Climate change in the Fertile Crescent and implications
of the recent Syrian drought
Colin P. Kelley
, Shahrzad Mohtadi
, Mark A. Cane
, Richard Seager
, and Yochanan Kushnir
University of California, Santa Barbara, CA 93106;
School of International and Public Affairs, Columbia University, New York, NY 10027; and
Earth Observatory, Columbia University, Palisades, NY 10964
Edited by Brian John Hoskins, Imperial College London, London, United Kingdom, and approved January 30, 2015 (received for review November 16, 2014)
Before the Syrian uprising that began in 2011, the greater Fertile
Crescent experienced the most severe drought in the instrumental
record. For Syria, a country marked by poor governance and un-
sustainable agricultural and environmental policies, the drought
had a catalytic effect, contributing to political unrest. We show
that the recent decrease in Syrian precipitation is a combination of
natural variability and a long-term drying trend, and the unusual
severity of the observed drought is here shown to be highly unlikely
without this trend. Precipitation changes in Syria are linked to rising
mean sea-level pressure in the Eastern Mediterranean, which also
shows a long-term trend. There has been also a long-term warming
trend in the Eastern Mediterranean, adding to the drawdown of soil
moisture. No natural cause is apparent for these trends, whereas
the observed drying and warming are consistent with model studies
of the response to increases in greenhouse gases. Furthermore,
model studies show an increasingly drier and hotter future mean
climate for the Eastern Mediterranean. Analyses of observations and
model simulations indicate that a drought of the severity and
duration of the recent Syrian drought, which is implicated in the
current conflict, has become more than twice as likely as
a consequence of human interference in the climate system.
Beginning in the winter of 2006/2007, Syria and the greater
Fertile Crescent (FC), where agriculture and animal herding
began some 12,000 years ago (1), experienced the worst 3-year
drought in the instrumental record (2). The drought exacerbated
existing water and agricultural insecurity and caused massive
agricultural failures and livestock mortality. The most significant
consequence was the migration of as many as 1.5 million
people from rural farming areas to the peripheries of urban
centers (3, 4). Characterizing risk as the product of vulnerability
and hazard severity, we first analyze Syria’s vulnerability to
drought and the social impacts of the recent drought leading to
the onset of the Syrian civil war. We then use observations and
climate models to assess how unusual the drought was within the
observed record and the reasons it was so severe. We also show
that climate models simulate a long-term drying trend for the
region as a consequence of human-induced climate change. If
correct, this has increased the severity and frequency of occur-
rence of extreme multiyear droughts such as the recent one. We
also present evidence that the circulation anomalies associated
with the recent drought are consistent with model projections of
human-induced climate change and aridification in the region
and are less consistent with patterns of natural variability.
Heightened Vulnerability and the Effects of the Drought
Government agricultural policy is prominent among the many
factors that shaped Syria’svulnerabilitytodrought.Despitegrowing
water scarcity and frequent droughts, the government of President
Hafez al-Assad (1971−2000) initiated policies to further increase
agricultural production, including land redistribution and irrigation
projects, quota systems, and subsidies for diesel fuel to garner
the support of rural constituents (5–9). These policies endangered
without regard for sustainability (10).
One critical consequence of these unsustainable policies is the
decline of groundwater. Nearly all rainfall in the FC occurs during
the 6-month winter season, November through April, and this
rainfall exhibits large year-to-year variability (Figs. 1Aand 2A). In
Syria, the rain falls along the country’s Mediterranean Sea coast
and in the north and northeast, the primary agricultural region.
Farmers depend strongly on year-to-year rainfall, as two thirds of
the cultivated land in Syria is rain fed, but the remainder relies
upon irrigation and groundwater (11). For those farms without
access to irrigation canals linked to river tributaries, pumped
groundwater supplies over half (60%) of all water used for irri-
gation purposes, and this groundwater has become increasingly
limited as extraction has been greatly overexploited (4). The
government attempted to stem the rate of groundwater depletion
by enacting a law in 2005 requiring a license to dig wells, but the
legislation was not enforced (6). Overuse of groundwater has
been blamed for the recent drying of the Khabur River in Syria’s
northeast (6). The depletion of groundwater during the recent
drought is clearly evident from remotely sensed data by the
NASA Gravity Recovery and Climate Experiment (GRACE)
Tellus project (Fig. 2C) (12).
