2006: the year that global warming came into sharp focus

It has been a hot year. The average temperature in Britain for 2006 was higher than at any time since records began in 1659. Globally, it looks set to be the sixth hottest year on record. The signs during the past 12 months have been all around us. Little winter snow in the Alpine ski resorts, continuing droughts in Africa, mountain glaciers melting faster than at any time in the past 5,000 years, disappearing Arctic sea ice, Greenland's ice sheet sliding into the sea. Oh, and a hosepipe ban in southern England.

You could be forgiven for thinking that you've heard it all before. You may think it's time to turn the page and read something else. But you'd be wrong. 2006 will be remembered by climatologists as the year in which the potential scale of global warming came into focus. And the problem can be summarised in one word: feedback.

During the past year, scientific findings emerged that made even the most doom-laden predictions about climate change seem a little on the optimistic side. And at the heart of the issue is the idea of climate feedbacks - when the effects of global warming begin to feed into the causes of global warming. Feedbacks can either make things better, or they can make things worse. The trouble is, everywhere scientists looked in 2006, they encountered feedbacks that will make things worse - a lot worse.

Next year, the UN's Intergovernmental Panel on Climate Change (IPCC) will publish its fourth assessment on the scale of the future problems facing humanity. Its last assessment, published in 2001, had little to say on the subject of climate feedbacks, partly because, at that time, they were such an unknown quantity. This year, scientists came to learn a little more about them, and they didn't like what they learnt.

During the past two decades, the IPCC has tended to regard the Earth's climate as something that will change gradually and smoothly, as carbon dioxide and global temperatures continue their lock-step rise. But there is a growing consensus among many climate scientists that this may be giving a false sense of security. They fear that feedback reactions may begin to kick in and suddenly tip the climate beyond a critical threshold from which it cannot easily recover.

Climate feedbacks could turn the Earth into a very different planet over a dramatically short period of time. It has happened in the past, scientists say, and it could easily happen in the future given the unprecedented scale of the environmental changes caused by man.