* Climate scientists seek Arctic early warning system
* Seek to understand triggers for Greenland thaw
* Better knowledge of thresholds would guide governments
By Alister Doyle, Environment Correspondent
TROMSOE, Norway, Jan 26 (Reuters) - Scientists sought on Wednesday to pin down triggers for abrupt climate shifts in the Arctic, such as a feared runaway melt of Greenland&${esc.hash}39;s ice sheet, to create an early warning system for governments.
"We need leading indicators to see when we are approaching a threshold so that we can stop before we reach it," Carlos Duarte, a professor at the Spanish Council for Scientific Research, told a conference on "Arctic Frontiers" in Norway.
Scientists at the conference sought to identify advance signs of instability that could lead to radical changes -- such as a disappearance of Arctic sea ice in summer or a melt of Greenland&${esc.hash}39;s ice that would raise world sea levels.
A 2009 study in the journal Nature showed that many systems, ranging from ocean currents, wildlife populations or even financial markets, often show signs of increased variability before radical shifts.
But in all cases, the problem is deciding what to watch.
"We have an enormous capacity to collect data, but not an enormous capacity to determine which of those data are the most relevant," Oran Young of the University of California, Santa Barbara, said of the Arctic climate.
"Our challenge is to think of a small number of indicators," he said.
Duarte said that unusual variability was likely to be the best single sign. Scientists studying ancient climates have found that there were short-term shifts between warm and cold before the Ice Age ended 10,000 years ago.
Other signs could be changes in correlations -- for instance how far rising temperatures match a decline in sea ice extent. As ever more reflective ice melts, it exposes water which soaks up more of the sun&${esc.hash}39;s energy, hastening a thaw.
TIPPING POINTS
Duarte said the Arctic Ocean might be ice-free in summers from 2020. "We are just about to go across this tipping point," he said, saying that big variations in the ice in recent years might be more worrying signals than a long-term shrinking trend.
Scientists are also working to pin down temperature rises that might trigger, for instance, a thaw of Greenland&${esc.hash}39;s ice sheet which holds enough water to raise world sea levels by 7 metres (23 ft) if it ever all melted, over centuries.
Stefan Rahmstorf, of the Potsdam Institute for Climate Impact Research, said recent studies indicated that a temperature rise of between 1.3 and 2.3 degrees Celsius (2.3-4.1 F) above pre-industrial times might trigger a runaway thaw.
That was a lower threshold than estimated by the U.N. panel of climate scientists in 2007, of 1.9-4.6 C. A melt of the 3,000-metre thick ice could be self-perpetuating, partly because the air on top would get warmer as it shrank to lower altitudes.
Almost 200 governments agreed last month in Cancun, Mexico, to a goal of limiting the average rise in world temperatures to below 2 degrees Celsius (3.6 F) above pre-industrial times. Temperatures have already gained by 0.8 degree Celsius.
"Greenland shows that 2 degree target may not be enough," Rahmstorf said. And the United Nations says promised curbs on greenhouse gas emissions, by major emitters led by China and the United States, are too small to stay below 2C.
For Reuters latest environment blogs, click on: http://blogs.reuters.com/environment/ (Editing by Philippa Fletcher)
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