The reduced supply of groundwater dramatically increased
Syria’s vulnerability to drought. When a severe drought began in
2006/2007, the agricultural system in the northeastern “bread-
basket”region, which typically produced over two-thirds of the
country’s crop yields, collapsed (13). In 2003, before the
drought’s onset, agriculture accounted for 25% of Syrian gross
domestic product. In 2008, after the driest winter in Syria’s ob-
served record, wheat production failed and the agricultural share
fell to 17% (14). Small- and medium-scale farmers and herders
There is evidence that the 2007−2010 drought contributed to
the conflict in Syria. It was the worst drought in the in-
strumental record, causing widespread crop failure and a mass
migration of farming families to urban centers. Century-long
observed trends in precipitation, temperature, and sea-level
pressure, supported by climate model results, strongly suggest
that anthropogenic forcing has increased the probability of se-
vere and persistent droughts in this region, and made the oc-
currence of a 3-year drought as severe as that of 2007−2010
2to3timesmorelikelythanbynatural variability alone. We
conclude that human influences on the climate system are
implicated in the current Syrian conflict.
Author contributions: C.P.K., S.M., M.A.C., R.S., and Y.K. designed research; C.P.K. per-
formed research; C.P.K., S.M., M.A.C., R.S., and Y.K. analyzed data; and C.P.K., S.M., M.A.C.,
R.S., and Y.K. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
To whom correspondence should be addressed. Email: email@example.com.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.
www.pnas.org/cgi/doi/10.1073/pnas.1421533112 PNAS Early Edition
AND PLANETARY SCIENCES
suffered from zero or near-zero production, and nearly all of their
livestock herds were lost (15). For the first time since self-suffi-
ciency in wheat was declared in the mid-1990s, Syria was forced to
import large quantities of wheat (13). The drought’s devastating
impact on vegetation is clearly evident in Moderate Resolution
Imaging Spectroradiometer (MODIS) Normalized Difference
Vegetative Index (NDVI) version 5 satellite imagery (Fig. 2D)
(16). Atieh El Hindi, the director of the Syrian National Agri-
cultural Policy Center, has stated that between 2007 and 2008,
drought was a main factor in the unprecedented rise in Syrian food
prices; in this single year, wheat, rice, and feed prices more than
doubled (17, 18). By February of 2010, the price of livestock feed
had increased by three fourths, and the drought nearly obliterated
all herds (16, 19). There was a dramatic increase in nutrition-
related diseases among children in the northeast provinces
(20), and enrollment in schools dropped by as much as 80% as
many families left the region (21). Bashar al-Assad, who suc-
ceeded his father in 2000, shiftedtoliberalizingtheeconomyby
cutting the fuel and food subsidies on which many Syrians had
become dependent. These cuts continued despite the drought,
further destabilizing the lives of those affected (22). Rural
Syria’s heavy year-to-year reliance on agricultural production
left it unable to outlast a severe prolonged drought, and a mass
migration of rural farming families to urban areas ensued.
Estimates of the number of people internally displaced by the
drought are as high as 1.5 million (3, 4, 13). Most migrated to the
peripheries of Syria’s cities, already burdened by strong pop-
ulation growth (∼2.5% per year) and the influx of an estimated
1.2–1.5 million Iraqi refugees between 2003 and 2007, many of
whom arrived toward the tail end of this time frame at the begin-
ning of the drought and remained inSyria(23).By2010,internally
displaced persons (IDPs) and Iraqi refugees made up roughly 20%
2002 was 8.9 million but, by the end of 2010, had grown to 13.8
million, a more than 50% increase in only 8 years, a far greater
rate than for the Syrian population as a whole (Fig. 1D) (24). The
population shock to Syria’s urban areas further increased the
strain on its resources (11).
The rapidly growing urban peripheries of Syria, marked by
illegal settlements, overcrowding, poor infrastructure, unemploy-
ment, and crime, were neglected by the Assad government and
became the heart of the developing unrest (13). Thus, the mi-
gration in response to the severe and prolonged drought exacer-
bated a number of the factors often cited as contributing to the
unrest, which include unemployment, corruption, and rampant
inequality (23). The conflict literature supports the idea that rapid
demographic change encourages instability (25–27). Whether it
was a primary or substantial factor is impossible to know, but
drought can lead to devastating consequences when coupled with
preexisting acute vulnerability, caused by poor policies and un-
sustainable land use practices in Syria’s case and perpetuated by
the slow and ineffective response of the Assad regime (13). Fig. S1
presents a timeline summarizing the events that preceded the
Fig. 1. (A) Six-month winter (November−April mean) Syria area mean precipitation, using CRU3.1 gridded data. (B) CRU annual near-surface temperature (red
shading indicates recent persistence above the long-term normal). (C) Annual self-calibrating Palmer Drought Severity Index. (D) Syrian total midyear pop-
ulation. Based on the area mean of the FC as defined by the domain 30.5°N–41.5°N, 32.5°E–50.5°E (as shown in Fig. 2). Linear least-squares fits from 1931 to
2008 are shown in red, time means are shown as dashed lines, gray shading denotes low station density, and brown shading indicates multiyear (≥3) droughts.
www.pnas.org/cgi/doi/10.1073/pnas.1421533112 Kelley et al.
