No single explanation accounts for all the trends and fluctuations indicated by various types of evidence. Major theories fall into four broad categories incorporating changes in insolation patterns, changes in the atmosphere's content, the Earth's surface features, and human interference. Variations in the energy emitted by the Sun - for example, those resulting from different levels of sunspot activity - also alter the amount (and perhaps the kind) of radiation received by the Earth.
The Milankovitch theory suggests that differences in the shape of the Earth's orbit, the tilt of the polar axis, and the time of year when the Earth is nearest the Sun effect changes in insolation. The periods of continental glaciation known as the ice ages have been linked to these effects by the Milankovitch theory. Substantial evidence now supports this link, although the theory continues to be debated.
Within the atmosphere, fluctuations in the amount and distribution of gases, clouds, and solid particles would be expected to alter the energy budget. Among the natural causes of such fluctuations are volcanic eruptions, as shown by Mexico's El Chichón in 1982 and by the Philippines' Mount Pinatubo in the early 1990s. Events such as comet or large meteor impacts, with resulting large-scale effects on climate, have also been theorized as the cause of several mass extinctions of life forms that have taken place in the Earth's history. Good evidence is accumulating for the validity of this theory.
The Earth's surface exerts immense influence on the heat and moisture budgets. Geologic changes in the size, position, and elevation of the continents have been studied as causes of paleoclimatic change. Changes in reflectivity resulting from shifting patterns of plant cover, water, or ice are other possible factors in climatic variability.
Human activity has the potential of affecting large-scale climate patterns through the introduction of materials into the atmosphere and the depletion of forest cover. Scientists now clearly recognize that the greenhouse effect, mentioned above, is being enhanced by human activities, including the massive burning of fossil fuels and consequent increase of carbon dioxide in the atmosphere. The reduction of the ozone layer by industrial fluorocarbons also has become a serious concern, because the ozone layer serves as a shield against ultraviolet radiation and plays a role in maintaining the Earth's heat balance. The regional effects on climate of large urban centers have also been the subject of research for many years.