The former Fertile Crescent region of the Middle East—where agriculture and animal herding are believed to have originated some 12,000 years ago—is home to several of the so-called “Arab Spring” countries including Syria, which devolved into a civil war shortly after the most severe multiyear drought (2007-2010) in the observed record, essentially since the beginning of the 20^th century.

In our recent study, published in the Proceedings of the National Academy of Science, we found that these events were strongly linked: The drought had a catalytic effect with dire consequences for Syrians. In addition, there is strong evidence that the drought was connected to long-term and human-induced drying and warming trends that increased the likelihood of climate-linked instability in the region.

In 2005, prior to the drought, Syria’s vulnerability to water and food security and ultimately political upheaval was acute. A dramatically increasing population, an unsustainable emphasis on wheat production, and declining groundwater were prominent among the factors that combined to push the country to the brink of its resilience. Beginning in 2006, as many as 1.5 million refugees fled unrest in Iraq to the cities in Syria’s west. This was the same year that the drought began. In the past it was not unusual for Syria to experience a cycle of a wet year followed by a dry year, etc., but multiyear droughts were much less common.

When this severe drought occurred, a cascade of events ensued. The agricultural system in the northeastern “breadbasket” region collapsed, many farmers uprooted their families and abandoned their villages, and a mass migration of as many as 1.5 million internally displaced people to the major cities in Syria’s west took place. Combined with strong natural population growth and the influx of refugees from the war in Iraq, this represented a shock of a roughly 50% increase in the population of Syria’s cities in only eight years, placing an untenable burden on resources. The rapidly growing urban peripheries of the country—marked by illegal settlements, overcrowding, poor infrastructure, unemployment, and crime—were neglected by the Assad government and became the heart of the developing unrest.  This chain of events combined with Syria’s existing vulnerability to push the country beyond its threshold of resilience and contributed to the uprising in early 2011.

In relation to the events in Syria, our study’s analyses relied on both observed data and output from the global climate models used in the most recent Intergovernmental Panel on Climate Change (IPCC) report to quantify the increased likelihood of severe multiyear droughts under climate change. We determined that the recent Fertile Crescent drought was not only the most severe during the last hundred years, but that there is strong evidence of long-term trends toward a drying and warming climate in the region.

There is very little evidence to suggest that these long-term trends can be explained by natural climate variability. However, they are consistent with the time evolution of increasing greenhouse gases, predominantly carbon dioxide. Additionally, three of the four most severe multiyear droughts in Syria’s observed record occurred during the last 30 years, when the rate of global carbon emissions has seen its largest increase. The global climate models, which perform well in their simulation of climate in this region, indicate that human-induced increases in greenhouse gases have acted to dry and warm this region and will continue to do so. The observed and modeled results were consistent. Severe droughts such as the recent one were two to three times more likely to occur under the effects of climate change than in its absence.

Last, we examined the mechanisms by which dry conditions occur in this region. Under normal conditions winds come from the west bringing moist air from the Mediterranean Sea—a necessary ingredient for rainfall. We found that in naturally occurring dry years and in the long-term trend the winds come more from the north, reducing the availability of moisture.  Essentially, during the recent severe drought, the trend served to greatly exacerbate a natural multiyear drought, making it the most severe in a hundred years of observations.

These results have important implications for the future stability of not only this region, but also any region with high vulnerability and low resilience to conflict. We know the Earth is getting warmer, and that humans, through burning of fossil fuels, deforestation, and other factors, are significantly contributing to this warming. What we are less certain about is how (and how much) the change in the global climate will affect different regions.

Our findings are highly relevant to policy makers as they try to mitigate these future changes and their effects, especially with respect to forced displacement. According to a representative of the United Nations High Commission on Refugees in a recent conference, the current international legal framework has no definition of a “climate-induced refugee,” and creating this definition along with an action plan is of vital importance going forward. About 20 million Bangladeshis alone are at risk of displacement according to some estimates of sea level rise over the next 50 years.

The US and other countries now consider emerging climate change to be a key concern. It was recently referred to by the Department of Defense as a “threat multiplier” that could lead to future instability in parts of the world, impacting national security. The IPCC has also stated that human security will be progressively threatened as the climate changes. Yet the humanitarian system has serious challenges, namely that there is not enough money or government commitments directed at this effort at a time when there are more displaced people than ever before.

The Syrian migration in response to the severe and prolonged drought exacerbated a number of the factors often cited as contributing to unrest, including unemployment, corruption, and rampant inequality. 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 ongoing Syrian conflict is no exception. Nonetheless, the conflict literature supports the idea that rapid demographic change encourages instability. Whether it was a primary or substantial factor in this case is impossible to know, but drought can lead to devastating consequences when coupled with preexisting acute vulnerability. In Syria’s case this was caused by poor policies and unsustainable land use practices and perpetuated by the slow and ineffective response of the current Assad regime.

Colin P. Kelley is a PACE Postdoctoral Fellow at the University of California, Santa Barbara. Shahrzad Mohtadi is a Masters candidate at the School of International and Public Affairs, Columbia University. Mark A. Cane, Richard Seager and Yochanan Kushnir are Professors at the Lamont–Doherty Earth Observatory, Columbia University.