The ocean, for instance, absorbs both carbon dioxide and heat, transports heat from one part of the planet to the other and provides moisture to the atmosphere. An even bigger limitation of the models than coarse resolution is the incomplete knowledge of the atmosphere's functioning, Hansen says. Still, the models have been steadily incorporating more and more features of the climate system. One of the most important, in terms of model realism, has been the effect of tiny solid particles and liquid droplets suspended in the atmosphere. The modern economy produces a lot of these aerosols, as they are called, most notably sulfur dioxide droplets emitted by industry.
Aerosols cool the parts of planet by reflecting sunlight.
Before this effect was included in the models, they consistently produced more global warming than was being observed. Afterward, the model representations and the observations agreed more closely. With the aerosol effect factored in, the intergovernmental science panel estimates that the average global temperature will rise by two to six degrees in the next century, with a best estimate of about three and a half degrees, barring cuts in greenhouse emissions. The warming will continue after that, the panel says. The biggest gap in knowledge, many scientists believe, is that the net effect of clouds on the planet's temperature is still unknown.