"This is good news in the sense that people are leaving drought-prone areas that are likely to see increasing vulnerabilities owing to climate change and variability," Alex de Sherbinin of Columbia University told environmentalresearchweb. "But on the flip side, people are moving towards coasts, which depending on the region, can put them at far greater risk of cyclones and floods."

It seems that migrants are ending up in areas that are better connected economically but that may be prone to different hazards.

De Sherbinin and colleagues examined the 1970s, 80s, and 90s using net-migration grids with a resolution of one square kilometre developed for a project on Global Environmental Change and Migration funded by the UK Government's Foresight Programme.

"Foresight wanted to know how many people were migrating into and out of various ecosystems globally," said de Sherbinin. "This is a difficult question to answer with available data on migration, which is scant. So we developed this inverse modelling approach to get net migration as a residual after accounting for that portion of population change that is owing to natural increase, or births minus deaths."

Inwards migration tended to be concentrated into cities such as Delhi or Beijing whereas net out-migration was generally a rural phenomenon and spread over a much larger area.

In developing countries migrants have tended to move out of drought-prone areas and marginal dryland and mountain ecosystems towards coastal ecosystems and areas that are prone to floods and cyclones. In North America, meanwhile, people are tending to move in the opposite direction, from the northeast towards the arid southwest and the Rocky Mountains. "Many are predicting that climate change will bring increasing water scarcity to this region," said de Sherbinin. "We have already seen that the region is becoming more and more vulnerable to forest fires, and research suggests that this is tied, in part, to climate factors."

According to de Sherbenin, the results largely confirmed what more limited case studies and anecdotal evidence had suggested. "People knew that population was growing in the coastal zone, for example, and that much of that growth in places like China was owing to migration," he said. "But at a global level we did not have a good understanding of how much of that growth was due to migration. Now we are able to quantify that."

Drylands cover around 40% of land worldwide and are home to more than two billion people, the bulk of them in developing countries. Around 11% of the world's population currently lives within 10 km of the coast.

Around five million to 15 million migrants left drylands across most developing countries in most decades, the team found. All regions except North America saw high levels of out-migration from mountain regions, especially in Asia, where more than 30 million people moved away from mountainous areas in the 1990s. Almost all coastal regions, on the other hand, saw a net influx. Asia topped the table with more than 18 million net migrants to the coast in each decade, and more than 60 million in the 1990s, a period when many Chinese people moved to industrial cities on the coast.

On average in low income countries 5.1 million people per decade migrated away from the highest-risk drought areas. But around 150,000 people per decade moved into cyclone-prone areas and around seven million per decade into areas at high risk of flooding.

"One of our concerns is that if people in the lowest income regions, with potentially the least adaptive capacity, are moving into areas that are prone to significant climate hazards, then that represents a form of maladaptation," said de Sherbinin. "We do find that for countries in the lowest income categories in Asia, people are moving into multi-hazard zones, but net migration is even higher in the wealthier Asian countries."

Now the team is exploring the log likelihoods of positive or negative net migration given various climatic, income and other factors. "Another line of research would be more policy oriented – to look at factors that might limit migration to the most hazard-exposed areas," said de Sherbinin. "The track record of past efforts to limit migration has not been particularly good. Only a few countries, such as China and the former Soviet Union, have overtly sought to control migration, particularly urban–rural migration, through residency permits." That said, de Sherbinin believes it is possible that "if markets functioned properly, and government subsidies for real-estate development in coastal areas were removed, that new housing would not be springing up in some of these areas".

De Sherbinin is also investigating the potential need to consider resettlement from the world's most drought- and flood-exposed areas to less risky regions. Many people may be trapped in such areas by poverty. "So there may be a need to consider planned relocations for those populations most at risk and with the least ability to move on their own," he said. "Here again, the Chinese have been pioneers, for better or worse, resettling thousands of people through their 'ecological migration' programme."

The team believes the results confirm that there needs to be a continued science and policy focus on the vulnerabilities of large cities in the coastal zone, and potential adaptive responses. "In the aftermath of Hurricane Sandy, it was noted by the media, including the New York Times, that New York City's government has been at the vanguard of thinking about climate change," said de Sherbenin, "but that it has been slow in implementing proposed responses, largely because those responses – such as sea walls, raised infrastructure, putting electrical lines underground, and ecosystem-based green infrastructure – can be costly. So when Sandy struck, the very locations and systems that were predicted to be most vulnerable were hardest hit. It often takes a disaster to underscore the vulnerabilities and to prompt politicians and society at large to take action."

According to Klaus Jacob of Columbia University, who is an expert on adaptation responses in New York City, population growth has been highest in some of the most vulnerable portions of the city, particularly lower Manhattan and Chelsea. Even with the best adaptation measures and additional storm preparedness, these movements of population will only put the city at increased risk, he believes.

"While our study cannot pinpoint the net-migration levels at a sub-city level with the same level of detail as one would get from analysing US Census data, the overall point is the same," said de Sherbinin. "If people continue to move into areas of substantial risk in the lowest elevation coastal zones, it will be very difficult to finance the infrastructure needed to protect them."

De Sherbinin and colleagues reported their findings in Environmental Research Letters (ERL) as part of the ERL Focus on Environmental Risks and Migration: Causes and Consequences.