The potential of agricultural land management to contribute to lower global surface temperatures

Abstract

Removal of atmospheric carbon dioxide (CO2) combined with emission reduction is necessary to keep climate warming below the internationally agreed upon 2°C target. Soil organic carbon sequestration through agricultural management has been proposed as a means to lower atmospheric CO2 concentration, but the magnitude needed to meaningfully lower temperature is unknown. We show that sequestration of 0.68 Pg C year⁻¹ for 85 years could lower global temperature by 0.1°C in 2100 when combined with a low emission trajectory [Representative Concentration Pathway (RCP) 2.6]. This value is potentially achievable using existing agricultural management approaches, without decreasing land area for food production. Existing agricultural mitigation approaches could lower global temperature by up to 0.26°C under RCP 2.6 or as much as 25% of remaining warming to 2°C. This declines to 0.14°C under RCP 8.5. Results were sensitive to assumptions regarding the duration of carbon sequestration rates, which is poorly constrained by data. Results provide a framework for the potential role of agricultural soil organic carbon sequestration in climate change mitigation.

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The potential of agricultural land management to
contribute to lower global surface temperatures
Allegra Mayer
1
*, Zeke Hausfather
2
, Andrew D. Jones
3
, Whendee L. Silver
1
Removal of atmospheric carbon dioxide (CO
2
) combined with emission reduction is necessary to keep climate warming
below the internationally agreed upon 2°C target. Soil organic carbon sequestration through agricultural management
has been proposed as a means to lower atmospheric CO
2
concentration, but the magnitude needed to meaningfully
lower temperature is unknown. We show that sequestration of 0.68 Pg C year
−1
for 85 years could lower global tem-
perature by 0.1°C in 2100 when combined with a low emission trajectory [Representative Concentration Pathway (RCP)
2.6]. This value is potentially achievable using existing agricultural management approaches, without decreasing land
area for food production. Existing agricultural mitigation approaches could lower global temperature by up to 0.26°C
under RCP 2.6 or as much as 25% of remaining warming to 2°C. This declines to 0.14°C under RCP 8.5. Results were sen-
sitive to assumptions regarding the duration of carbon sequestration rates, which is poorly constrained by data. Results
provide a framework for the potential role of agricultural soil organic carbon sequestration in climate change mitigation.
INTRODUCTION
The uptake of atmospheric carbon (C) by plants and subsequent storage
in soils may be an effective means to lower atmospheric carbon dioxide
(CO
2
) concentrations and to help mitigate climate change. Integrated
assessment models (IAMs), which are used to explore future energy,
land-use, and greenhouse gas (GHG) emission scenarios, currently rely
on bioenergy with carbon capture and storage (BECCS) as a principal
negative emission technology to reach climate change mitigation targets,
butgenerallydonotconsiderthepossibility of C drawdown and soil
organic C (SOC) sequestration from improved land management (1,2).
Improved land management, without changing land use, may be an ad-
ditional C sequestration option that does not require more land con-
version. Land-use change and poor management practices have resulted
in the loss of more than 130 Pg C from agricultural soil (3), leaving
>1 billion hectares of degraded soil worldwide (4). Site-based studies
and ecosystem-scale models have shown that degraded and managed
agricultural lands have great potential to contribute to increased SOC
sequestration through improved management (5–7). We define soil C
sequestration as a net increase in SOC storage. Several agricultural
(cropland and grazing land) management practices have been shown
to increase soil C sequestration including organic amendments (8–10),
cover crops, reduced tillage, improved crop rotations (5,11,12), and im-
proved grazing management (13,14). Citing these proven practices and
others, France and 33 other countries recently instituted a challenge to
increase soil C by 4 per mil per year (15). However, the actual potential
of these practices to contribute to lowering global temperature over time
is poorly understood, despite recent efforts to quantify the amenable
global land area and near-term sequestration rates associated with various
practices (16–18). This is primarily due to uncertainty regarding the
maintenance of soil C sequestration rates over time, the C sequestration
capacity of different soils under different managements, and the sen-
sitivity of global temperature changes to CO
2
emission and sequestration.
