A self-perpetuating cycle, triggered by global warming, has appeared in weather data gathered from 1952 to 2007 over a 3-million-square-kilometre expanse of ocean off Mexico, Amy Clement of the University of Miami and colleagues wrote in this week’s edition of Science.
Clouds, like forest fires and Arctic permafrost, are studied for their potential to amplify warming. There is still enough mystery in how they function that scientists have said it’s difficult to produce a consensus forecast for temperatures, which in turn may determine the severity of future storms, droughts and Arctic ice melting.
“There’s a wide range of predicted warming for the 21st century,” Clement, a professor of meteorology and oceanography, said in a podcast on the Science website. “‘This study indicates that perhaps we should be giving serious consideration to the high range of future warming.”
Trying to stem climate change, a 17-member group of the Earth’s most polluting nations including the US and China agreed this month to limit the global average temperature increase to no more than 2 degrees Celsius above pre-industrial times. They couldn’t agree on targets for reducing greenhouse-gas emissions, leaving that for climate treaty talks that are set to conclude in December.
The average global temperature already has risen by about 0.8 degrees, and more than double that in the Arctic.
The researchers studied 18 models used by the United Nations’ Intergovernmental Panel on Climate Change. They found only one model, developed by the British Met Office, accurately predicted changes observed in low-level clouds.
The British model projects a 4.4-degree warming in a scenario in which the concentration of carbon dioxide in the atmosphere doubles. That forecast was the highest increase among the various models, Professor Clement said.
The latest IPCC report estimated a range of 2 to 4.5 degrees with a doubling of carbon dioxide. Carbon dioxide in the atmosphere amounts to about 385 parts per million molecules of air, up from about 280 parts per million at the beginning of the industrial age.
The scientists cautioned that their study is not conclusive, and that more work needs to be done to develop better models to simulate the effect of cloud cover on the climate system. “‘Relying on one model is a shaky platform,” Professor Clement said.
Water vapor in clouds also functions like carbon dioxide in keeping heat trapped in the earth’s atmosphere, while at the same time reflecting solar radiation, producing a net effect that’s difficult to calculate.
Clouds actually may cause much of global warming as part of an ocean- and air-current cycle called the Pacific Decadal Oscillation that has increased northern hemisphere temperatures, said former National Aeronautics and Space Administration scientist Roy Spencer.
He said on his website that the earth might be at the end of a cycle that started in 1977 and will enter a new phase that contributes to cooling.
A “‘small change”’ in global average cloudiness could cause global warming, Spencer said, adding that observations have not been complete or accurate enough to document such changes.
“‘The big problem is trying to understand the roles of clouds in climate change,” Professor Clement said. “‘Carbon dioxide alone would amount to just a modest warming of the earth. The big issue is whether feedback from the climate system amplifies or dampens the warming.”
