Global warming - sometimes called the greenhouse effect - is caused by gases in the atmosphere that absorb and then re-emit infrared radiation, thereby 'trapping' the heat on the Earth instead of allowing it to escape into space. The three most common greenhouse gases are carbon dioxide, water vapour, and ozone. However, some greenhouse gases have larger impacts than others: methane, for example, is 21 times more damaging than carbon dioxide over a 100-year period. Other greenhouse gases include the sulphur hexafluoride used in semiconductor manufacturing, and the methane produced by landfill sites and livestock. Carbon dioxide emissions can be countered by 'carbon sinks', such as trees, but it is difficult to model this process and therefore to predict how successful such measures will be.

Reilly and colleagues analysed the climate, financial and environmental effects of three possible emissions policies until 2100: controls on fossil fuel emissions; a multi-gas target with controls only on carbon dioxide emissions; and controls and targets on multi-gas emissions. They found that there were no significant differences in the impact on the climate and ecosystem of the second and third approaches. Moreover, the multi-gas strategy was cheaper over this time scale and, if new stricter emission targets are set, may also be the only way to meet these targets.