Evidence that Earth is heating is incontrovertible. Glaciers and permafrost are melting. Sea level is rising. Deserts are spreading. Growing seasons are getting longer in far northern latitudes. Migratory species arrive at their summer breeding grounds earlier and remain later. Animals, including some of the less-pleasant snakes and disease-carrying insects, are increasing their ranges. Meteorological records show that the past decade has been the warmest on record. So many independent observations from different areas of science make an exceptionally strong case that a real effect is occurring.

Like any other system, temperatures on Earth are governed by a simple heat balance:

(Heat in)–(Heat out) = (Heat retained in system).

Several sources provide heat. These include incoming solar radiation, heat generated by human activity, and heat from decay of radioactive species in the Earth’s interior. Of these, solar radiation dominates, by far. It is estimated that the entire yearly energy needs of all of humankind could be met by capturing and converting all of the solar energy falling on Earth for about 45 minutes. Heat is lost primarily by radiative heat transfer back into space, much in the infrared. The balance between heat coming in, mainly solar energy, and heat going out, mainly infrared radiation to space, maintains the average global temperature. Any change in either term necessarily results in a change in the amount of heat retained, which in turn eventuates in a change in average global temperature. Because temperature has a major role in affecting climate, the net effect is a change in global climate.