内容摘要：Europe is currently struggling to absorb an influx of refugees from war-torn Syria, Afghanistan, Ira

Europe is currently struggling to absorb an influx of refugees from war-torn Syria, Afghanistan, Iraq, and parts of Africa. Germany alone has taken in more than a million people since 2015, many of whom have sought asylum status. This wave of immigration has led to political upheaval, with the rise of right-wing political parties in Germany, Poland, Austria, and Hungary, among others.

Now a new study, published in the journal Science, shows that the current surge in refugees may be a preview of what's to come due in large part to global warming. Studies have already tied climate change to the civil war in Syria and raised red flags about how climate change can serve as a threat multiplier, increasing the odds of conflict. However, this new work, from researchers at Columbia University, is the first to examine climate change's upcoming impact on migration, as measured via asylum applications to the European Union.

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One central lesson of the study is that climate change will have a significant spillover effect that, if not prepared for ahead of time, could result in far more political instability than one might otherwise expect.

Assuming the study's links between climate change and migration are correct, then even wealthy, relatively stable nations that successfully adapt to climate change on a national level won't be immune from the destabilizing influence from countries that don't fare so well, particularly areas already experiencing food and water scarcity.

Homs, Syria, after the battles, in April 2017.Credit: IBL/REX/Shutterstock

Specifically, the study uses previously established research on how climate change affects crop yields and economic productivity to conclude that asylum applications to the E.U. will undergo an accelerated increase under scenarios in which global greenhouse gas emissions stay high, and average temperatures spike by several more degrees Fahrenheit.

Assuming everything else affecting migration stays constant (a major assumption), the study found that asylum applications by the end of the century would increase by an average of 28 percent even if emissions are sharply reduced and additional global warming is limited. However, along a business as usual path, which the world is currently hewing closest to, there would be a 188 percent spike in additional asylum applications per year. This would translate into 660,000 more applications each year, which is comparable to the uptick Germany saw in 2016.

The researchers studied how climate variability during the period from 2000 to 2014 in 103 "source regions" for European migration affected asylum applications. They then used statistical techniques and a suite of computer models to project how asylum applications could change, based on global warming, by the end of the century.

The key variable, the researchers found, was how temperature alters maize yields, since they concluded that temperature is a stronger predictor of human migration than precipitation changes. Once the temperature gets too warm, crop yields diminish, prompting people to move. The optimal temperature, they found, was 20 degrees Celsius, or 68 degrees Fahrenheit, during the growing season. Once temperatures far exceeded this, migration would pick up.

Anouch Missirian, a doctoral student at Columbia University and coauthor of the new study, said the research is significant because it sheds light on how climate change can worsen migration crises that have multiple triggers, such as the ongoing one.

"We were surprised to find a robust relationship between asylum applications and temperature over the maize (and other staple crops) growing area and season of such a diverse panel of countries," Missirian said in an email.

Migrants and refugees gather outside of the German consulate in Thessaloniki, Greece on Sept. 1, 2016 to protest against the difficult living conditions in the camps.Credit: NurPhoto via Getty Images

While the new study does not cover the time period of the current refugee crisis, Missirian says it does suggest some conclusions about the role climate change may have played in it.

"What I think can safely be said is that the current refugee crisis proceeds from a series of factors in interaction with one another, among which [are] environmental drivers, and we show here that temperature matters — but there are many others (political, economic, etc.)," she said.

"Would the crisis have happened without the weather shocks and their aggravation by climate change? Perhaps. Would it have had the same magnitude? Probably not. Our finding [about] the relationship between asylum applications and temperatures [that] we describe in the paper also suggests that, all else being equal, climate change should make things worse," she added.

Researchers who are not affiliated with the new study said it's an important addition to our understanding of how climate change may prompt waves of climate refugees. Prior to this new paper, there has been a dearth of research tying climate change and migration in a scientifically rigorous manner.

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Marshall Burke, an assistant professor at Stanford University, who has studied the links between climate change and conflict, said the study makes "a huge contribution."

