Scientists examine causes for lull in warming

LONDON/OSLO Thu Feb 25, 2010 10:06am EST

Cars drive through snow on a highway outside Varnamo, southern Sweden, February 22, 2010. REUTERS/Johan Nilsson/Scanpix

Cars drive through snow on a highway outside Varnamo, southern Sweden, February 22, 2010.

Credit: Reuters/Johan Nilsson/Scanpix

LONDON/OSLO (Reuters) - Climate scientists must do more to work out how exceptionally cold winters or a dip in world temperatures fit their theories of global warming, if they are to persuade an increasingly skeptical public.

At stake is public belief that greenhouse gas emissions are warming the planet, and political momentum to act as governments struggle to agree a climate treaty which could direct trillions of dollars into renewable energy, away from fossil fuels.

Public conviction of global warming's risks may have been undermined by an error in a U.N. panel report exaggerating the pace of melt of Himalayan glaciers and by the disclosure of hacked emails revealing scientists sniping at skeptics, who leapt on these as evidence of data fixing.

Scientists said they must explain better how a freezing winter this year in parts of the northern hemisphere and a break in a rising trend in global temperatures since 1998 can happen when heat-trapping gases are pouring into the atmosphere.

"There is a lack of consensus," said Kevin Trenberth, head of the Climate Analysis Section at the U.S. National Center for Atmospheric Research, on why global temperatures have not matched a peak set in 1998, or in 2005 according to one U.S. analysis. For a table of world temperatures: [ID:nLDE6050Y5]

Part of the explanation could be a failure to account for rapid warming in parts of the Arctic, where sea ice had melted, and where there were fewer monitoring stations, he said.

"I think we need better analysis of what's going on on a routine basis so that everyone, politicians and the general public, are informed about our current understanding of what is happening, more statements in a much quicker fashion instead of waiting for another six years for the next IPCC report."

The latest, fourth Intergovernmental Panel on Climate Change (IPCC) report was published in 2007 and the next is due in 2014.

The proportion of British adults who had no doubt climate change was happening had dropped in January to 31 percent from 44 percent in January 2009, an Ipsos MORI poll showed this week.

HOTTEST DECADE ON RECORD

The decade 2000-2009 was the hottest since 1850 as a result of warming through the 1980s and 1990s which has since peaked, says the World Meteorological Organization.

British Hadley Center scientists said last year that there was no warming from 1999-2008, after allowing for extreme, natural weather patterns. Temperatures should have risen by a widely estimated 0.2 degrees Centigrade, given a build up of manmade greenhouse gases.

"Solar might be one part of it," said the Hadley's Jeff Knight, adding that changes in the way data was gathered could be a factor, as well as shifts in the heat stored by oceans.

The sun goes through phases in activity, and since 2001 has been in a downturn meaning it may have heated the earth a little less, scientists say.

"We've not put our finger precisely on what has changed," Knight said. "(But) If you add all these things together ... there's nothing really there to challenge the idea that there's going to be large warming in the 21st century."

Melting Arctic ice was evidence for continuing change, regardless of observed temperatures, said Stein Sandven, head of the Nansen Environmental and Remote Sensing Center in Norway.

"The long-term change for the Arctic sea ice has been very consistent. It shows a decline over these (past) three decades especially in the summer. In the past 3-4 years Arctic sea ice has been below the average for the last 30 years."

Rajendra Pachauri, chair of the IPCC, told Reuters that the IPCC stood by its 2007 findings that it is more than 90 percent certain that human activities are the main cause of global warming in the past 50 years.

"I think the findings are overall very robust. We've made one stupid error on the Himalayan glaciers. I think that there is otherwise so much solid science." The IPCC wrongly predicted that Himalayan glaciers could vanish by 2035.

NATURAL CAUSES?

One long-running doubter of the threat of climate change, Richard Lindzen, meteorologist at the Massachusetts Institute of Technology, said a lull in warming was unsurprising, given an earlier "obsessing about tenths of a degree" in the 1980s and early 1990s.

The world warmed 0.7-0.8 degrees Celsius over the last century. Lindzen expected analysis to show in a few years' time that recent warming had natural causes. "It just fluctuates. I think the best explanation is the ocean. The timescale for ocean circulations can be decades."

He dismissed recent ice melt over a short, 30-year record.

Pachauri said that scientists had to unpick manmade global warming from natural influences -- such as the sun and cyclical weather patterns -- also dubbed "natural variability."

"Natural variability is not magic, there is movement of energy around the climate system and we should be able to track it," said Trenberth.

Trenberth attributed the cold winter to an extraordinary weather pattern not seen since 1977 which had curbed prevailing westerly winds across the northern hemisphere, and said that the underlying cause was "one we don't have answers to."

(For Reuters latest environment blogs, click on: blogs.reuters.com/environment/)

(Editing by Elizabeth Fullerton)