Here, we use a climate model emulator to translate SOC sequestra-
tion from agricultural management into a range of potential global
mean surface temperature changes over time, consistent with global-
scale outputs from the latest generation of Earth system models (ESMs;
see Materials and Methods and fig. S1). Much of the research to date on
the potential of land use–based SOC sequestration has focused on quan-
tifying current sequestration rates with the implicit assumption that
rates remain constant over time, often assuming a constrained time pe-
riod of 20 to 50 years (12,16). The potential for SOC sequestration to
contribute to a portfolio of mitigation strategies aimed at reducing cli-
mate change depends not only on the rates soon after C sequestering
practices are implemented but also on the time-integrated dynamics of
those rates, that is, how quickly land-use changes can be adopted, how
long they remain in effect, and how SOC stocks change over time (19).
This temporal dynamic is poorly understood but is critical to accurately
estimate the potential for land-based management to slow climate change.
In practice, rates of SOC sequestration are likely to decline over time
at any one site as soils reach new equilibria (20), but the time scale and
shape of these declines are not well constrained by data and are likely to
vary significantly among locations and management practices. To help
bound this uncertainty, we model the effects of SOC sequestration on
global surface temperature with and without consideration of effective
sequestration years, defined as the number of years it would take to
reach the maximum SOC stock (SOC max) at the current potential
sequestration rate (see fig. S3). The SOC max is a concept proposed
by Six et al.(21) and is poorly constrained by data at both site and global
scales. An SOC max provides a theoretical limit on the amount of SOC
storage in soils. As opposed to applying an arbitrary SOC max at a fixed
time period (for example, 20 or 50 years), we model effective sequestra-
tion years as a continuum of time periods required to reach an SOC max
(from 0 to 85 years) for a range of SOC sequestration rates. The current
potential sequestration rate was taken from values reported in the liter-
ature (table S1). Our analysis focuses specifically on the temperature re-
sponse to SOC sequestration. SOC storage is sensitive to a suite of global
change factors such as elevated atmospheric CO
2
concentration and
changes in climate, among others (22,23). These factors will likely have
additional, albeit poorly constrained, impacts on management-induced
SOC sequestration (24). Our goal here is not to quantify the ecological
controls on SOC storage and loss in agricultural ecosystems but to de-
termine the magnitude of SOC sequestration needed at a global scale to
meaningfully affect temperatures and to explore the sensitivity of atmo-
spheric temperature change to a range of possible temporal limits to soil
C sequestration (effective sequestration years).
1
Department of Environmental Science, Policy and Management, University of
California, Berkeley, Berkeley, CA 94720, USA.
2
Energy and Resources Group, Uni-
versity of California, Berkeley, Berkeley, CA 94720, USA.
3
Climate and Ecosystem
Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94705, USA.
*Corresponding author. Email: allegramayer@berkeley.edu
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RESULTS
The model shows that a global SOC sequestration rate of 0.68 Pg C year
−1
from 2015 to 2100 would be required to yield a 0.1°C reduction in mean
surface temperature in target year 2100 when coupled with an aggressive
emission reduction scenario [Representative Concentration Pathway
(RCP)2.6;Fig.1].Theseresultsassumed a constant sequestration rate
tothetargetyear.The0.68PgCyear
−1
global sequestration rate is at
the low end of the ranges of published estimates, which vary from a
low of 0.36 Pg C year
−1
to a high of 1.56 Pg C year
−1
(12,17,25,26). There
is considerable uncertainty in the actual time horizon of soil C sequestra-
tion rates within sites, which is likely to vary in response to social, econom-
ic, and biophysical factors (17,27). Few studies have measured long-term
(>20 years) patterns in soil C sequestration with agricultural management,
and those have shown a wide range of continued sequestration rates from
20 to over 150 years (11,28–30).Forthisreason,weappliednoapriori
assumption on the time horizon required of soil C sequestration and, in-
stead, modeled a continuum of effective sequestration years at SOC
sequestration rates from 0 to 2.0 Pg C year
−1
. It is important to note that,
while the model assumes constant annual rates of SOC sequestration at a
global scale, it does not require constant rates at individual sites.