"The potential link between climate and migration has been talked about a lot, but there haven't been any studies that have been able to make a clear link at a broad scale, and at the same time understand what the potential mechanism is  -- and this paper does both," he said.

Colin Kelley, a climate scientist at Columbia's International Research Institute of Climate and Society, who published a 2005 study showing the link between the Syrian civil war and a historic drought, said even though this is a relatively poorly studied area, the study's results are convincing.

"I think the study is quite thorough and that the link between climate change and the number of migrants increasingly seeking asylum is very solid," he said in an email. "This is an unsurprising result and it really drives home the point that wealthier countries are not immune to climate and extreme weather effects that are experienced in less developed nations."

Projected relationship between temperature change and asylum seekers into the European Union.Credit: Science magazine

This dynamic, in which climate change impacts in some parts of the world lead to repercussions far away, can be thought of as the spillover effect. Climate change projections consistently show that the nations that will experience the most severe impacts from global warming lie close to the equator, with poorer nations in Latin America, Africa, and Asia suffering the most from rising temperatures, sea level rise, and other effects.

It is a cruel irony that these nations are the ones that contributed the least to global warming, with the industrialized world faring far better under most warming scenarios.

The new study, along with other climate research, suggests that, in addition to cutting greenhouse gas emissions that are causing global warming in the first place, it would be wise to prepare now for an influx of climate refugees. The way European governments have been destabilized from the wave of migration during the past few years is one indication that a lack of planning can result in negative outcomes.

Mike Hulme, a geographer at the University of Cambridge, who has been critical of studies tying climate to conflict, faulted the new study for elevating temperature above other factors that are likely to be more significant in driving asylum-seeking behavior. "I would have thought that civil war, political repression, weak civil institutions and low levels of educational attainment, etc., are more powerful predictors of asylum-seeking," he said in an email. "But this is a question these authors don’t ask. Yet it matters."

"To adopt an aphorism – if the only tool in your tool-kit is a predictive statistical climate model then every problem is going to look like a climate-shaped one," he said.

However, Solomon Hsiang, an economics professor at the University of California at Berkeley who is a prominent scholar of how climate change may influence conflict and economic productivity, said the new study offers lessons that we should heed. Hsiang, like Hulme, was not involved in the new research.

"When the climate changes in a location far away, that still will affect people who feel otherwise protected because the families trying to escape need a place to go," he said in an email.

"One approach to managing a rising influx of asylum seekers would be to build walls and close ourselves off from the world. That would very likely be a profound mistake," Hsiang said.

"The wealthy regions of the world have created security and economic opportunity for their citizens through the extensive use of energy, primarily fossil fuels. A side effect of that success is global climate change, which will be the most devastating to poorer populations in hotter regions. As communities flee the instability and violence that has resulted from this global transformation, it seems a moral imperative that we, the beneficiaries, find ways to welcome, protect, and accommodate them."

"We will need to build new institutions and systems to manage this steady flow of asylum seekers. As we have seen from recent experience in Europe, there are tremendous costs, both for refugees and their hosts, when we are caught flat-footed. We should plan ahead and prepare," Hsiang said.

The new work adds to studies published in the past several years that show there are complex, yet clear, links between climate change and conflict. The U.S. military views global warming as a threat multiplier, since it is likely to increase instability and get the military involved in more conflicts or humanitarian missions.

The findings on climate change and asylum seekers, which rest on the relationship between temperature change and crop productivity, add yet another reason for experts to be concerned about what a dystopian future.

According to Marc Levy, the deputy director of the Center for International Earth Science Information Network, who researches the connections between climate change and conflict, the new study is particularly noteworthy.

In an email, he said the study "... Is a textbook example of the sobering observation that studies that make use of the most robust methods and the best available data find the most alarming impacts.  That ought to frighten the hell out of everyone," he said. 

"For most social concerns, when science weighs in with increasing rigor the risks tend to become more nuanced or less severe than the public was worried about. This is the opposite."

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