What are Greenhouse Gases?
Just like the glass on a greenhouse, a blanket of water vapour and other 'greenhouse gases' - notably carbon dioxide, methane and nitrous oxide - trap some of the sun's radiation as it bounces off the surface of the earth.
This natural greenhouse effect has enabled life to develop in all its complexity, without it the planet would be frozen solid. But since the industrial revolution, the amount of greenhouse gas has risen. As a result more heat has been trapped, causing a warming of the atmosphere.
During the 20th Century, global atmospheric temperatures have risen by about 0.7°C. However, since the mid-1970s the rate of warming has tripled. The last decade has seen nine of the warmest ten years since records began in 1861. In fact, 2004 was the fourth-warmest ever. The five warmest-ever years were, in descending order, 1998, 2003, 2002, 2004, 2001. And new evidence - compiled from 'proxy data' like ice cores and tree rings - suggests that temperatures are now higher than they have been for over a 1,000 years.
At the same time, ever greater amounts of the key greenhouse gas carbon dioxide have been pumped into the atmosphere by the burning of fossil fuels and the destruction of forest cover. Over the past two centuries, CO2 concentrations in the atmosphere have risen by a third, to levels unmatched in at least 420,000 years, and probably in the last 20 million years. This shows that we have pushed our atmosphere outside natural cyclical limits, and the climate is unlikely to remain stable as a result.
Another highly significant greenhouse gas is methane, which traps heat 30 times more efficiently than carbon dioxide. Over the past two centuries, methane concentrations in the atmosphere have more than doubled. A fifth of methane emissions today come from the cultivation of rice. Pipeline leaks, the flatulence of cattle and termites and forest fires also contribute to rising methane levels.
Significant synthetic chemicals which contribute to global warming include halons like the chlorofluorocarbons (CFCs) and long-lived gases like sulphur hexafluoride. Some of these synthetic gases are several thousand times more powerful as greenhouse gases than CO2 - and last almost indefinitely.