Results were dependent on the underlying RCP scenario, assuming
3°C warming per doubling of CO
2
. In scenarios with greater emission
trajectories and thus higher atmospheric CO
2
concentrations (RCP 8.5
and RCP 6), greater SOC sequestration rates were required to reach the
same reduction in global surface temperatures due to the logarithmic
nature of CO
2
forcing (31). To achieve a 0.1°C reduction by the year
2100, the RCP 6.0 emission scenario would require a sequestration of
0.98 Pg C year
−1
, while 1.25 Pg C year
−1
would be required in the RCP
8.5 scenario. Management-based SOC sequestration had the highest
efficacy in the RCP 2.6 scenario, indicating the importance of simulta-
neous emission reductions and SOC sequestration activities resulting
from management. The sensitivity of climate to changes in atmospheric
CO
2
concentration is a key uncertainty in the model and had a large
influence on the temperature effect of C sequestration activities, as sce-
narios with lower climate sensitivity would require increased sequestra-
tion to result in the same temperature reduction (Fig. 1, black bars).
Estimates of climate sensitivity range from 1.5° to 4.5°C warming per
doubling CO
2
, with a median estimate of around 3°C (32).
A synthesis of the literature yielded a mean annual SOC sequestra-
tion potential from agriculture of 0.83 Pg C year
−1
, with an upper value
of 1.78 Pg C year
−1
(table S1). This value is greater than our estimate of
0.68 Pg C year
−1
needed to reduce global temperatures by 0.1°C in 2100
under RCP 2.6 (Fig. 1), and results in a temperature reduction of 0.12°C
in 2100 if sustained for 85 years. Greater SOC sequestration and associated
temperature reduction may potentially be achieved with biochar amend-
ments (see the Supplementary Materials); this approach is less well constrained
than the other approaches and could require at least some utilization of
abandoned lands (10,33),andisthusnotconsideredfurtherhere.
Fig. 1. Impact of constant global rates of C sequestration (Pg C year
−1
) on mean surface temperatures by target year (2016–2100) for a climate sensitivity of
3°C per doubling of atmospheric CO
2
.A 0.1°C reduction is highlighted by white lines. Different graphs indicate different RCP scenarios. Bars show the range of
continued C sequestration rates needed to achieve a 0.1°C reduction in 2050, 2075, and 2100, respectively, under a range of alternative climate sensitivities from 1.5°C
per doubling (upper bound) to 4.5°C per doubling (lower bound) (32). Upward arrows represent low CO
2
sensitivity upper bounds that are higher than the range of C
sequestration rates (0 to 2 Pg C year
−1
) considered in this study; error bars are not symmetric around the 0.1°C reduction line due to nonlinearities in CO
2
forcing.
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Uncertainties in potential global rates of SOC sequestration with
improved land management can be partitioned into two primary
factors: the range of field-scale SOC sequestration rates reported for each
practice and the global land area over which the technique was considered
effective. Both area and rate assumptions affect the estimate of total SOC
sequestration potential. Area-dependent SOC sequestration rates varied from
0.02 to 1.15 Mg C ha
−1
year
−1
through improved cropland management
(12,34) and from 0.03 Mg C ha
−1
year
−1
(35)to0.62MgCha
−1
year
−1
through improved grazing land management (Table 1) (10,26). The
range of amenable land area varied from approximately 2900 Mha
for grazing land (36) to 400 Mha for cropland (16). We used the bio-
physical potential as the upper limit of SOC sequestration potential and
the minimum reported SOC sequestration estimate as the lower limit
(16,17,37). Nutrient availability at the field scale could theoretically limit
SOC sequestration rates (38,39), although the increase in soil organic
matter content can alleviate at least some of this limitation (18). We note
that agricultural management to increase SOC storage can interact
with soil inorganic C, by increasing storage or facilitating losses to the
atmosphere (40). However, the long-term impacts of SOC sequestration
on soil inorganic C dynamics are poorly understood. Economic con-
straints influenced the uncertainty regarding the amount of land
amenable to improved management (17,37), as well as whether man-
agement strategies could be implemented quickly and maintained over
multiple decades on the available and amenable land area. The manage-
ment practices explored here are likely to simultaneously provide the
co-benefit of improved soil fertility and water holding capacity, thus
increasing the financial desirability for implementation.
