Clouds won't counter global warming
Don't count on clouds to come and rescue us from global warming, says a NASA researcher, who claims that the minimum amount of warming predicted by scientists should be revised upwards by half a degree Celsius.
Some climate theories predict that a warmer atmosphere would evaporate more water forming thicker and more opaque low clouds over the land which would reflect sunlight and counter the warming effects of greenhouse. However, research published in the Journal of Climate provides some of the first substantial evidence that this is not the case.
"This is very important research because clouds are crucial to our understanding of climate change. They are the biggest contributor to uncertainty and yet how clouds behave is not given much attention," comments Dr Leon Rotstayn of CSIRO's Division of Atmospheric Research.
Dr Anthony Del Genio of NASA's Goddard Insitute for Space Studies in New York City analysed data from 3,000 individual cloud "snapshots" - each measuring the air temperature, thickness of the cloud and the amount of liquid water in it. He found that when air temperatures were higher, clouds were thinner and thus less capable of reflecting sunlight. These thinner clouds occurred regardless of weather conditions.
The Intergovernmental Panel on Climate Change (IPCC) currently estimates that if levels of carbon dioxide doubled temperatures would rise by between 1.5 and 4.5 degrees Celsius. However Genio asserts that when real-world cloud behaviour is taken into account, the minimum amount of warming predicted should be increased to about 2 degrees Celsius.
However, according to some, Genio's study has weaknesses.
"The limitation of the study is that it has only considered how the properties of low cloud change with global warming, but not how high cloud changes," says Rotstayn.
Under current conditions, high cloud has mainly a heat trapping effect, but scientists are uncertain about what will happen under global warming conditions. While Genio's study offers no obvious clues, Rotstayn says preliminary research suggests that, as global temperatures rise, high clouds would increase their heat trapping effect.
Rotstayn is involved in researching yet another lever in the climate change picture - aerosols. These small airborne particles - from both natural and human-made sources - have a localised cooling impact over different areas of the Earth.
"Critics of global warming theory - some with vested interests - have tried to discredit the climate change models on the basis that the observed temperature drops were lower than expected," he says. "But that's because until recently, we only used changes in carbon dioxide levels to calculate temperature change. Once we put in the aerosol data we found the models were in much better agreement with the observed temperatures".
So with aerosols cooling, clouds not providing the protection we thought, and other factors such as changes in solar radiation and ozone, how does Rotstayn see the overall impact on IPCC estimates?
For Rotstayn, it's still just too early to say.
