* Asia is most disaster-prone continent, report says
* Hard to measure impact of slow-onset crises like drought
LONDON, Sept 22 (Reuters) - Floods, storms, drought and other climate-related natural disasters drove 20 million people from their homes last year, nearly four times as many as were displaced by conflicts, a new U.N. report said on Tuesday.
The study tried to quantify for the first time the number of people forced to flee their homes because of climate change.
Global warming is increasing the frequency and intensity of storms and otherwise altering weather patterns, so disasters are now "an extremely significant driver of forced displacement globally", it said.
The study said a total of 36 million people were driven from their homes by rapid-onset natural disasters in 2008. China's Sichuan earthquake accounted for 15 million of these, but climate-related disasters displaced 90 percent of the rest.
The report said many more people were probably being forced from their homes by slower-onset crises like droughts.
The report was compiled jointly by the U.N. Office for the Coordination of Humanitarian Affairs and the Internal Displacement Monitoring Centre (IDMC), a body which normally tracks displacement caused by conflict.
The aim was "to see whether it was possible to put numbers to the problem and come up with a methodology that will enable us to do that over time", said IDMC head Kate Halff.
The answer was a qualified "yes", though Halff warned that the monitoring effort so far "doesn't give us any idea of what time period these people have been displaced or what their needs are. At this stage it's just about a number."
MIGRATE OR FLEE?
Accurately tracking displacement resulting from slower-onset crises like rising sea levels is also expected to prove difficult, largely because it is hard to judge when voluntary movement from a problem zone becomes forced fleeing, she said.
Determining what role climate change may have played in a natural disaster will also undoubtedly remain controversial.
Still, "an increase in the number of people temporarily displaced will be an inevitable consequence of more frequent and intense extreme weather events affecting more people globally," the report said.
Last year, more than five million people were displaced by flooding in India, attributed in part to changes in that country's monsoon cycle.
In the Philippines, nearly two million people were forced from their homes by severe storms. China and Myanmar also saw large-scale displacements due to storms.
Asia accounted for over 90 percent of disaster-related displacements last year, which the report said "may simply be because Asia is the most disaster-prone region".
By comparison, 4.6 million people were internally displaced last year by conflict, according to Halff's centre.
Altogether 42 million people were living as refugees or internally displaced persons last year because of fighting, she said.
(Editing by Tim Pearce) (For more stories on climate change and humanitarian relief please visit www.alertnet.org) (Laurie Goering, Reuters AlertNet, tel + 4420 7542 8067)
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