Global warming this century could trigger a runaway thaw of Greenland's ice sheet and other abrupt shifts such as a dieback of the Amazon rainforest, scientists said on Monday.

They urged governments to be more aware of "tipping points" in nature, tiny shifts that can bring big and almost always damaging changes such as a melt of Arctic summer sea ice or a collapse of the Indian monsoon.

"Society may be lulled into a false sense of security by smooth projections of global change," the scientists at British, German and U.S. institutes wrote in a report saying there were many little-understood thresholds in nature.

"The greatest and clearest threat is to the Arctic with summer sea ice loss likely to occur long before, and potentially contribute to, Greenland ice sheet melt," they wrote in the journal Proceedings of the National Academy of Sciences.

"Tipping elements in the tropics, the boreal zone, and west Antarctica are surrounded by large uncertainty," they wrote, pointing to more potential abrupt shifts than seen in a 2007 report by the U.N. Climate Panel.

A projected drying of the Amazon basin, linked both to logging and to global warming, could set off a dieback of the rainforest.

"Many of these tipping points could be closer than we thought," lead author Timothy Lenton, of the University of East Anglia in England, told Reuters of the study.

Other sudden changes linked to climate change, stoked by human use of fossil fuels, included a dieback of northern pine forests, or a stronger warming of the Pacific under El Nino weather events that can disrupt weather worldwide, they wrote.

A possible greening of parts of the Sahel and the Sahara, if monsoon rains in West Africa were disrupted, was one of the few positive abrupt shifts identifed by the scientists.

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Even a moderate warming could set off a thaw of Greenland's ice sheet that could then vanish in 300 years -- raising sea levels by 6 metres (20 ft), or 2 metres a century and threatening coasts, Pacific islands and cities from Bangkok to Buenos Aires.

The U.N. Climate Panel foresees a rise in world sea levels ranging up to about 80 cms this century and reckons that a thaw of Greenland would take hundreds of years longer.

The new study said a disappearance of Arctic sea ice in summertime could happen in coming decades -- earlier than projected by the U.N. panel. That could stoke further global warming as dark water soaks up more heat than ice and snow.

The report also identified risks such as damage to northern pine forests -- widely exploited by the pulp industry -- because of factors such as more frequent fires and vulnerability to pests in warmer, drier conditions.

But it played down some other fears, such as of a runaway melt of Siberian permafrost, releasing stores of methane which is a powerful greenhouse gas.

And it said a shutdown of the Gulf Stream in the Atlantic Ocean that brings warm water north to Europe "appears to be a less immediate threat".