The results were sensitive to effective sequestration years (Fig. 2),
with the reduction in warming increasing roughly linearly with the
number of years that soils could continue to sequester C. If we assume
a limited effective sequestration period of 20 years (11,28), irrespective
of the mechanism limiting C storage, then the climate impact in the year
2100 associated with a sequestration rate of 0.83 Pg C year
−1
decreased
from 0.12° to 0.03°C under RCP 2.6. If the effective sequestration period
is 50 years (12), then the climate impact in the year 2100 was 0.07°C
under RCP 2.6. These values highlight the potential negative impact
of short effective sequestration years. However, much of the world’s
soils are degraded with regard to SOC (4). Sanderman et al.(3)esti-
matedthat75and133PgChavebeenlostgloballyfromthetop1to
2 m of soil, respectively, due to human land use. If we assume that the
maximum soil C sequestration potential is equivalent to the amount of
C that has been lost from soils due to land use, then soils globally would
have the capacity to store an additional 0.9 Pg C year
−1
until 2100 in the
top meter alone and 1.62 Pg C year
−1
in the top 2 m. To determine the
impact of effective sequestration years on a given soil C sequestration
rate from any combination of management strategies, we assessed the
effect of the number of effective sequestration years (from 1 to 85) on
the year 2100 global mean surface temperatures for the range of SOC
sequestration rates (0 to 2.0 Pg C year
−1
) previously considered (Fig. 2).
In practice, SOC sequestration rates would likely exhibit a gradual de-
clineovertime,andtheshapeofthisdeclinewouldlikelyvarybyland
use, climate, and soil type and is not well understood (41,42). Some
studies have found that SOC stocks reach an asymptote over time (28).
This can be indicative of the development of equilibrium conditions
under a given rate of inputs (11). It has also been hypothesized that
soils may become saturated with regard to SOC, with stocks ceasing
to increase even at increasing input rates (21,41,43,44). Understanding
of the long-term potential for soilsto sequester C is limited, and mech-
anisms such as C stabilization and protection are not consistently
represented in coupled climate–C cycle models and major ESMs (45).
Our approach using effective sequestration years allows us to bound the
uncertainty arising from these poorly understood mechanisms.
DISCUSSION
Our results show that existing management strategies on current ag-
ricultural lands have the potential to reduce global temperatures by
the end of the century, sequestering as much as 1.78 Pg C year
−1
.This
rate of sequestration could result in warming reductions of as much as
0.26°C in 2100 (assuming an effective sequestration period of at least
85 years). These model results are dependent on the effective seques-
tration years at a global scale and concurrent trends in emission reduc-
tion. The time horizon of SOC sequestration is poorly understood but
is critical for determining the long-term viability of these approaches.
In particular, we note that climate change is a millennial-scale phenom-
enon that stretches beyond the 2100 target. Therefore, the residence time
of organic C in soils, which can affect the long-term efficacy of SOC
sequestration (46), is an additional concern. SOC has the potential to
be stored for millennia [as evidenced by numerous radiocarbon studies
(47,48)], particularly when considering the entire soil depth profile (49),
but this may require a long-term commitment to maintaining soil C in
the future.
Management-based soil C sequestration strategies were significantly
more effective at reducing global warming under RCP 2.6 due to the
atmospheric saturation of CO
2
in high emission scenarios (for example,
in RCP 8.5). This result points to the value of combining aggressive
emission reduction with C removal strategies for climate change mitiga-
tion. The management strategies we evaluated here are different from
the measures assumed in the modeled RCP scenarios (50,51)andthere-
fore provide additional negative emissions if applied simultaneously
with emission reductions. The RCP scenarios can be realized through
many alternative energy, land-use, and land-cover pathways, but current
IAMs largely rely on land-cover change (for example, afforestation) and
C capture and storage technology combined with bioenergy (BECCS) to
reach goals of reduced radiative forcing (50). In contrast, this study
examined the effects of improving management practices on agricultural
Table 1. Published global estimates of management amenable agricultural land and the C sequestration potential of land management techniques.
