First, the basic physics. It is beyond doubt that certain gases in the atmosphere, most importantly water vapour and carbon dioxide, trap infrared radiation emitted by the Earth's surface and so have a greenhouse effect. This in itself is no bad thing. Indeed, without them the planet would freeze. There is also no doubt that human activity is pumping CO2 into the atmosphere, and that this has caused a sustained year-on-year rise in CO2 concentrations. For almost 60 years, measurements at the Mauna Loa observatory in Hawaii have charted this rise, and it is largely uncontested that today's concentrations are about 35 per cent above pre-industrial levels (see Graph).
The effect this has on the planet is also measurable. In 2000, researchers based at Imperial College London examined satellite data covering almost three decades to plot changes in the amount of infrared radiation escaping from the atmosphere into space - an indirect measure of how much heat is being trapped. In the part of the infrared spectrum trapped by CO2 - wavelengths between 13 and 19 micrometres - they found that between 1970 and 1997 less and less radiation was escaping. They concluded that the increasing quantity of atmospheric CO2 was trapping energy that used to escape, and storing it in the atmosphere as heat. The results for the other greenhouse gases were similar.
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