The greenhouse effect The sun provides radiant energy to the earth. On the surface this energy is absorbed and re-emitted as longer wavelength infrared radiation. Some of this radiation escapes the atmosphere but the majority is absorbed by molecules of atmospheric gases (particularly carbon dioxide and water vapour) and is reradiated back towards the earth, warming the surface. These processes, collectively known as the ‘greenhouse effect’, are all parts of the natural climatic cycle which maintains the steady state of the global ecosystem. They are also processes of great importance to life on earth, for if the infrared radiation from the earth were not trapped in the atmosphere in this way the average surface temperature of the earth would not be around +15°C but rather around –15°C: a chilly prospect. In the years following the Second World War, concern began to mount among scientists that the burning of fossil fuels might be contributing carbon dioxide to the atmosphere in sufficient quantities to affect the earth’s temperature. If so, this would represent an anthropogenic enhancement of the natural warming effect. In observations now regarded as classic, advanced air monitoring instruments atop the Mauna Loa volcano in Hawaii demonstrated that atmospheric carbon dioxide concentrations were indeed increasing over time; and sampling of carbon dioxide trapped in air bubbles in drill cores from below Antarctic ice sheets showed that rises and falls in the atmospheric concentration of carbon dioxide were synchronous with warm and cold eras in the earth’s history (Weart 2003). But atmospheric scientists were in for a major shock. In unrelated investigations it was discovered that minor industrial gases called CFCs, even at very low concentrations, could single-handedly destroy the earth’s protective ozone layer in the stratosphere; and further, these gases could be over 10,000 times more efficient as absorbers of infrared radiation than was carbon dioxide itself. Subsequent investigations showed that there were also other gases and families of gases that were far more efficient absorbers of infrared radiation than carbon dioxide, and which could have significant effects on global temperatures despite the very low concentrations in which they might occur. Prominent in this respect were the CFCs, methane, and nitrous oxide.