The Drought in Context
Having established Syria’s vulnerability to droughts, we now ex-
amine the 2007–2010 drought itself. The severity and persistence
of the drought can be seen in the area mean of FC rainfall
according to the University of East Anglia Climatic Research
Unit (UEA CRU) data (Fig. 1A) and in the two Global Historical
Climatology Network (GHCN) stations located closest to Syria’s
northeastern agricultural region, Deir ez-Zor on the Euphrates
River and Kamishli near the Turkish border (Materials and
Methods). The 2007/2008 winter was easily the driest in the ob-
served records. Multiyear drought episodes, here defined as three
or more consecutive years of rainfall below the century-long
normal, occurred periodically over the last 80 years (CRU), in the
late 1950s, 1980s, and 1990s (Fig. 1A, brown shading). Although
less severe, these droughts raise the question of why the effects of
the recent drought were so much more dramatic. We offer three
reasons: (i) the recent demand for available resources was dis-
proportionately larger than in the 1950s; in addition to the recent
emphasis on agricultural production, the total population of
Syria (Fig. 1D) grew from 4 million in the 1950s to 22 million in
recent years; (ii) the decline in the supply of groundwater has
depleted the buffer against years with low rainfall; and (iii) the
recent drought occurred shortly after the 1990s drought, which
was also severe; Syria was far more vulnerable to a severe drought
in the first decade of the 21st century than in the 1950s, and the FC
never fully recovered from the late 1990s drought before collapsing
again into severe drought. In fact, the region has been in moderate
to severe drought from 1998 through 2009, with 7 of 11 years re-
ceiving rainfall below the 1901–2008 normal. It is notable that three
of the four most severe multiyear droughts have occurred in the last
25 years, the period during which external anthropogenic forcing
has seen its largest increase.
Regional Climate Variability and Trend
Agriculture in Syria depends not only on the precipitation that
falls within Syria and on local groundwater but also on water
from the Euphrates and Tigris rivers and their numerous tributaries.
These rivers have long provided water to the region via precipitation
in their headwaters in the mountains of eastern Turkey. Despite
through its upstream placements of dams, Syria and Turkey have
cooperated in recent years, and Turkey increased water flow to
Syria during the recent drought (28). It has been previously
shown that natural winter-to-winter rainfall variability in western
Turkey is due largely to the influence of the North Atlantic
Oscillation (NAO) (29). For eastern Turkey and in Syria and
the other FC countries, however, the NAO influence is weak
or insignificant. This has allowed observational analyses to identify
an externally forced winter drying trend over the latter half of
the 20th century that is distinguishable from natural variability
(30–32). Furthermore, global coupled climate models over-
whelmingly agree that this region will become drier in the future
as greenhouse gas concentrations rise (33), and a study using
a high-resolution model able to resolve the complex orography of
the region concluded that the FC, as such, is likely to disappear by
the end of the 21st century as a result of anthropogenic climate
That the neighboring regions of southeast Turkey and northern
Iraq also experienced recent drought, to a lesser extent, perhaps
begs the question as to why the effects in Syria were so grave.
Syria was far more vulnerable to drought, given its stronger de-
pendence on year-to-year rainfall and declining groundwater for
agriculture. Water scarcity in Syria has been far more severe than
in Turkey or Iraq, with Syria’s total annual water withdrawal as a
percentage of internal renewable water resources reaching 160%,
with Iraq at 80% and Turkey at around 20% in 2011 (35). Fur-
thermore, Turkey’s geographic diversity and investment in the
southeast region’s irrigation allowed it to better buffer the drought,
whereas the populace in northwest Iraq is far less dependent on
agriculture than their counterparts in northeast Syria (36, 37).
To address the question of whether the recent drought was
made more severe by a contribution from long-term trends, we
first determined the long-term change in winter rainfall. The FC
as a whole has experienced a statistically significant (P<0.05)
winter rainfall reduction (13%) since 1931 (Fig. 1A). Observa-
tional uncertainty was large before 1930 due to sparseness of
station data. Further examination of the linear trends present in
the individual GHCN stations for the FC corroborate the drying
trend, as 5 of 25 stations exhibited a statistically significant (P<
0.1) negative rainfall trend (Fig. 2B). The pattern of this trend
(Fig. 2B) is similar to the climatological rainfall pattern (Fig. 2A),
concentrated along the coast and in northeastern Syria. The long-
term drying trend is closely mirrored by recent changes in satellite
measurements of groundwater (measured in terms of liquid water
equivalent) (Fig. 2C) and, to a lesser extent, by estimates of veg-
etation changes (Fig. 2D).
