toward climate change were the next strongest positive predictor of enrollment after annual and cost-share payments. By simulating landowner enrollment in six policy relevant program scenarios, we illustrate that carefully designed forest carbon incentive programs for NIPF owners could have tangible carbon protection benefits (16.25 to 50.31 MMT CO2e cumulative) at relatively low costs per MT CO2e ($3.60 to $7.70). We highlight tradeoffs between maximizing enrollment in forest carbon incentive programs and providing longer term protection of carbon. This research contributes to the literature on the design of potential forest carbon incentive programs and communication about forest carbon management, as well as aims to aid policy makers and program administrators that seek ways to engage private landowners in carbon-oriented forest management.
Saline tidal wetlands are important sites of carbon sequestration and produce negligible methane (CH4) emissions due to regular inundation with sulfate‐rich seawater. Yet, widespread management of coastal hydrology has restricted tidal exchange in vast areas of coastal wetlands. These ecosystems often undergo impoundment and freshening, which in turn cause vegetation shifts like invasion by Phragmites, that affect ecosystem carbon balance. Understanding controls and scaling of carbon exchange in these understudied ecosystems is critical for informing climate consequences of blue carbon restoration and/or management interventions. Here, we (1) examine how carbon fluxes vary across a salinity gradient (4–25 psu) in impounded and natural, tidally unrestricted Phragmites wetlands using static chambers and (2) probe drivers of carbon fluxes within an impounded coastal wetland using eddy covariance at the Herring River in Wellfleet, MA, United States. Freshening across the salinity gradient led to a 50‐fold increase in CH4 emissions, but effects on carbon dioxide (CO2) were less pronounced with uptake generally enhanced in the fresher, impounded sites. The impounded wetland experienced little variation in water‐table depth or salinity during the growing season and was a strong CO2 sink of −352 g CO2‐C m−2 year−1 offset by CH4 emission of 11.4 g CH4‐C m−2 year−1. Growing season CH4 flux was driven primarily by temperature. Methane flux exhibited a diurnal cycle with a night‐time minimum that was not reflected in opaque chamber measurements. Therefore, we suggest accounting for the diurnal cycle of CH4 in Phragmites, for example by applying a scaling factor developed here of ~0.6 to mid‐day chamber measurements. Taken together, these results suggest that although freshened, impounded wetlands can be strong carbon sinks, enhanced CH4 emission with freshening reduces net radiative balance. Restoration of tidal flow to impounded ecosystems could limit CH4 production and enhance their climate regulating benefits. Widespread management of coastal hydrology has restricted tidal exchange in vast areas of coastal wetlands, which often leads to impoundment, freshening, and vegetation shifts like invasion by Phragmites. Using both static chambers and eddy covariance, we demonstrate that freshening causes enhanced methane emission and increases radiative forcing from greenhouse gases. Restoration of tidal flow to impounded ecosystems could suppress methane production and combined with conversion to native salt marsh vegetation, serve as an effective blue carbon climate change mitigation opportunity.
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Urban trees reduce respirable particulate matter (PM 10) concentrations and maximum daytime summer temperatures. While most cities are losing tree cover, some are considering ambitious planting efforts. Maximizing PM 10 and heat mitigation for people from such efforts requires cost-effective targeting. We adapt published methods to estimate the impact of a decade (2004-2014) of tree cover change on city-level PM 10 and heat mitigation in 27 U.S. cities and present a new methodology for estimating local-level PM 10 and heat mitigation by street trees and tree patches. We map potential tree planting sites in the 27 cities and use our local-level PM 10 and heat mitigation methods to assess the population-weighted return on investment (ROI) of each site for PM 10 and heat abatement for nearby populations. Twenty-three of the 27 cities lost canopy cover during 2004-2014, reducing estimated city-level PM 10 removal by 6% (168 tons) and increasing city-level average maximum daily summer temperature by 0.1 °C on average across cities. We find large potential for urban reforestation to increase PM 10 and heat abatement. Intra-city variation in planting site ROI-driven primarily by differences in population density around planting sites-exceeds four orders of magnitude, indicating large scope for targeting to increase PM 10 and heat abatement from reforestation. Reforesting each city's top 20% ROI sites could lower average annual PM 10 concentrations by > 2 μg/m 3 for 3.4-11.4 million people and average maximum daily summer temperatures by > 2 °C for 1.7-12.7 million-effects large enough to provide meaningful health benefits-at a combined annual cost of $102 million.
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Many areas of the natural and social sciences involve complex systems that link together multiple sectors. Integrated assessment models (IAMs) are approaches that integrate knowledge from two or more domains into a single framework, and these are particularly important for climate change. One of the earliest IAMs for climate change was the DICE/RICE family of models, first published in Nordhaus (Science 258:1315–1319, 1992a), with the latest version in Nordhaus (2017, 2018). A difficulty in assessing IAMs is the inability to use standard statistical tests because of the lack of a probabilistic structure. In the absence of statistical tests, the present study examines the extent of revisions of the DICE model over its quarter-century history. The study finds that the major revisions have come primarily from the economic aspects of the model, whereas the environmental changes have been much smaller. Particularly, sharp revisions have occurred for global output, damages, and the social cost of carbon. These results indicate that the economic projections are the least precise parts of IAMs and deserve much greater study than has been the case up to now, especially careful studies of long-run economic growth (to 2100 and beyond). Additionally, the approach developed here can serve as a useful template for IAMs to describe their salient characteristics and revisions for the broader community of analysts.
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Carbon stocks in vegetation have a key role in the climate system. However, the magnitude, patterns and uncertainties of carbon stocks and the effect of land use on the stocks remain poorly quantified. Here we show, using state-of-the-art datasets, that vegetation currently stores around 450 petagrams of carbon. In the hypothetical absence of land use, potential vegetation would store around 916 petagrams of carbon, under current climate conditions. This difference highlights the massive effect of land use on biomass stocks. Deforestation and other land-cover changes are responsible for 53-58% of the difference between current and potential biomass stocks. Land management effects (the biomass stock changes induced by land use within the same land cover) contribute 42-47%, but have been underestimated in the literature. Therefore, avoiding deforestation is necessary but not sufficient for mitigation of climate change. Our results imply that trade-offs exist between conserving carbon stocks on managed land and raising the contribution of biomass to raw material and energy supply for the mitigation of climate change. Efforts to raise biomass stocks are currently verifiable only in temperate forests, where their potential is limited. By contrast, large uncertainties hinder verification in the tropical forest, where the largest potential is located, pointing to challenges for the upcoming stocktaking exercises under the Paris agreement.
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Forest resilience to climate change is a global concern given the potential effects of increased disturbance activity, warming temperatures and increased moisture stress on plants. We used a multi-regional dataset of 1485 sites across 52 wildfires from the US Rocky Mountains to ask if and how changing climate over the last several decades impacted post-fire tree regeneration, a key indicator of forest resilience. Results highlight significant decreases in tree regeneration in the 21st century. Annual moisture deficits were significantly greater from 2000 to 2015 as compared to 1985–1999, suggesting increasingly unfavourable post-fire growing conditions, corresponding to significantly lower seedling densities and increased regeneration failure. Dry forests that already occur at the edge of their climatic tolerance are most prone to conversion to non-forests after wildfires. Major climate-induced reduction in forest density and extent has important consequences for a myriad of ecosystem services now and in the future.
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Better stewardship of land is needed to achieve the Paris Climate Agreement goal of holding warming to below 2 °C; however, confusion 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, restoration , and improved land management actions that increase carbon 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 e y −1) represents cost-effective climate mitigation, assuming the social cost of CO 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 warming 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 estimates. 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