Circumstantial evidence for human involvement in global warming is easy to find. Due to the burning of fossil fuels such as coal and oil, the amount of carbon dioxide in the atmosphere has steadily increased for at least 150 years. The greenhouse effect explains how, other factors remaining equal, an increase of certain gases such as carbon dioxide will lead to higher temperatures at the Earth's surface. The century-long upward trend in temperature contains numerous wiggles and bumps. The record is punctuated by a 10-year cooling episode in the 1950s, and a leveling off in the early 1980s.
Observed temperatures represent the combined effects of human-induced and natural processes.
Ground cover, which determines the amount of sunlight the surface will absorb, also affects the temperature. Atmospheric aerosols are involved as well. Such models take into account as much as possible about the way the atmosphere, oceans, land surface, and biosphere function. Information about changing conditions, such as the varying amount of atmospheric carbon dioxide, is put into the model. The model then calculates the resulting temperature and other climatic consequences.