Management Land type Range of published amenable
land area estimates (Mha)
Range of published potential C
sequestration rates (Mg C ha
−1
year
−1
)Sources
Improved cropland management Cropland 380–1910 0.08–1.85 (16,17,26,34,36)
Improved grazing land management Grazing land 500–2900 0.09–1.70 (16,17,26,35,36)
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land currently in production. We note that future changes in the overall
size of the agricultural land area will affect the area amenable to these
practices and that heavy reliance on bioenergy could compete with some
management activities on cropland or grazing land. This is one reason
that we emphasize consideration of a large range of possible sequestration
rates and time frames. Extensive adoption of land management strategies
could moderately reduce the need for BECCS, which is considered ex-
tensively in most 1.5° and 2°C target scenarios (52). Sequestering an
additional 1.78 Pg C year
−1
through BECCS would require devoting
89 Mha of agricultural land to bioenergy production [equivalent to
roughly half of current global maize area harvested (53)]. Growing bio-
energy crops for BECCS on abandoned agricultural land could reduce
the impact on food prices and ecosystem carbon storage (33), although
lower crop yields and economic limitations on the use of abandoned
lands must be accounted for in this context (54).
Our analysis also points to the importance of the long-term potential
for SOC sequestration. The largely underappreciated scientific un-
certainty of effective sequestration years greatly affected the climate
change mitigation potential of land management strategies. A better
understanding of the long-term regional potential of specific man-
agement applications for C sequestration, as well as the controls and
limits on C sequestration, will facilitate better predictions of future land-
atmosphere C cycle feedbacks and also inform the potential for long-
term stabilization of C beyond the 2100 temperature target. As new
strategies are identified for sequestering C through land management
such as repurposing urban and rural nutrient and C waste streams
(7,55), our model provides a framework for translating these into
warming reductions.
MATERIALS AND METHODS
Climate model
We used a climate model that has been used previously in the literature
(56–59) to evaluate the impact of emissions or emission reductions/C
sequestration on future transient climate responses (fig. S1). The model
takes a particular emission scenario for three major GHGs (CO
2
,CH
4
,
and N
2
O) and converts these emissions into atmospheric concentra-
tions, radiative forcing, and transient temperature response. This
emulator model has the benefit of enabling us to consider many possible
permutations of sequestration rates and effective sequestration years
while matching the global mean surface temperature response found
in more complex ESMs such as those included in the Coupled Model
Intercomparison Project 5 (CMIP5) featured in the Intergovernmental
Panel on Climate Change 5th Assessment Report (IPCC AR5; fig. S1).
Fig. 2. Impact of SOC sequestration rate (Pg C year
−1
) and effective sequestration years on 2100 global mean surface temperature for a climate sensitivity of
3°C per doubling CO
2
with a 0.1°C reduction (highlighted by a white line). A range of potential C sequestration rates are shown in the center of the chart, as well as
their combined potential (black solid). The vertical dashed line shows the mean estimated potential of 0.83 Pg C year
−1
for reference.
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As a single run of a coupled ESM would take 5 days, it would be pro-
hibitively difficult to perform the 6800 separate combinations of se-
questration rates and effective sequestration years examined in this paper.
To translate the rate of SOC sequestration into a transient global mean
surface temperature response, we perturbed the emission scenarios in the
RCPs by all possible sequestration rates between 0 and 3.0 Pg C year
−1
from 2016 to 2100. We used simplified atmospheric lifetime functions for
each GHG (60) to calculate both perturbed and unperturbed atmospheric
CO
2
concentrations, translated these CO
2
concentrations into radiative
forcing (61), and used a continuous diffusion slab ocean model to
estimate transient temperature response (62). Reduction in warming
associated with sequestration rates was calculated from the difference
between the temperatures at a given point in time between the un-
perturbed and perturbed RCP scenario. We did not consider the
potential effects of temperature change on SOC dynamics as this was
beyond the scope of this study. These effects are poorly constrained,
and different studies find both increases and decreases of the SOC stock
with warming (24).
The model approximates the life cycle of each GHG using atmo-
spheric lifetime functions adapted from Joos et al.(60). These model
the percent of a discrete pulse remaining in the atmosphere after tyears
GCO2ðtÞ¼0:217 þ0:259et=172:9þ0:338et=18:51 þ0:186et=1:186
GCH4ðtÞ¼et=10
GN2OðtÞ¼et=114
CH
4
and N
2
O were assumed to have e-foldingtimesof10and114years
(for example, the time scale for a quantity to decrease to 1/eof its
initial value), respectively, while CO
2
reflects more complex C cycle
dynamics. These were converted into atmospheric concentrations
A
gas
(t) by treating each annual emission (or emission reduction) as
a discrete pulse and summing all pulse responses over the time pe-
riod of interest t
AgasðtÞ¼∑
n
i¼1EiGðttiÞ
where E
i
is the emissions in year iand G(t−t
i
) is the fraction of the
gas remaining in the atmosphere after time t−t
i
. The mass of each
gas in the atmosphere was converted into concentrations in parts per
million (ppm) [or parts per billion (ppb)] based on their respective
molar mass.