The annual surface temperature in the FC also increased sig-
nificantly (P<0.01) during the 20th century (Fig. 1B). The warming
in this region since 1901 has outpaced the increase in global
mean surface temperature, with much of this increase occurring
over the last 20 years (all years from 1994 through 2009 were
above the century-long mean) (Fig. 1B, red shading). The trend
during the summer half year (1.2 degrees, Fig. S2) is also impor-
tant, as this is the season of highest evaporation, and winter crops
such as wheat are strongly dependent on reserves of soil moisture.
Reductions in winter precipitation and increases in summer
evaporation both reduce the excess of precipitation over evapo-
ration that sustains soil moisture, groundwater and streamflow.
The recent strong warming is concomitant with the three most
recent severe multiyear droughts, together serving to strongly dry
the region during winter and summer.
The century-long, statistically significant trends in both pre-
cipitation and temperature seen in Fig. 1 suggest anthropogenic
influence and contributed to the severity of the recent drought.
The FC area mean of the self-calibrating Palmer Drought Severity
Fig. 2. (A)Observedwinter(November−April) precipitation climatology,
1931–2008, UEA CRU version 3.1 data. (B) The spatial pattern of the CRU
change in 6-month winter precipitation from 1931 to 2008 based on
a linear fit (shading); those GHCN stations that indicate a significant (P<0.1)
trend over their respective records are shown as circles and crosses (in-
dicating drying/wetting). (C) The difference in liquid water equivalent (LWE)
between 2008 (annual) and the mean of the previous 6 years using the NASA
GRACE Tellus project data. (D)ThedifferenceintheNormalizedDifference
Vegetation Index (NDVI) between 2008 (annual) and the mean of the previous
Kelley et al. PNAS Early Edition
AND PLANETARY SCIENCES
Index (38), which combines precipitation and temperature as a
proxy for cumulative soil moisture change, also exhibits a signifi-
cant long-term trend (Fig. 1C). Although natural variability on
timescales of centuries or longer cannot be entirely ruled out
for this region, the long-term observed trends and the recent in-
crease in the occurrence of multiyear droughts and in surface
temperature is consistent with the time history of anthropogenic
climate forcing. The case for this influence is supported by
additional modeling and theoretical and observational evi-
dence (see Frequency of Multiyear Droughts,Mechanisms,and
Frequency of Multiyear Droughts
For Syria and for the greater FC, natural multiyear droughts—
here defined as three or more consecutive years of rainfall below
the long-term normal—occurred periodically during the 20th
century (Fig. 1A). It is a generic property of a time series con-
sisting of a natural oscillatory part and a downward trend that
the minimum is most likely to occur toward the end of the time
period when the negative influence of the trend is greatest and
when the oscillation is also at a minimum. The century-long
trends in precipitation and temperature, here implicated as evi-
dence of anthropogenic influence, point toward them being key
contributors to the recent severe drought. We therefore esti-
mated the increased likelihood of an extreme 3-year drought
such as the recent one due to anthropogenic trend.
We did this in two ways. First we separated the observed an-
thropogenic precipitation trend from the residual, presumably
natural, variability by regressing the running 3-year mean of ob-
served (CRU) 6-month winter precipitation onto the running
3-year mean of observed annual global atmospheric carbon dioxide
) mixing ratios from 1901–2008 (39, 40). The latter time
series was used as an estimate of the monotonic but nonlinear
change in total greenhouse gas forcing (Materials and Methods).
After removing the CO
fit from the total observed winter pre-
cipitation timeseries (Fig. 3A), we constructed frequency dis-
tributions of the total and residual timeseries (Fig. 3B) and
applied gamma fits to the distributions. The difference in the
total and residual distributions is significant (P<0.06), based on
a Kolmogorov−Smirnoff test, and is due almost entirely to the
difference in the means. Thresholds are shown at 10%, 5%, and
2% (in percent of the total sample size of 76 3-year means) in the
dry tail for the timeseries (Fig. 3A) and for the distribution of the
total (Fig. 3B). The result is that, when combined, natural vari-
ability and CO
forcing are 2 to 3 times more likely to produce
the most severe 3-year droughts than natural variability alone.
Residual, or natural, events exceeding the 10% threshold of the
total occur less than half as often (3 versus 8, out of 76). For the
residual alone, no values exceed the 5% threshold of the total.
