The G8 summit that gets under way today could be a key step towards a global agreement on climate change, and steer the 25 countries responsible for 80% of carbon emissions on a course to a new treaty to replace Kyoto after 2012.

Yet there is also an enormous danger at Heiligendamm. If the summiteers compromise on what the science is telling us we have to do, or agree to a US-style plan for warm words but little action, the whole trajectory of the talks will go awry. Far from averting dangerous change, we will have decided to inflict incalculable consequences on our own prosperity and - worse - on millions in the developing world.

If there is one leader who personifies that danger, it is Tony Blair. This is his last G8 summit. He has been determined to ensure that it is seen in Britain as his show, even rejecting appeals to take Gordon Brown with him. Who better to forge a compromise between Europe and the US than the midwife of so many of George Bush's other unacceptable policies? Blair wants his legacy - but it must not poison a sound successor to the Kyoto protocol.

The danger was highlighted by Sir David King, Blair's chief scientific adviser, when he reiterated last week that a key goal of the summit should be greenhouse gas stabilisation in the atmosphere. Quite right. But Sir David added ominously that the British position - for 450-550 parts per million (ppm) - would be an implicit weakening of the EU agreement in March and wave goodbye to averting climate chaos.

This range is simply inadequate to stop global warming of more than 2C above pre-industrial levels. Yet two degrees - we are already at 0.7 degrees - is widely recognised as the threshold of unacceptably dangerous change. It could mean the loss of the Greenland and west Antarctic ice sheets, and a rise of seven metres in sea levels - a catastrophe for delta cultures such as Bangladesh and the Netherlands.

For many others, droughts, floods and storm damage will rise sharply. There could be millions of refugees. Thawing permafrost in Alaska, Canada and Russia could lead to large releases of methane, a greenhouse gas four times more powerful than carbon dioxide. Global warming above 2C is a world that human beings should not want to visit.

Last month the UN Intergovernmental Panel on Climate Change found that 445-490 ppm - the lower end of the British position - would lead to warming of 2-2.4 degrees; the upper range would lead to warming of 2.4-2.8 degrees. A paper by the Swiss scientist Malte Meinshausen shows there is an 85% risk of overshooting 2C if we go for the top end of the British range. Even at the bottom end, there is just a 53% chance of staying within 2C.

Thankfully, Angela Merkel, the summit host, is a physicist, and may be the only participant to understand what is at stake. She is the first G8 premier to have been an environment minister, and was her country's Kyoto negotiator.

However, the pressures for global compromise will be intense, not least because Germany itself will only hold the presidency of the EU for another month, and will not host another G8 summit for years. The EU must not relent. If Bush does not move enough - and he is moving - it would be better to isolate the US and wait for a more enlightened administration than agree a trajectory for a new protocol that would be doomed to fail because it is doomed to pursue the wrong objective. You cannot split the difference with a disaster. They do not come in halves.

Chris Huhne MP is the Liberal Democrats' environment spokesman huhnec@parliament.uk

© 2007 The Guardian