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* 26 scientists say ice melting, CO2 emissions up sharply
* Seas could rise up to 2 metres at worst by 2100
* Experts urge Copenhagen summit to cap greenhouse emissions
By Alister Doyle, Environment Correspondent
OSLO, Nov 24 (Reuters) - Global warming is happening faster than expected and at worst could raise sea levels by up to 2 metres (6-1/2 ft) by 2100, a group of scientists said on Tuesday in a warning to next month's U.N. climate summit in Copenhagen.
In what they called a "Copenhagen Diagnosis", updating findings in a broader 2007 U.N. climate report, 26 experts urged action to cap rising world greenhouse gas emissions by 2015 or 2020 to avoid the worst impacts of climate change.
"Climate change is accelerating beyond expectations," a joint statement said, pointing to factors including a retreat of Arctic sea ice in summer and melting of ice sheets on Greenland and Antarctica.
"Accounting for ice-sheets and glaciers, global sea-level rise may exceed 1 metre by 2100, with a rise of up to 2 metres considered an upper limit," it said. Ocean levels would keep on rising after 2100 and "several metres of sea level rise must be expected over the next few centuries."
Many of the authors were on the U.N.'s Intergovernmental Panel on Climate Change, which in 2007 foresaw a sea level rise of 18-59 cms (7-24 inches) by 2100 but did not take account of a possible accelerating melt of Greenland and Antarctica.
Coastal cities from Buenos Aires to New York, island states such as Tuvalu in the Pacific or coasts of Bangladesh or China would be highly vulnerable to rising seas.
"This is a final scientific call for the climate negotiators from 192 countries who must embark on the climate protection train in Copenhagen," Hans Joachim Schellnhuber, Director of the Potsdam Institute for Climate Impact Research, said in a statement.
Copenhagen will host a Dec. 7-18 meeting meant to come up with a new U.N. plan to succeed the Kyoto Protocol beyond 2012. But a full legal treaty seems out of reach and talks are likely to be extended into 2010.
"Delay in action risks irreversible damage," the researchers wrote in the 64-page report, pointing to a feared runaway thaw of ice sheets or possible abrupt disruptions to the Amazon rainforest or the West African Monsoon.
The researchers said global carbon dioxide emissions from fossil fuels were almost 40 percent higher in 2008 than in 1990.
"Carbon dioxide emissions cannot be allowed to continue to rise if humanity intends to limit the risk of unacceptable climate change," said Richard Somerville of the Scripps Institution of Oceanography at the University of California.
In a respite, the International Energy Agency has said emissions will fall by up to 3 percent in 2009 due to recession.
The report said world temperatures had been rising by an average of 0.19 Celsius a decade over the past 25 years and that the warming trend was intact, even though the hottest year since records began in the mid-19th century was 1998.
"There have been no significant changes in the underlying warming trend," it said. A strong, natural El Nino weather event in the Pacific pushed up temperatures in 1998. ((For a TAKE A LOOK about the Road to Copenhagen, click on [ID:nLL527527]. For an overview of climate change stories, click [nCLIMATE])) (For an Interactive factbox, click here) (For Reuters latest environment blogs click on: blogs.reuters.com/environment/ (Editing by Janet Lawrence) ((email@example.com; +47 900 87 663; Reuters Messaging: rm://firstname.lastname@example.org))