OSLO (Reuters) - Rising temperatures trigger a runaway melt of Greenland's ice sheet, raising sea levels and drowning Pacific islands and cities from New York to Tokyo.
In Siberia, the permafrost thaws, releasing vast frozen stores of greenhouse gases that send temperatures even higher. In the tropics, the Amazon rainforest starts to die off because of a warmer, drier climate.
Such scenarios may read like the script of a Hollywood disaster movie but many scientists say there are real risks of "tipping points" -- sudden, catastrophic changes triggered by human activities blamed for warming the planet.
"Even small risks in the climate need to be considered, just as we try to avert accidents at nuclear power plants," said Stefan Rahmstorf, a professor at the Potsdam Institute for Climate Impact Research and an expert in ocean currents.
"I don't think this is scaremongering. We don't really understand the system," he said of risks that the warm Gulf Stream current in the North Atlantic might shut down in one possible "tipping point" scenario.
Melting ice in Greenland could send a sudden flow of cool water into the North Atlantic, disrupting the giant current that pulls warm water northwards to create the Gulf Stream.
This might shut down the warm current and could also make parts of Europe and North America sharply colder, despite an overall warming of the climate.
Scenarios like this, and the uncertainty surrounding them, will provide a dramatic backdrop to a United Nations climate change meeting in Montreal, Canada, from Nov. 28-Dec. 9.
Around 190 countries will debate how to expand a U.N.-led fight against global warming to include developing nations such as China and India and sceptic countries, led by the United States and Australia.
Under the U.N.'s Kyoto Protocol, about 40 rich nations have agreed to curb emissions of heat-trapping gases released by burning fossil fuels in power plants, factories and cars by 5.2 percent below 1990 levels by 2012.
What will happen after 2012 is unclear.
Many environmentalists say the risk of "tipping points" makes it ever more urgent to curb climate change, already widely predicted to cause more storms and floods and even drive some species of animals and plants to extinction.
But there are those who disagree.
"Environmentalists talk about 'tipping points' because they are frustrated," said Fred Singer, head of the U.S. Science and Environmental Policy Project. He believes humans can adapt to any warming caused by a buildup of greenhouse gases.
"All the climate models that I've seen show only a gradual warming as the level of greenhouse gases increases," he said.
Even so, records of the ancient climate found in ice caps and ocean sediments show there have been staggeringly big shifts in the past. "Past climate change is ringing alarm bells," Rahmstorf said, referring to the climate's fragility.
During the last Ice Age, temperatures in the North Atlantic region once soared by 12 Celsius in just 10 years, perhaps because of swings in ocean currents linked to small shifts in the sun.
Such drastic changes have stopped since the end of the Ice Age about 10,000 years ago, maybe because ocean currents are more stable outside Ice Ages, according to a study co-authored by Rahmstorf and published in the Nature journal this month.
There are more obvious examples of "tipping points" in nature, like the collapse of cod stocks off Newfoundland, Canada, in the early 1990s from overfishing.
And, at some point in the 17th century, hunting of the flightless dodo in Mauritius doomed the birds to extinction.
Concerns about "tipping points" today focus on the Artic.
Experts say Greenland's 3,000 metre thick ice sheet, which has been melting at ever higher altitudes in summers in recent years, may be vulnerable to a runaway thaw.
If the Greenland sheet melted entirely over the next few centuries, world sea levels would rise by about 7 metres. Antarctica's far bigger ice cap is likely to be more resilient as the giant continent acts as a deep freeze.
A melting of the Arctic "may happen very abruptly. It's one of the big unknowns and would be irreversible," said Paal Prestrud, head of the Centre for International Climate and Environmental Research in Oslo.
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"The concern is that there are tipping points out there that could be passed before we're halfway through the century," said Tim Lenton, an earth systems modeller at Britain's University of East Anglia.
Risks of abrupt change seem greatest in the Arctic, where warming uncovers dark ground or sea that soaks up more heat than reflective snow and ice, accelerating the melt, he said.
Other reports have pointed to risks that Siberia's permafrost could start to thaw, releasing heat-trapping methane, or that the Amazon rainforest could shrink.
There could be some benefits from change: Lenton said some models suggested a shift of monsoon rains in West Africa towards the Sahara desert, possibly making that region more fertile.
Assessing risks of "tipping points" is almost impossible.
Rahmstorf said he recently polled 12 experts on the chances of a collapse of the Gulf Stream: four said risks were above 50 percent if world temperatures rose by 5C by 2100.
"That was unexpected for me, I reckon the risks are lower," he said of the so far unpublished survey. A rise of 5C is at the top of a range forecast for global warming by 2100 by the scientific panel that advises the United Nations.