The influx of Middle Eastern immigrants to the United Kingdom through the European Union undoubtedly played a significant role in Brexit, coupled with many British citizens’ resentment against what they perceived as intrusive and ill-advised rules imposed by unelected and unaccountable European Union officials and bureaucrats (among other reasons).

It is sometimes said that climate change, in turn, has been an important driver of migration from the Middle East, and consequently that it was a significant cause of Britain’s vote to leave the EU. For instance, the authors of “Climate change in the Fertile Crescent and implications of the recent Syrian drought,” PNAS, March 2, 2015, summarized their findings by saying, “[T]he 2007−2010 drought contributed to the conflict in Syria. It was the worst drought in the instrumental record, causing widespread crop failure and a mass migration of farming families to urban centers.”

They continued, “Century-long observed trends in precipitation, temperature, and sea-level pressure, supported by climate model results [emphasis added], strongly suggest that anthropogenic forcing has increased the probability of severe and persistent droughts in this region, and made the occurrence of a 3-year drought as severe as that of 2007−2010 2 to 3 times more likely than by natural variability alone.”

Their summary concluded, “[H]uman influences on the climate system are implicated in the current Syrian conflict.” And that conflict, of course, contributes to the flow of refugees.

But the case isn’t quite so clear.

Consider temperature and rainfall trends for the region. Are those enough to explain the drought — or even much of it? A graph in the PNAS paper suggests not.

In the Fertile Crescent, which includes Syria, the Palmer Drought Severity Index (a scale from +3 to -3) worsened from about positive 0.2 to about negative 0.8 since 1930. That’s significant but not enough to explain the severe 2007–2010 drought.

More important, what caused the drought?

The Fertile Crescent experienced about a 7% decline in winter rainfall since 1930, most before 1980, leaving only about 3% during the period of allegedly man-made warming, post-1980. Not much there to explain.

If you accept the paper’s data, which come from the Climatic Research Unit at East Anglia, annual surface temperature in the Fertile Crescent rose by about 0.5 C˚ since 1930, again about half before 1980, leaving about 0.25 C˚ since then, but that’s not enough to explain the drought.

So, with so little change in precipitation and temperature, why the increase in drought, and, more important, what caused the conflict over water? The answer will be more understandable if first we understand that “drought” designates not necessarily a time of low precipitation but a time of water shortage — which can be caused by increased consumption or accelerated runoff.

Retired U.S. Navy Rear Admiral David Titley, a meteorologist and professor at Pennsylvania State University, explained that after longstanding bad water policy “there was no resilience left in the system.” He added, “It’s not to say you could predict ISIS out of that, but you just set everything up for something really bad to happen.”

So in addition to the not-so-important rise in temperature and decline in precipitation, another cause of the drought was longstanding bad water policy.

But there’s another, more important cause, part of it apparent in the bottom panel of the very graph that depicts the temperature and precipitation. From 1930 to 2010, Syria’s population multiplied 11 times, from about 2 million to about 23 million. At the same time, its industrial and agricultural water use multiplied even more. Eleven times as many people coupled with burgeoning industry and agriculture mean greatly increased water consumption — and hence face water shortages, especially with “poor water policy” — even without any rise in temperature or decline in rainfall.

But assume that not population, industrial and agricultural growth but higher temperature and lower rainfall drove the drought. That doesn’t explain what caused either of those, and the U.N. Intergovernmental Panel on Climate Change concluded in its 2012 report on extreme weather that it was impossible to demonstrate a connection between global warming, man-made or natural, and increasing frequency or severity of extreme weather events, including droughts.

Even assuming that global warming contributed somewhat to the rise in annual surface temperature and the decline in rainfall, that doesn’t mean human activity drove it. The computer models on which the IPCC depends simulate warming from rising atmospheric CO₂ at two to three times the observed rate, and none simulated the absence of observed warming from early 1997 to late 2015, so they provide only weak reason to believe human activity was the main driver.

At most, human activity has contributed only a fraction of the global warming observed over the last 30, 50, 100, or 150 years, so it can have contributed only a fraction of the rise in temperature and decline in rainfall in the Fertile Crescent, and hence only a fraction of a fraction of the drought, and so only a fraction of a fraction of a fraction of the conflict over water.

In short, rising population and expanding industry and agriculture were far greater causes of conflict for access to water in Syria than climate change — and even those pale into insignificance compared with religio-political conflicts as causes of Syria’s civil war, the rise of ISIS, and the consequent refugee crisis, and the refugee crisis was but one among many reasons Britons voted to leave the EU.

So, was Brexit the result of a “climate refugee crisis”? Only slightly, if at all.