Clouds and Climate Change

Justin Gillis has a nice summary in the Times of the debate over the role of clouds in climate change:

On balance, in today’s climate, clouds cool the earth. Dense, low-lying clouds are responsible for most of that effect, because they reflect considerable sunlight back to space. Many high, thin clouds have the opposite influence, allowing incoming sunshine to pass through but effectively trapping heat that is trying to escape. “It’s like putting a lid on a pot on the stove,” said Andreas Muhlbauer, a cloud researcher at the University of Washington.

Humans are perturbing the earth’s heat balance by releasing greenhouse gases. Chemists proved in the 19th century that these gases, especially the carbon dioxide that results from burning fossil fuels, work like an invisible blanket in the atmosphere, blocking some heat that is attempting to escape to space. In the mid-20th century, as it became clear how fast carbon dioxide levels were rising, some scientists began to predict a warming of the planet. But they also realized that an exact forecast was difficult for several reasons, especially the question of how clouds would react.

Researchers are virtually certain the amount of water vapor in the atmosphere will rise with temperature, and evidence suggests this is already happening. But that does not say much about the type or location of clouds that will condense from the vapor. Scientists use sophisticated computer programs to forecast future climate, but the computers are not yet powerful enough to predict the behavior of individual clouds across the whole earth over a century, which forces the researchers to use rough approximations. The most elaborate computer programs have agreed on a broad conclusion: clouds are not likely to change enough to offset the bulk of the human-caused warming. Some of the analyses predict that clouds could actually amplify the warming trend sharply through several mechanisms, including a reduction of some of the low clouds that reflect a lot of sunlight back to space. Other computer analyses foresee a largely neutral effect. The result is a big spread in forecasts of future temperature, one that scientists have not been able to narrow much in 30 years of effort. 

This uncertainty has been seized on by people who don't want to worry about global warming. Nothing to fear, they say; cloud cover will serve as a buffer for any warming, keeping the climate more or less steady. The problem with this isostatic model is that the global temperature has historically tracked the level of atmospheric CO2 fairly closely:

So while clouds may buffer the effect of high CO2 concentrations to some degree, they do not prevent greenhouse warming. If we keep burning fossil fuels, the planet is going to get warmer; the only question is how much warmer.