The incoming solar radiation is normally given as an amount of energy per square meter per second averaged over the entire earth. But due to the different distances between the sun and points on the earth’s surface, the forcing is not at all homogeneous around the globe. Approximately 30% of this energy is reflected back to space by clouds, aerosols and the surface of the earth.
Estimate of the Earth’s annual and global mean energy balance, from IPCC, Source: Kiehl and Trenberth (1997)
Due to the reflecting properties of aerosols natural events like volcanic eruptions or aerosol-containing industrial emissions may induce considerable changes to the earth’s energy balance. The remaining two thirds of this energy is absorbed by the earth and, to achieve a balance, emitted back as longwave radiation. If this longwave radiation was directly emitted to space the average surface temperature of the earth would -19C which is 34C less than it actually is. However, only ten per cent of the outgoing surface radiation goes directly into space. This is due to greenhouse gases and some types of clouds that absorb a small part of the surface radiation and reflect more than 80% back to earth. It is the same effect that the glass roof of a greenhouse produces. The most important greenhouse gases are water vapour, existing only for days, and carbon dioxide, which can survive for millenia.