In the last Democratic debate, Sen. Bernie Sanders said that the United Nations is projecting “hundreds of millions of climate refugees” in the “years to come” as a result of climate change. The U.N., however, doesn’t currently endorse a particular estimate, and the term “climate refugee” is in many ways problematic.

In the debate, which was hosted by the Washington Post and MSNBC on Nov. 20, moderators asked a few presidential contenders a question about climate change: How would the candidates ensure support for climate action beyond a two-term presidency?

As part of his reply, Sanders said, “The United Nations is telling us that in the years to come there are going to be hundreds of millions of climate refugees causing national security issues all over the world.”

The Sanders campaign did not respond to our requests for a source for his statement, but Sanders’ remark is similar to a comment the U.N. deputy high commissioner for refugees made in 2008. While at a climate change conference in Poland, the official said that between 200 million and 250 million people would be displaced by 2050 because of global warming.

As we’ll explain, that figure is likely flawed — and it was also more than a decade ago. More recent statements from U.N. officials do not indicate the agency endorses an estimate.

In 2011, for instance, the high commissioner for refugees said there was “no consensus” on a number, and a 2016 FAQ available on the agency’s website says “it is hard to say” how many people will be displaced by climate change. A 2015 blog post by the United Nations University’s Institute for Environment and Human Security also states that there are “no reliable estimates of the number of people on the move today or in the future as a result of environmental factors” and that “[n]o one knows how many climate migrants will exist.”

Similarly, reports from the International Organization for Migration, the U.N.’s migration agency, have discussed a range of estimates for the number of climate migrants — IOM’s preferred term — in the future. But the group has not produced its own estimate, or validated any figure. Instead, reports emphasize that too little is known to make a reliable estimate. We contacted IOM’s top migration, environment and climate change official, but did not receive a reply.

“Such statements have been made frequently over the last 20 years, but there is little evidence to back them up,” University of Sydney migration expert Stephen Castles said in an email, of Sanders’ comment. “People do move to adapt to climate issues,” he said, “but generally move with[in] their own countries or regions.”

We’ll run down how some of the existing estimates came to be and why experts have said they’re not dependable. But first, let’s clarify the nomenclature.

Problematic Terms

Scholars in the field don’t use the term “climate refugee” because “refugee” has a very specific meaning in international law — and currently would not apply to most people displaced because of climate change.

The U.N. Refugee Agency explains on its website that a “refugee” is a person who has crossed an international border “owing to well-founded fear of being persecuted for reasons of race, religion, nationality, membership of a particular social group or political opinion” or someone fleeing “events seriously disturbing public order.”

Environmental concerns aren’t listed here, and research shows people affected by climate change usually move within a country first, according to the agency. The agency says it does not endorse the term “climate refugee,” and says it is “more accurate” to talk about “persons displaced in the context of disasters and climate change.”

That’s fairly cumbersome, so we’ll stick with “climate migrant” to refer to anyone leaving their home because of climate change — although figuring out how many or who those people are isn’t always so clear, since migration is usually due to a mix of reasons, including economic, political and social factors. IOM defines climate migration as any temporary or permanent movement away from a place of residence “predominantly for reasons of sudden or progressive change in the environment due to climate change.”

A related, but distinct term is “environmental migration,” which would cover any environmental reason, not just those due to climate change. Both terms, however, do not have any legal standing.

Fuzzy Numbers

The most recent IOM report to address estimates of climate or environmental migrants was published in 2014, and stated that there was “great uncertainty about the figures.” It noted that the forecasts for the number of environmental migrants by mid-century vary widely — by as much as a factor of 40 — or between 25 million and 1 billion.

The actual figure, the report explained, will depend on a host of factors that are largely unknowable at this point, including which climate scenarios will play out, how humans will respond and adapt, and political and demographic factors. As a result, the agency said it “does not advance an estimated figure.”

The same wide-ranging estimates have appeared in a variety of IOM reports over the years, including particular mention of one estimate of 200 million environmental migrants by 2050. But each time, the reports have put the figures into context and noted the uncertainty.

In a section titled “Lack of reliable estimates,” a 2009 IOM report said the “most widely cited figure predicts 200 million environmental migrants by 2050” and pointed to the 40-fold range of estimates between 25 million and 1 billion.

Another 2009 IOM report also referred to the 200 million estimate, but noted that it “has been dismissed as apocalyptic and based on no more than anecdotal evidence and intuitive judgement” and said that “[p]redictions of migration flows caused by environmental factors are impossible to make” based on the available literature.

A 2008 IOM report also called the 200 million number “the most widely repeated prediction,” adding, “[b]ut repetition does not make the figure any more accurate.”

Similarly, a 2016 briefing from the IOM’s Global Migration Data Analysis Centre said the 200 million to 1 billion estimates “are known for likely overestimating the phenomenon and represent guesstimates rather than actual evidence.”

So where did these estimates originate? None of them were created by the U.N. or IOM. Instead, they largely come from one Oxford professor who first predicted in 1993 that there would be 150 million “environmental refugees in a greenhouse-affected world” by 2050. In that paper, environmental scientist Norman Myers proposed approximate numbers based on projected populations and a loose tallying of the number of people who might be affected by sea level rise in places including Bangladesh, India, Egypt and China. An extra 50 million people were then added in for presumed climate-induced food shortfalls. There are few details about how each of the numbers is derived, and Myers acknowledged in the paper that his estimate was “rough.”

In subsequent publications, including an influential 1995 report, Myers revised his number upward to 200 million. And in a 2007 interview with the British charity Christian Aid, Myers said he believed the figure would be closer to 250 million. It’s likely Myers’ 200 million and 250 million figures that the U.N. deputy high commissioner for refugees had in mind in 2008. As documented in a 2011 report put together for the U.K.’s Government Office of Science, most of the existing estimates of the number of climate or environmental migrants stem from Myers’ work, which over time did not incorporate new methodology. And many experts have critiqued his findings, noting that his predictions are based on the number of people in a region who could be at risk, rather than those who would actually migrate. Such predictions also assume that it’s possible to distinguish a climate or environmental migrant, despite the fact that migration usually has multiple causes.

Castles, the University of Sydney migration expert, said there is “no basis” for Myers’ projections. After a dustup surrounding another Myers climate migrant prediction — 50 million by 2010, which did not come to pass but was shared on the U.N. Environment Programme’s website — Myers told the BBC in 2011 that it was “really difficult” to predict the number of climate refugees. But he added that “in the long run I do believe very strongly that it will be better for us to find that we have been roughly right than precisely wrong.”

Beyond Numbers

To be clear, many scientists are concerned about climate change’s effect on migration. But there isn’t a consensus on the number of people who will move — and it’s important to recognize that not all climate migration is necessarily forced.

As the IOM’s Migration, Environment and Climate Change Division head Dina Ionesco explains in a 2019 blog post, given the slow onset of climate change in many places, migration is still a choice for many.

And migrating away from an affected area is not always the worst outcome. The 2011 U.K. report notes that the people who move away from environmentally dangerous areas often have more resources than those who stay put.

As a result, the report argues that focusing on the number of environmental migrants “could mean neglecting the major humanitarian issues surrounding those who stay behind, and indeed those who are unable [to] migrate and who become trapped in parlous environmental circumstances.” The report concludes: “Trying to produce global estimates of ‘environmental migrants’ is methodologically unsound, unhelpful for policy purposes and may even be counterproductive.”

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