The resulting atmospheric GHG concentrations closely mirror the
results of CMIP5 runs (fig. S2) for the most part, although there is some
divergence in high emissions (RCP 6 and RCP 8.5) scenarios where
changes in ocean chemistry associated with acidification reduce the
airborne fraction in a manner not captured by our emulator model.
For this analysis, however, because we are looking at small perturbations
in the net CO
2
emissions of underlying RCP scenarios, the limitations of
the simple atmospheric carbon cycle model used should be minimal. We
did not explicitly consider feedbacks or interactions between carbon
sequestration and other GHG emissions (methane and nitrous oxide, in
particular). These fluxes were poorly constrained for most of the land uses
considered here, and thus, this was beyond the scope of the current analysis.
To convert atmospheric GHG concentrations into direct radiative
forcing, we used the simplified radiative forcing functions from the
IPCC AR5 (61). These are functional fits to more complicated absorp-
tion models derived from line-by-line radiative transfer functions that
have relatively small uncertainties: about 1% for CO
2
radiative forcing
and 10% for CH
4
radiative forcing calculations (31).
Forcing from a change in atmospheric concentration of CO
2
is
given by
DFCO2¼5:35⋅ln ðPCO2þaCO2Þ
PCO2
Here, PCO2represents the initial concentration of CO
2
in the atmo-
sphere before the industrial era, while aCO2represents the additional
parts per million CO
2
added for any given scenario. For the purposes
of this analysis, PCO2was set to 277 ppm, the approximate value for the
preindustrial era (for example, 1765).
The direct radiative forcing of a given increase of CH
4
and/or
N
2
O in the atmosphere can be approximated by (61)
DFCH4¼0:036ðffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
PCH4þbCH4
pffiffiffiffiffiffiffiffiffiffi
PCH4
pÞ
fðPCH4þbCH4;PN2OÞþfðPCH4;PN2OÞ
DFN2O¼0:12ðffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
PN2OþbN2O
pffiffiffiffiffiffiffiffiffiffi
PN2O
pÞ
fðPCH4;PN2OþbN2OÞþfðPCH4;PN2OÞ
where
fðM;NÞ¼0:47 lnð1þ2:01⋅105ðMNÞ0:75 þ
5:31⋅1015MðMNÞ1:52Þ
In this equation, PCH4is the initial concentration of atmospheric
CH
4
, while bCH4is the addition being evaluated. PN2Ois the initial
concentration of N
2
O, and bN2Ois the addition being evaluated. The
radiative forcing of both CH
4
and N
2
O is a function of the combination
of both, reflecting their interacting atmospheric chemistry. For this anal-
ysis, PCH4was set to 722 ppb and PN2Owas set to 272 ppm, reflecting
preindustrial atmospheric concentrations. f(M,N) is a function that
accounts for the interrelationship between CH
4
and N
2
Oforcing(61).
Radiative forcing was translated into a transient temperature re-
sponse by using a continuous diffusion slab ocean model adapted from
Caldeira and Myhrvold (62) and based on the study of Hansen et al.
(63). It is governed by the equations
∂DT
∂t¼kv
∂2DT
∂z2
∂DT
∂z



z¼0¼ðlDTDFðtÞÞ
pf cp;wkv



z¼0
DTt¼0¼0
j
∂DT
∂z



z¼zmax ¼0
where fis the fraction of the earth covered by ocean (0.71), pis the
density of water, c
p
is the heat capacity of water, z
max
is the maximum
ocean depth (4000 m), lis the feedback parameter (DT¼DF
lat equi-
librium, with l=1.25 chosen to reflect a climate sensitivity of 3 C per
doubling CO
2
), and k
v
is the ocean vertical thermal diffusivity (assumed
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to be 5.5 × 10
−5
m
2
s
1
). The land fraction of the earth was assumed to
follow its equilibrium temperature response, with global surface tem-
peratures being the area-weighted average of the two.