The trend contribution would be quite similar if we simply
calculated a linear time trend. There is no apparent natural ex-
planation for the trend, supporting the attribution to anthropo-
genic greenhouse gases. Further support comes from model
simulations. We used 16 Coupled Model Intercomparison Proj-
ect phase five (CMIP5) models (Materials and Methods and
Table S1) to construct similar distributions, providing a larger
sample size than for the observed 3-year droughts. In this case,
rather than removing the CO
forcing as in the observed case, we
compare the historical and historicalNat runs. The former in-
clude all external forcings during the 20th century, including the
change in greenhouse gas concentrations, whereas the latter in-
clude only the natural forcings (Materials and Methods). In this
analysis, the models were normalized to the observed CRU mean
and standard deviation (SD) (see Fig. S3 for model comparison
before normalizing). The resulting distributions support the ob-
served finding, as the driest 3-year events occur less than half as
often under natural forcing (historicalNat runs) alone (Fig. 3C).
The agreement between the model and observational analysis
results supports the attribution of the century-long negative trend
in precipitation to the rise in anthropogenic greenhouse forcing
and to the role of the latter in the devastating early 21st century
We examine the low-level (850 hPa) regional atmospheric circu-
lation by comparing a composite of driest minus wettest winters
(Fig. 4B) to the difference between the periods 1989–2008 and
1931–1950, representing the long-term change, or trend (Fig. 4C).
Climatologically, the flow is from the west, bringing moist air
(shading represents specific humidity) in from the Mediterra-
nean Sea and allowing moisture convergence that sustains pre-
cipitation (Fig. 4A). In both the composite dry anomalies and the
trend, the climatological westerly flow is weakened. In both
cases, there is a positive geopotential height anomaly over the
Mediterranean Sea (consistent with higher surface pressure) and
an anomalous anticyclonic (clockwise) circulation (arrows). In
the composite case, this anomaly extends over Turkey and be-
yond the eastern Black Sea, resulting in anomalous northeasterly
flow over the FC, advecting dry air and generating anomalous
moisture divergence. In the trend case, by contrast, the positive
geopotential height anomaly does not extend over most of
Turkey, and the flow anomaly is more northerly over most of the
FC. This difference between the composite and trend anomalies
can be seen in the specific humidity anomalies (Fig. 4 Band C,
1930 1950 1970 1990 2010
2, 5, 10% quantiles
30 35 40 45 50 55 60 65
residual (CO2 removed)
total (including CO2)
CRU observed clim. 1931−2008
2, 5 and 10% quantiles (of total)
30 35 40 45 50 55 60 65
histNat (all runs)
hist (all runs)
CRU observed clim. 1931−2004
2, 5 and 10% quantiles (hist−obs)
Three−year running means of Fertile Crescent precipitation
(six−month winters, Nov−Apr)
Fig. 3. (A) Timeseries of observed (CRU) 3-year running mean 6-month
winter FC (area mean) precipitation: total (red), CO
fit from regression
(black), and the residual or difference between these (dashed blue). Fre-
quency distributions based on gamma fits of 3-year running mean 6-month
winter FC (area mean) precipitation, for the (B)observeddata(corre-
sponding with above) and (C) CMIP5 model simulations, comparing histori-
cal and histNat runs. Quantile thresholds based on the total (in B) and
historical (in C) are shown at 2%, 5%, and 10% (dotted lines). The tables
indicate the percentage of actual (B) observed (sample size 76) and (C)
model simulated (sample size 46 ×72 for histNat and 69 ×72 for historical)
occurrences exceeding the respective thresholds.
www.pnas.org/cgi/doi/10.1073/pnas.1421533112 Kelley et al.
shading); in the composite, the center of the anomaly is located
over the FC and southeastern Turkey and northern Syria, Iraq,
and Iran, whereas in the trend case, it is centered over western
Turkey. Thus, the trend in the circulation enhances drying in
naturally occurring FC dry years by strengthening the northerly
flow, dry air advection, and moisture divergence anomalies. In
2005–2008, the long-term trend combined with a dry phase of
natural variability to produce the most severe drought in the
instrumental record over the greater FC.
We have here pointed to a connected path running from human
interference with climate to severe drought to agricultural collapse
and mass human migration. This path runs through a landscape of
vulnerability to drought that encompasses government policies
promoting unsustainable agricultural practices, and the failure of
the government to address the suffering of a displaced population.
Our thesis that drought contributed to the conflict in Syria draws
support from recent literature establishing a statistical link between
climate and conflict (25–27). We believe that the technical
challenges to this work (41) have been adequately answered
(42, 43). A more fundamental objection (27) is that data-driven
methods do not provide the causal narrative needed to anoint a
“theory”of civil conflict, and the quantitative work on climate
and conflict has thus far not adequately accounted for the effects
of poor governance, poverty, and other sociopolitical factors.
Our analysis of the conflict in Syria shows an impact of an extreme
climate event in the context of government failure, exacerbated by
the singular circumstance of the large influx of Iraqi refugees.
