Alister Doyle, Reuters 4 Feb 08;
OSLO (Reuters) - 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 identified 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 meters (20 ft), or 2 meters 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".
(Editing by Janet Lawrence)
Climate set for 'sudden shifts'
Pallab Ghosh, BBC News 4 Feb 08;
Many of Earth's climate systems will undergo a series of sudden shifts this century as a result of human-induced climate change, a study suggests.
A number of these shifts could occur this century, say the report's authors.
They argue that society should not be lulled into a false sense of security by the idea that climate change will be a gradual process.
The work by an international team appears in the Proceedings of the National Academy of Sciences journal.
"Our findings suggest that a variety of tipping elements could reach their critical point within this century under human-induced climate change," he said.
"The greatest threats are tipping of the Arctic sea-ice and the Greenland ice sheet, and at least five other elements could surprise us by exhibiting a nearby tipping point."
The bulk of climate scientists now believe that human induced global warming has begun to affect some aspects of our climate.
Risk assessment
But that change is the start of a series of more dramatic changes if global warming continues, according to a group of more than 50 scientists.
In a formal survey the researchers said that a number of systems that influence the Earth's weather patterns could begin to collapse suddenly if there's even a slight increase in global temperatures.
At greatest risk is arctic sea ice, the Greenland ice sheet and the west Antarctic ice sheet.
The researchers have listed and ranked nine ecological systems that they say could be lost this century as a result of global warming. The nine tipping elements and the time it will take them to undergo a major transition are:
* Melting of Arctic sea-ice (about 10 years)
* Decay of the Greenland ice sheet (about 300 years)
* Collapse of the West Antarctic ice sheet (about 300 years)
* Collapse of the Atlantic thermohaline circulation (about 100 years)
* Increase in the El Nino Southern Oscillation (about 100 years)
* Collapse of the Indian summer monsoon (about 1 year)
* Greening of the Sahara/Sahel and disruption of the West African monsoon (about 10 years)
* Dieback of the Amazon rainforest (about 50 years)
* Dieback of the Boreal Forest (about 50 years)
The paper also demonstrates how, in principle, early warning systems could be established using real-time monitoring and modelling to detect the proximity of certain tipping points.
