Any way you look at it, the numbers on climate migration are staggering. The problem is, there are a lot of ways to look at it.
One study says 100 million people will be displaced by global warming. Another puts it at 250 million. Meanwhile, a sweeping report from Christian Aid warns that 1 billion people, an almost unthinkable crush of humanity, could be forced from their homes by midcentury because of climate change and the increase in natural disasters, which will exacerbate regional conflicts.
How can the numbers be so wildly disparate? The truth is, researchers acknowledge, that though climate migration may be the defining issue of the century, it is calculated with fuzzy math.
"The most widely accepted estimate, and it's really a guesstimate, of how many people could be on the move because of environmentally related factors, including climate change, is an extra 200 million," said Koko Warner, who heads the U.N. University's migration section within the Institute for Environment and Human Security in Bonn, Germany.
"You could see maybe a doubling. Maybe more. But we don't know. There's so much we don't know about climate change," she said.
Researchers say credible numbers are key because politicians are just waking up to the knowledge that floods, droughts and other products of a warmed world could eject millions from their homelands. Nervous governments are pushing academics to predict how many will leave, which countries they will flee and -- most importantly -- where the displaced are likely to go.
There are no easy answers, and there is no agreed-upon methodology for calculating the projections.
Demographers say early models were crude exercises in number crunching. Climate scientists with no expertise in migration patterns simply measured the population levels of threatened areas and factored in each affected country's gross domestic product to estimate a group's propensity to move. Then they wiped people off the map.
But that's not how humans tick.
Dominic Kniveton, a senior lecturer in geography at the University of Sussex who is devising a statistical methodology on climate migration for the United Nations, said people don't migrate just because they are pushed out by dire conditions. They also need to be pulled by the lure of a job, a waiting family member, a chance to succeed elsewhere.
"You don't just get up and go unless you have someplace to go. There's a whole mixed bag of evidence that points to whether people will go when conditions get bad," Kniveton said.
Indeed, many researchers are starting to sound alarms that environmentalists may be overplaying climate change's role in migration, and to accuse green groups of using scanty evidence to support wild claims.
"Individuals and communities have quite a lot of coping mechanisms," said Clionadh Raleigh, a professor at Trinity College Dublin. "There is very little evidence there will be an increase in international migration."
A recent U.N. University study of 22 nations confirmed a growing body of research that migration is out of reach altogether for those living in the worst conditions. The report was the first empirical study of climate migration trends.
So, by midcentury, how many climate migrants are we really talking about?
Said Kniveton, "I have no idea."