During the twentieth 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. 2004 was the fourth-warmest ever. In descending order of heat the five warmest-ever years were: 1998, 2003, 2002, 2004, 2001. Moreover, 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 thousand years.
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.
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