Manmade global warming helped spark the brutal civil war in Syria by doubling to tripling the odds that a crippling drought in the Fertile Crescent would occur shortly before the fighting broke out, according to a groundbreaking new study published on March 2.

The study, published in the journal Proceedings of the National Academy of Sciences, is the first to attribute the drought in Syria in large part to global warming.

In doing so, it provides powerful evidence backing up the Pentagon and intelligence community’s assessments that climate change is likely to play the role of a “threat multiplier” in coming decades, pushing countries that are already vulnerable to upheaval over the edge and into open conflict.

Previous studies had shown that the drought, along with other factors such as an influx of refugees from the Iraq War next door, helped prime Syria for conflict by 2011, when the uprising began, before transitioning into an all-out civil war. Today, once-cosmopolitan Syria has been reduced to rubble, with the terrorist group known as ISIS taking over large swaths of territory.

At least 200,000 people are estimated to have died in this conflict so far.

The drought, which gripped the country between 2007 and 2010, forced 1.5 million farmers and herders in northeastern Syria to flee their lands and travel to urban areas in search of food and work.

This profound demographic shift helped further destabilize the country, the study says.

The study also found that much of the eastern Mediterranean, including Syria, parts of Turkey, Lebanon, Israel and Jordan — no bastions of stability today — may face an even more tenuous security situation in the coming decades as global warming increases temperatures and reduces rainfall throughout much of the region.

According to this study and others, global warming along with unsustainable water use is causing the Fertile Crescent, where agriculture and animal herding first began 12,000 years ago, to lose its fertility.

“This region is going to continue to get drier and continue to get hotter, so this is only a problem that is going to continue to get worse in that region,” says Colin Kelley, the study’s lead author and a postdoctoral researcher at the University of California at Santa Barbara.

Although the drought did not directly cause the war and subsequent rise of ISIS, which the U.S. and its allies are combating using military force, it formed a significant part in the cascading series of events that led to the deadly Syrian conflict.

Government policies that encouraged the unsustainable use of water resources and provided inadequate aid to displaced persons, among other factors, also ratcheted up Syria’s vulnerability to conflict around the time of the drought.

“We would not say and did not even attempt to say that the uprising was caused by climate change,” Kelley says. Rather, the drought was one in a chain of events that led to the breakout of hostilities.

“Syria’s vulnerability was so acute that all it took was something to push them over the edge,” he said in an interview.

Kelley says the drought set in motion a series of events that wound up sparking one of the deadliest conflicts of the 21st century to date. “A lot of these farming communities abandoned their villages and went to the cities at the same time that Iraqi refugees were coming in,” Kelley says.

“There was a very big population shock to these urban areas.”

Urban areas in Syria saw a population jump of 50% between 2002 to 2010, from 8.9 million to 13.8 million, as a result of refugees fleeing fighting in Iraq as well as those who were abandoning their land in northeastern Syria.

“That’s a tremendous increase in people to these urban areas over this period,” Kelley told Mashable. “It’s not at all surprising that the uprising happened shortly after this.”

Richard Seager, a climate scientist at Columbia University’s Lamont-Doherty Laboratory in New York and a co-author of the study, told Mashable that farmers were prepared to cope with a one-to-two-year dry spell, but three years exceeded their ability to cope.

“The length and severity of this drought [that was] made more likely by human climate change was absolutely key in driving the agricultural community toward a threshold where they had no other opportunity but to pick up and leave,” Seager says.

Kelley says the long-term trends favor warmer and drier conditions in the Fertile Crescent, which helps load the dice in favor of more severe and longer-lasting droughts. Such trends, he said, don’t have “a natural explanation,” but rather are consistent with what is expected from global warming.

Data shows that droughts are already changing in the region. For example, the study found that three of the four most severe three-year droughts in Syria since 1901 have occurred in the last 25 years.

There has been a long-term increase in sea level pressure in the eastern Mediterranean, which is consistent with weaker storms and sinking air that stifles incipient storm systems. Also, storm tracks seem to be shifting in the region, too, in response to a global warming-related expansion of air circulation patterns between the tropics and the poles, Kelley says.

The particular air circulation pattern in question is known as the Hadley Cell, and it helps fuel tropical rainstorms just north and south of the equator, with deserts located to the north and south of those areas, where the air is sinking back down. Seager and others say the Hadley Cell is expanding as the world warms, putting parts of the Mediterranean and southern Europe in its dry zone of descending air .

“It does seem there is a shift… toward increasing subsidence that is continually sucking moisture out of that region,” Seager says. “That has all the hallmarks of looking like a Hadley Cell expansion.”

The study provides a comprehensive overview of the factors that led to the Syrian Civil War. In so doing, the authors make clear that it’s not easy to assign a significance to each individual factor, including government policies that led to wildly unsustainable water use.

“It may be that all of this would have occurred without the drought,” Seager says. However, the way it actually played out was clearly related to the drought and the influx of internally displaced people associated with it and the Iraq War next door, he says.

