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 unsustainable 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. , 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 occurrence 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.

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