* Soot darkens ice, causing widespread Greenland melt - study
* Extreme thaw similar to 2012's could happen almost yearly by 2100
By Alister Doyle, Environment Correspondent
OSLO, May 19 (Reuters) - Forest fires and global warming caused an extreme melt of Greenland's ice in 2012, according to a study on Monday that said such thaws may happen almost yearly by 2100, threatening the survival of the entire ice sheet.
Clouds of soot from forest blazes in Siberia and North America dumped a dark layer onto Greenland in 2012 and made it absorb more of the sun's heat, it said. Greenland's ice, the second largest body of ice after Antarctica's, is already thawing, raising world sea levels.
Last week, another report found signs that glaciers in West Antarctica had begun an unstoppable slide. Over several centuries, that thaw could raise seas by 1.2 metres (4 feet), swamping coasts from Bangladesh to Florida.
Monday's study by scientists in the United States said an unusual number of forest fires helped explain a melt across 97 percent of Greenland's surface in July 2012, the most widespread since 1889. The role of the blazes had previously been unclear.
"Warmer temperatures and more frequent Northern Hemisphere forest fires driven by climate change may increase the frequency of these widespread melt events, contributing to the further demise of the Greenland ice sheet," the scientists wrote.
Such widespread melt events "may begin to occur almost annually by the end of (the) century", they wrote in the U.S. journal Proceedings of the National Academy of Sciences.
Parts of Greenland are 3,000 metres (9,800 feet) high, and any meltwater there quickly re-freezes. Ice cores show that 1889 was also a warm year - before man-made climate change was an issue - when forest fires added large amounts of soot.
"Our data shows that currently you need both warm temperatures and black carbon in order to cause melting at the surface of the centre of the Greenland ice sheet," said lead author Kaitlin Keegan of Dartmouth College in New Hampshire.
She told Reuters that the chemical make-up of the soot showed that it came from forest fires, rather than industrial pollution from factories, power plants or vehicles.
Climate change may cause more fires by drying forests in some areas, U.N. reports say. In addition, some insect pests have moved north as temperatures warm, killing trees and making forests more vulnerable to fires started by lightning.
A U.N. panel of experts says it is at least 95 percent likely that man-made emissions of greenhouse gases, rather than natural variations in the climate, are to blame for global warming. Almost 200 governments have agreed to work out a deal by the end of 2015 to limit climate change.
On Sunday, a report in the journal Nature Geoscience showed that parts of Greenland's ice were resting on bedrock below sea level. That meant the ice was "probably more vulnerable" than has been believed to erosion from below by warmer seas, they wrote.
Greenland lost ice equivalent to about 8 mm (0.3 inch) of world sea levels in the past two decades, with record losses in 2012, the U.N. panel of climate experts says. If all Greenland's ice ever melted, a process that would take many centuries, it would raise world sea levels by 7 metres (23 feet).
(Editing by Kevin Liffey)