But even the most sophisticated models are only simplified descriptions of Earth's climate system. They leave out some processes that are not well understood, and abbreviate others to keep the size of models within the limits of existing computers. The models have had a tough time reproducing the past century's global average temperatures. Feeding in the measured change in greenhouse gases, most models predicted that the temperature would increase at about twice the observed rate. However, in the last few years, aerosols have been added to the computer descriptions of the climate.
Aerosols reflect sunlight, making the surface cooler.
They account for the lack of warming in the early 1980s, when Mexico's El Chichon volcano erupted, creating a blanket of reflecting particles in the upper atmosphere that persisted for more than a year. Without aerosols, global average temperatures would be even warmer. By including the effects of aerosols, the models show unprecedented agreement with the 100-year historical record. Increased confidence in the model results is the key element behind the IPCC's new findings. Scientists are searching past and present climate data for other ways to improve our confidence in the greenhouse warming models.