By 2010, the study says, internally displaced persons and Iraqi refugees made up a staggering 20% of the Syrian population. “It was very clear then what was going on,” Seager says. “The social unraveling in 2010-11 was influenced by this drought,” he said.

Scholars who specialize in the links between climate and conflict say that it’s impossible to know whether the drought-related displacement of more than a million Syrians helped spark the uprising and subsequent civil war.

Like other research in this area, the study does not provide the smoking gun pinpointing the precise mechanism through which the uprising influenced the war, instead providing a wealth of what might be referred to in a courtroom as circumstantial evidence, since there is plenty of research suggesting that the influx of poor farmers to urban areas helped destabilize an already fragile country.

So, while the study presents a convincing case that global warming helped influence the severity and duration of the drought, there is greater uncertainty about how solid the link is between the drought and the conflict itself, experts said.

“What we don’t know, just yet, is how exactly that displacement contributed to civil unrest in Syria in 2011. That’s very difficult to tell, as there’s a civil war underway, and it’s almost impossible to investigate the motivations of people in hindsight,” said Francesco Femia and Caitlin Werrell, co-founders of the Center for Climate and Security, in a statement to Mashable.

“Plus, conflict is a messy confluence of multiple factors and grievances.”

Solomon Hsiang, a researcher at the University of California at Berkeley who has published many studies on the relationship between climate and conflict, describes the dilemma differently.

“The central challenge that we still face, analytically, is figuring out what would have happened in Syria had we not caused the climate to change,” said Solomon Hsiang, a researcher at the University of California at Berkeley who has published many studies on the relationship between climate and conflict. “It is very likely that a large drought would have occurred and that this drought would have made conflict more likely,” he said in an email.

The paper, Hsiang says, “shows that the drought was made worse from climate change, so presumably the risk of conflict was also made worse.” He cautioned against attributing the entire conflict “just to the portion of the drought cause by humans,” Hsiang said.

Hsiang noted that drought has likely caused “major upheaval” in the Fertile Crescent before. “Evidence suggests that the Akkadian Empire (spanning Syria and Iraq) very likely collapsed during a multi-year mega-drought,” he said.

Femia and Werrell said the new study is especially significant for its projections that the future will be even hotter and drier in the Fertile Crescent region, which may prevent farmers from ever fully returning to their lands and lead to more political instability.

Seager, too, has his eye on the near future as the Mediterranean climate continues to change. Mentioning Lebanon and Jordan, in particular, as countries that are vulnerable to these warming and drying trends. These countries are also being strained by influxes of refugees, this time from the Syrian civil war, Seager says. This could result in a domino of conflict radiating outward from Syria and Iraq.

“We may not have seen the end of this,” he said.

For years, national security experts have warned that global warming could serve as a destabilizing force that pushes vulnerable regions over the edge and into open conflict. The Pentagon produced a climate change “road map” in 2014 that laid out the case for viewing global warming impacts as a threat multiplier and, in some cases, as an immediate threat.

“In our defense strategy, we refer to climate change as a ‘threat multiplier’ because it has the potential to exacerbate many of the challenges we are dealing with today — from infectious disease to terrorism. We are already beginning to see some of these impacts,” said then-Secretary of Defense Chuck Hagel when the report was released in October. “A changing climate will have real impacts on our military and the way it executes its missions.”

The intelligence community has also warned of the risks that global warming poses, particularly in states that have fragile governments and rapid population growth.

The new study backs up these warnings.

Martin Hoerling, a climate scientist with the National Oceanic and Atmospheric Administration whose own work has detected climate change-related drying trends in the eastern Mediterranean, says the new study should prompt additional research and planning for more political instability. Hoerling was not involved in the new research.

“This study invites one to seriously ask what other regions in the world, especially across the southern Mediterranean, could also experience tipping point outcomes triggered by human caused drought?” Hoerling said in an email conversation.

Many experts urge viewing global warming as a risk management challenge, and when seen through that prism, it’s clear we’re failing to reduce climate-related risks in parts of the Middle East.

“We’ve added another risk to what are already parts of the world that are pretty unstable,” says David Titley, who directs the center for solutions to weather and climate risk at Penn State University. Titley, who rose to the rank of Rear Admiral in the Navy, has co-authored reports on climate change and national security. He was not involved in the study published Monday.

Thomas Homer-Dixon, a professor at the University of Waterloo in Canada, who also was not involved in the new study, told Mashable that the research “is the latest in a stream of thorough analyses that establish, beyond doubt, that climate change will gravely affect international security.”

Dixon disagrees with the way the study characterizes the role that drought played in the conflict chain, however. He says the drought wasn’t just a “catalyst” of conflict, as the study says, but that it “had a deep precipitating role.”

“Without the drought, the mass migration of farmers to the cities wouldn’t have occurred, and without that migration, it’s likely the civil unrest would have been much less severe (in fact, it might not have occurred at all),” he said.

For his part, Kelley, the lead author, is now examining the roles of drought, water scarcity and climate change in contributing to the political instability in Yemen, where the government was recently overthrown by a rebel group and the U.S. has been involved in fighting a formidable al-Qaeda branch.