Multiyear droughts occur periodically in the FC due to natural
causes, but it is unlikely that the recent drought would have been
as extreme absent the century-long drying trend. We argued,
with support from analyses of observations and climate model
simulations, that the observed long-term trends in precipitation
and temperature are a consequence of human interference with
the climate system. The attribution to anthropogenic causes is also
supported by climate theory and previous studies (see Supporting
Information). Fortunately for this line of argument, this is a region
where models compare reasonably well with 20th century obser-
vations in terms of simulation of the climatology of precipitation
and its trend (44). The strong agreement between observations
and climate model simulations in century-long trends in pre-
cipitation, temperature, and sea-level pressure (Fig. S4) adds
confidence to the conclusion that in this region, the anthropogenic
precipitation signal has already begun to emerge from the natural
“noise”and that the recent drought had a significant anthropo-
genic component. It also implies that model future projections of
continued drying for Syria and the FC are reliable.
An abundance of history books on the subject tell us that civil
unrest can never be said to have a simple or unique cause. The
Syrian conflict, now civil war, is no exception. Still, in a recent
interview (45), a displaced Syrian farmer was asked if this was
about the drought, and she replied, “Of course. The drought and
unemployment were important in pushing people toward revo-
lution. When the drought happened, we could handle it for two
years, and then we said, ‘It’s enough.’” This recent drought was
likely made worse by human-induced climate change, and such
persistent, deep droughts are projected to become more com-
monplace in a warming world.
Materials and Methods
In this study, the winter and summer seasons are represented by the 6-month
periods November through April and May through October, respectively.
Timeseries of FC area means are here defined by the domain 30.5°N−41.5°N,
32.5°E−50.5°E (as shown in Fig. 2). Three datasets were used for observed
precipitation: the UEACRU version 3.1 (46, 47) and Global Precipitation Cli-
matology Centre v6 (48) gridded (0.5° by 0.5° horizontal resolution) pre-
cipitation data sets and the GHCN beta version 2 station precipitation data
(49). CRU v3.1 was also used for observed surface temperature. We used 16
CMIP5 global climate models (Table S1) assessed in the Intergovernmental
Panel on Climate Change Fifth Assessment Report. For the 20th century, we
compare the “historical”using all forcings and “historicalNat”simulations
including natural forcings only. To compare with 20th century observations,
Fig. 4. The 6-month winter low-level (850 hPa) horizontal winds (arrows)
and specific humidity (shading) for the period 1931–2008. Shown are the (A)
climatology, (B) composite difference between driest and wettest years (those
outside of ±1SD)and(C) the change, or difference between the recent 20
years and the 20 years at the beginning of the period.
Kelley et al. PNAS Early Edition
AND PLANETARY SCIENCES
we first linearly interpolated the models to the same 0.5° by 0.5° horizontal
grid as the CRU observations. To determine the change due to trend, we
applied linear least-squares fits, except in the case of the estimation of mul-
tiyear droughts, when regression onto global CO
mixing ratios was used. For
the latter, this nonlinear detrending provided a more conservative estimate of
the residual than linear detrending. We also examined the sensitivity of using
global mean surface temperature rather than CO
and found almost no dif-
ference in the resulting residual. For analysis of the regional circulation, we
used the Twentieth Century Reanalysis Project, with a horizontal resolution of
2° by 2° (50). For composites, dry and wet years are here defined as those
outside of ±1 SD (based on the CRU 1931–2008 period).
ACKNOWLEDGMENTS. We thank Dr. Dipali Mukhopadhyay for her gener-
ous assistance and outstanding advice, as well as Yuma Shinohara for his
assistance in creating Fig. S1.ThanksalsogotoDr.YotamMargalitand
to Lina Eklund for feedback and advice, and to Dr. Tahsin Tonkaz for field-
work support. We thank the Global Decadal Hydroclimate group at the
Lamont–Doherty Earth Observatory and the Bulletin of Atomic Scientists
for their support and funding. The authors were supported by the follow-
ing grants: Office of Naval Research Award N00014-12-1-0911, National
Oceanic and Atmospheric Administration Award NA10OAR4310137
(Global Decadal Hydroclimate Predictability, Variability and Change),
and Department of Energy Award SC0005107. This is Lamont–Doherty
Earth Observatory Contribution 7871.
1. Salamini F, Ozkan H, Brandolini A, Schäfer-Pregl R, Martin W (2002) Genetics and
geography of wild cereal domestication in the near east. Nat Rev Genet 3(6):429–441.
2. Trigo RM, Gouveia CM, Barriopedro D (2010) The intense 2007–2009 drought in the
Fertile Crescent: Impacts and associated atmospheric circulation. Agric Meteorol
3. Integ rated Regional Information Networks (November 24, 2009) Syria: Drought re-
sponse faces funding shortfall. IRIN. Available at irinnews. org/report/87165/syria-
drought-response-faces-funding-shortfall. Acces sed May 1, 2014.