Literature analysis
We used published global estimates of C sequestration potential in
grazing and croplands (table S1). Improved management approaches
included conversion to reduced or conservation till (12,17,64), crop
residue management (64,65), crop rotation and cover crop manage-
ment (12,17,66),optimizedirrigationandnutrient amendment strate-
gies (6,12,17), biochar amendments (10,16,26), increased productivity
of both cropland and grasslands (6,12,17), and improved grazing
management (17,25,26,35,67). To better determine the impact of soil
C sequestration on temperature, we used only estimates of soil C sinks,
and not addition or avoidance of CO
2
emissions due to management.
However, some global estimates account for nitrous oxide emissions
stimulated or avoided due to management, and report C estimates in
units of CO
2
eor C equivalents. We therefore compared global estimates
using the unit CO
2
e-C, which was converted using the ratio of atomic
mass: 1 Pg C = 12/44 × 1 Pg CO
2
e. We reported estimates for biochar
separately because of continuing interest in biochar as a means to se-
quester atmospheric CO
2
despite poorly constrained estimates of its
persistence in soil and notwithstanding its potential land conversion re-
quirements (10,25,33,68). Other management strategies, including
compost and other organic amendments, could also be applied over
large and diverse areas, but large-scale estimates for the potential of C
sequestration from these strategies are lacking.
We differentiated between total soil C sequestration potential and
the combined potential. Total C sequestration potential is an aggrega-
tion of literature estimates of the total global agricultural C sink potential
(12,17,25,26,37). For the combined potential, we summed recent land-
use and management-specific estimates for potential sequestration in
cropland (16,17,26,34,69) and grazing lands (16,17,26,35,69)together
with the available land area given in the same sources for these practices.
We report this combined potential with and without biochar contribu-
tions, as biochar application can be used alone or coupled with other
land-use practices for the same land area. Values given in the text are
means ± 1 SE when multiple estimates were available.
SUPPLEMENTARY MATERIALS
Supplementary material for this article is available at http://advances.sciencemag.org/cgi/
content/full/4/8/eaaq0932/DC1
Fig. S1. SimMod emulator climate model transient (solid red) temperature response compared
to CMIP5 multimodel mean (black line) and 2.5 to 97.5% spread (gray area) for each RCP
scenario.
Fig. S2. RCP (solid lines) and SimMod emulator climate model (dashed lines) atmospheric
concentrations of CO
2
,N
2
O, and CH
4
for each scenario (53).
Fig. S3. A schematic illustration of the concept of effective sequestration years (ESY).
Fig. S4. Same as Fig. 2 in the text but with the inclusion of biochar and for a range of
sequestration rates from 0 to 3 Pg C year
−1
.
Table S1. Summary of global soil C sequestration potential (Pg C year
−1
) by agricultural land
management.
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Acknowledgments: We thank P. Smith for advice, support, and help with data
interpretation. We also thank B. Collins for supporting the conception of the project, I. Fung
for her challenge and friendly review, and J. Wick for encouragement. Funding: We thank the
Rathmann Family Foundation for funding this project. This work was supported in part by the
U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research
under contract DE-AC02-05CH11231. Author contributions: A.M. contributed to conceptual
development, conducted literature review, made figures, and led the writing effort. Z.H.
contributed to conceptual development, updated and ran the carbon-climate model, made
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figures, and contributed to writing. A.D.J. and W.L.S. guided conceptual development and
contributed to writing. W.L.S. conceived the project. Competing interests: The authors declare
that they have no competing interests. Data and materials availability: The simple CMIP
emulator model used in this analysis along with the underlying RCP scenario emissions
are available on GitHub (https://github.com/hausfath/SimMod). 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 29 September 2017
Accepted 23 July 2018
Published 29 August 2018
10.1126/sciadv.aaq0932
Citation: A. Mayer, Z. Hausfather, A. D. Jones, W. L. Silver, The potential of agricultural land
management to contribute to lower global surface temperatures. Sci. Adv. 4, eaaq0932 (2018).
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temperatures
The potential of agricultural land management to contribute to lower global surface
Allegra Mayer, Zeke Hausfather, Andrew D. Jones and Whendee L. Silver
DOI: 10.1126/sciadv.aaq0932
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