BONN, Germany - Tens of millions of people will be displaced by climate change in coming years, posing social, political and security problems of an unprecedented dimension, a new study said on Wednesday.
"Unless aggressive measures are taken to halt global warming, the consequences for human migration and displacement could reach a scope and scale that vastly exceed anything that has occurred before," its authors warned.
"Climate change is already contributing to migration and displacement.
"All major estimates project that the trend will rise to tens of millions of migrants in coming years. Within the next few decades, the consequences of climate change for human security efforts could be devastating."
The report, "In Search of Shelter," was compiled by specialists from Columbia University in New York and the United Nations University, and from a non-governmental organisation, CARE International.
It was presented to journalists on the sidelines of the UN climate talks in Bonn, a staging post to an envisioned new global pact for tackling global warming and its impacts.
The study swung the spotlight on several regions that, according to projections, will be badly hit by rising sea levels, flood or drought.
Rather than a migration from poor countries to rich ones, the exodus is most likely to unfold within poor nations, with a movement mainly from the countryside to cities, thus further burdening urban infrastructure, it said.
In central Mexico, where tens of millions of people live, rainfall in some areas could decline by up to 50 percent by 2080, "rendering many livelihoods unviable and dramatically raising the risk of chronic hunger," the report said.
South Asia faces both short- and long-term threats.
Warming will accelerate melting from Himalayan glaciers in springtime, thus heightening the probability of flooding. But glacier shrinkage will eventually affect the flow of major rivers that wind down from the Himalayan foothills.
"This has a lot of consequences for agricultural production in one of the world's most populous regions," said Charles Ehrhart, climate-change coordinator at CARE.
The Ganges Delta, small island states and other low-lying areas, meanwhile, are in peril from rising sea levels.
If ocean levels rise by two metres (seven feet), "9.4 million people would be completely flooded out" in Bangladesh alone, said Ehrhart.
A two-metre (seven-feet) rise is seen by most climate scientists as being at the top end of predictions for what could happen this century.
In 2007, the UN's Intergovernmental Panel for Climate Change (IPCC) predicted sea levels will rise by up to 59 centimeters (23 inches) before 2100 due the expansion of warmer waters.
But this figure does not factor in a partial melting of massive ice sheets in western Antarctica and Greenland, a scenario now identified by more recent research.
The new report urged policymakers to develop tools to identify regions and populations at risk of being displaced by climate change.
And they said funds mustered to help cope with climate change under the future global treaty must also be directed at poor migrants.
The new pact, designed to run from 2012, would chiefly slash emissions from fossil fuels and deforestation that are warming Earth's atmosphere, affecting weather patterns.
The report admits that the definition of a climate migrant is complex, as poverty, a run of bad harvests or civil strife are usually the immediate, and thus most visible, triggers for displacement.
Estimates of the likely numbers range from 25 to 50 million people by 2010, while the International Organisation for Migration (IOM) has pitched a figure of 200 million by 2050.
The term "climate refugee" is shunned by UN organisations, as "refugee" is a term with legal connotations under the 1851 Geneva Convention.