4. Solh M (September 27, 2010) Tackling the drought in Syria. Nature Middle East.
Available at natureasia.com/en/nmiddleeast/article/10.1038/nmiddleeast.2010.206.
Accessed November 17, 2012.
5. Nguyen H (1989) Agricultural planning policy and variability in Syrian cereal pro-
duction. Variability in Grain Yields, eds Anderson JR, Hazell PBR (Int Food Policy Res
Inst, Washington, DC).
6. Rodriguez A, Salahieh H, Badwan R, Khawam H (1999) Groundwater Use and Sup-
plemental Irrigation in Atareb, Northwest Syria (Int Cent Agric Res Dry Areas, Aleppo,
Syria), Soc Sci Pap 7.
7. Salman M, Mualla W (2008) Water demand management in Syria: Centralized and
decentralized views. Water Policy 10(6):549–565.
8. Barnes J (2009) Managing the waters of Baath country: The politics of water scarcity in
Syria. Geopolitics 14(3):510–530.
9. Hinnebusch R (2012) Syria: From ‘authoritarian upgrading’to revolution? Int Aff
10. de Châtel F (January 2010) Mining the deep. Syria Today,pp48−51.
11. Erian W (2011) Dro ught Vulnerability in th e Arab Region (UN Off Disas ter Risk Re-
duction, Geneva) . Available at reliefweb. int/sites/reliefweb.i nt/files/resources/ Full_
Report_3074.pd f. Accessed March 1, 2014.
12. Joodaki G, Wahr J, Swenson S (2014) Estimating the human contribution to ground-
water depletion in the Middle East, from GRACE data, land surface models, and well
observations. Water Resour Res 50(3):2679–2692.
13. Massoud A (2010) Years of Drought: A Report on the Effects of Drought on the Syrian
Peninsula (Heinrich Böll Stiftung, Berlin). Available at lb.boell.org/downloads/
Drought_in_Syria_En.pdf. Accessed August 1, 2014.
14. US Department of Agriculture Foreign Agricultural Service (2014) Production, Supply
and Distribution Online (US Dep Agric, Washington, DC). Available at fas.usda.gov/
psdonline/psdQuery.aspx. Accessed August 1, 2014.
15. International Federation of Red Cross and Red Crescent Societies (2009) Syria:
Drought Emergency Appeal No. MDRSY001 Operations Update No. 1 (IFRC, Geneva).
Available at reliefweb.int/report/syrian-arab-republic/syria-drought-emergency-appeal-
no-mdrsy001-operations-update-no-1. Accessed January 10, 2014.
16. Huete A, et al. (2002) Overview of the radiometric and biophysical performance of
the MODIS vegetation indices. Remote Sens Environ 83(1-2):195–213.
17. Nehme N (2008) The Contribution of Agriculture to the Process of Economic Reforms
in Syria (Natl Agric Policy Cent, Damascus, Syria).
18. Integrated Regional Information Networks (22 February, 2009) Syria: Drought blamed
for food scarcity. IRIN. Available at irinnews.org/report/83069/syria-drought-blamed-
for-food-scarcity. Accessed August 5, 2014.
19. Integrated Regional Information Networks (22 February, 2009) Syria: Overa million people
affected by drought. IRIN. Available at irinnews.org/fr/report/88139/syria-over-a-million-
people-affected-by-drought. Accessed August 5, 2014.
20. United Nations Office for the Coordination of Humanitarian Affairs (2008) Syria
DroughtAppeal (UN Off Coord Humanitarian Affairs, New York). Available at reliefweb.
september-2008. Accessed August 5, 2014.
21. De Schutter O (2010) UN Sp ecial Rapporteur on the Right to Food: Mission to Sy ria
from 29 August to 7 Sep tember 2010 (UN Off Coord Humanitar ian Affairs, New
York). Available at www.ta ndfonline.com/doi/abs/10 .1080/00263206.2013.850076# .
VNl_tFPF8kQ. Accesse d August 5, 2014.
22. de Chatel F (2014) The role of drought and climate change in the Syrian uprising:
Untangling the triggers of the revolution. Middle East Stud 50(4):521–535.
23. United Nations High Commissions for Refugees (2010) Iraqi Refugees in Syria Reluctant
to Return to Home Permanently: Survey (UN High Comm Refugees, Geneva). Available
at unhcr.org/4caf376c6.html. Accessed March 1, 2014.
24. US Census Bureau (2014) International Database (US Census Bur, Washington, DC).
Available at census.gov/population/international/data/idb/informationGateway.php.
Accessed July 15, 2014.
25. Goldstone J (2002) Population and security: How demographic change can lead to
violent conflict. J Int Aff 56(1):3–22.
