The Earth's thermostat is a complex and delicate mechanism, at the heart of which lie the greenhouse gases in the atmosphere. Carbon dioxide (CO2), a colourless and odourless gas, is the principal well-mixed greenhouse gas. It is through emissions of this gas that human activities exert their greatest influence on climate. Increased concentrations of carbon dioxide disturb the natural radiative balance of the atmosphere and lead to warming of the Earth's surface. The latest report from the Intergovernmental Panel on Climate Change, the Fourth Assessment Report (2007), has confirmed the assertion that “warming of the climate system is unequivocal” and that most of the observed 20th century increase in globally averaged temperatures is “very likely” due to the observed increases in anthropogenic greenhouse gas concentrations. A discernable human influence on the climate system is now apparent and extends to oceanic warming, temperature extremes and wind patterns. Concentrations of the main greenhouse gases (GHGs) are higher than at any time in the past 650,000 years. CO2 concentrations are presently over 380 ppmv (parts per million volume) compared with pre-industrial levels of 280ppm, while methane concentrations have already doubled from their pre-industrial values. In the absence of strict emissions controls, a doubling of atmospheric concentrations of CO2 is likely by the end of the present century. As a consequence, global temperatures are projected to increase by between 1.8 to 4oC over the same period depending on the climate sensitivity to increased levels of GHG.