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OSLO (Reuters) - Before a summit on climate change in Paris next week, many governments are citing scientific studies indicating that their plans to curb greenhouse gas emissions until 2030 will come within 0.7 degrees Celsius of an agreed 2C (3.6 Fahrenheit) target for limiting global warming this century.
Yet the studies they choose to quote are only the most optimistic of a range of projections, and presume that governments will go on to make even deeper emission cuts after 2030, which is far from certain.
With no action, a U.N. scientific panel estimates that the global average surface temperature in 2100 will be around 4.8C (8.6F) above pre-industrial times, dramatically increasing the frequency of extreme weather events and raising the sea level.
To avoid the worst of these effects, a ceiling of 2C has been agreed, and about 170 governments have submitted national plans before the Nov. 30-Dec. 11 summit to curb emissions from 2020-30.
Keen to show their policies will work, many cite two estimates that the pledges so far could limit the rise to 2.7C (4.9F).
U.S. Climate Envoy Todd Stern mentioned 2.7C in testimony to a Senate sub-committee last month, saying national policies marked "a powerful move in the right direction".
Christiana Figueres, the head of the U.N. Climate Secretariat, summed up the national plans in a report last month by saying they "have the capability of limiting the forecast temperature rise to around 2.7 degrees Celsius by 2100".
Yet Bill Hare, one of the scientists behind Climate Action Tracker (CAT), a group of four European institutes that first estimated 2.7C, said promises for action until 2030 "mark progress, but current policies are far from enough".
He said the CAT estimate required all countries to continue deeper curbs on emissions right up to 2100 - far stricter than the assumptions by most other research institutes.
The International Energy Agency also estimates an increase of 2.7C. But projections by at least 10 research groups range up to a rise of 3.7C (6.7F).
Thomas Spencer, of the Institute for Sustainable Development and International Relations in France, noted that there were huge uncertainties in all projections beyond 2030:
"It's like trying to predict the winner of a marathon after only the first 10 km."
Bjorn Lomborg, head of the Copenhagen Consensus Center who won fame with his 2001 book "The Skeptical Environmentalist", reckons current national plans will only make a fraction of a degree of difference to warming this century.
"It's like saying Greece is on track to solve its debt crisis after paying a first instalment of a loan," he said.
This year is on track to be the warmest on record, already about 1.0C (1.8F) above pre-industrial times.
Andrew Jones of U.S.-based experts Climate Interactive, which estimated with MIT Sloan that the existing pledges put the globe on track for 3.5C (6.3C) of warming by 2100, said 94 percent of the difference with CAT hinged on less optimistic projections about what happens after 2030.
Climate Interactive reckons, for instance, that overall greenhouse gas emissions from China, the world's biggest emitter, will rise after 2030, while CAT says they will fall.
(This story has been refiled to remove erroneous conversion of 2C in paragraph 4.)
Reporting By Alister Doyle; Editing by Kevin Liffey