26. Hsiang SM, Burke M, Miguel E (2013) Quantifying the influence of climate on human
conflict. Science 341(6151):1235367.
27. Solow AR (2013) Global warming: A call for peace on climate and conflict. Nature
28. Kibaroglu A, Scheumann W (2011) Euphrates-Tigris rivers system: Political rapproche-
ment and transboundary water cooperation. Turkey’s Water Policy (Springer,
Berlin), pp 277−299.
29. Cullen HM, deMenocal PB (2000) North Atlantic influence on Tigris–Euphrates stream-
flow. Int J Climatol 20(8):853–863.
30. Kelley C, Ting M, Seager R, Kushnir Y (2012) The relative contributions of radiative
forcing and internal climate variability to the late 20th century winter drying of the
Mediterranean region. Clim Dyn 38(9-10):2001–2015.
31. Kelley C, Ting M, Seager R, Kushnir Y (2012) Mediterranean precipitation climatology,
seasonal cycle, and trend as simulated by CMIP5. Geophys Res Lett 39(29):L21703.
32. Hoerling MP, et al. (2012) On the increased frequency of Mediterranean drought.
J Clim 25(6):2146–2161.
33. Intergovernmental Panel on Climate Change (IPCC) (2007) Climate Change 2007: The
Physical Science Basis. Contribution of Working Group I to the Fourth Assessment
Report of the Intergovernmental Panel on Climate Change,edsSolomonS,etal.
(Cambridge Univ Press, Cambridge, UK).
34. Kitoh A, Yatagai A, Alpert P (2008) First super-high-resolution model projection that
the ancient “Fertile Crescent”will disappear in this century. Hydrol Res Lett 2:1–4.
35. Breisinger C, Ecker O, Al-Riffai P, Yu B (2012) Beyond the Arab Awakening: Policies
and Investments for Poverty Reeducation and Food Security (Int Food Policy Res Inst,
36. US Department of Agriculture (2008) Commodity Intelligence Report: Middle East:
Deficient Rainfall Threatens 2009/10 Wheat Production Prospects (US Dep Agric,
37. Eklund L, Pilesjö P (2012) Migration patterns in Duhok Governate, Iraq, 2000-2010.
Open Geogr J 5:48–58.
38. Sousa PN, et al. (2011) Trends and extremes of drought indices throughout the 20th
century in the Mediterranean. Nat Hazards Earth Syst Sci 11:33–51.
39. Etheridge DM, et al. (1996) Natural and anthropogenic changes in atmospheric
over the last 1000 years from air in Antarctic ice and firn. JGeophysRes 101:
40. Keeling CD, et al. (2001) . Exchanges of Atmospheric CO
with the Ter-
restrial Biosphere and Oceans from 1978 to 2000. I. Global Aspects.SIOReference
Series (Scripps Inst Oceanogr, San Diego, CA), No 01-06.
41. Buhaug H, et al. (2014) One effect to rule them all? A comment on climate and
conflict. Clim Change 127(3-4):391–397.
42. Hsiang SM, Meng KC (2014) Reconciling disagreement over climate-conflict results in
Africa. Proc Natl Acad Sci USA 111(6):2100–2103.
43. Cane MA, et al. (2014) Temperature and violence. Nat Clim Change 4:234–235.
44. Seager R, et al. (2014) Causes of increasing aridification of the Mediterranean region
in response to rising greenhouse gases. J Clim 27:4655–4676.
45. Friedman T (May 18 2013) Without water, revolution. New York Times.Availableat
nytimes. com/2013/05/1 9/opinion/su nday/friedma n-without-w ater-revoluti on.html?
pagewanted=all&_r=0. Accessed May 1, 2014.
46. New M, Hulme M, Jones PD (2000) Representing twentieth century space-time climate
variability. Part 2: Development of 1901–96 monthly grids of terrestrial surface cli-
mate. J Clim 13:2217–2238.
47. Jones P, Harris I (2008) CRU Time Series (TS) High Resolution Gridded Datasets (Clim
Res Unit, Univ East Anglia, Norwich, UK). Available at iridl.ldeo.columbia.edu/expert/
48. Schneider U, Fuchs T, Meyer-Christoffer A, Rudolf B (2008) Global Precipitation
Analysis Products of the GPCC (Dtsch Wetterdienst, Offenbach, Germany).
49. Vose RS, et al. (1992) The Global Historical Climatology Network: Long-Term Monthly
Temperature, Precipitation, Sea Level Pressure, and Station Pressure Data (Oak Ridge
Natl Lab, Oak Ridge, TN) CDIAC-53:NDP-041.
50. Compo GP, et al. (2011) The Twentieth Century Reanalysis Project. QJRMeteorolSoc
www.pnas.org/cgi/doi/10.1073/pnas.1421533112 Kelley et al.