The effect is illustrated in the diagram on the right. Incoming solar radiation passes unimpeded through the atmosphere (except for UV, which is mostly stopped by the ozone layer), and warms the earth. The warmed earth then emits heat -- infra-red electromagnetic radiation. Crucially, this radiation has a different wavelength from the incoming solar radiation, and a much larger proportion is trapped by 'greenhouse gases' in the atmosphere -- carbon dioxide (CO2), methane (CH4) and ordinary water vapour (H20). The more of these gases there are in the atmosphere, the more heat is trapped, and the more the earth's surface warms up.

The red line on the graph shows the level of CO2 in the atmosphere, measured in Hawaii, in the middle of the Pacific Ocean. It fluctuates each year, decreasing between March and September as plants grow new leaves in the northern hemisphere, where there is more vegetation than in the south, and rising again after northern autumn leaf fall. The overall rise is due to our burning fossil fuels -- coal, oil and gas.

At the current rate of increase, we will have doubled the pre-industrial level of atmospheric CO2 by 2060. The climate sensitivity is the temperature increase this doubling would lead to. It is hard to estimate how much this will be, because of the complexity of atmospheric processes and the many feedbacks involved -- for example, the melting of snow and ice darkens the polar and high mountain regions, leading to more rapid warming. Other feedbacks may speed up warming or slow it down. Overall, the climate would take a very long time -- perhaps even millennia - to reach a new equilibrium, even if  CO2 levels stabilised.