The world's target for stopping global warming should be based on the point at which the melting of the great Greenland ice sheet becomes irreversible, says the Government's chief scientific adviser, Sir David King.

The loss of Greenland's ice would be a global catastrophe, raising sea levels by more than 20ft, swamping vast regions of low-lying land from East Anglia to Bangladesh.

The international community must limit the atmospheric level of the principal greenhouse gas, carbon dioxide (C02), to below the point where the Greenland ice begins to melt in a runaway manner, Sir David said. This figure is not yet precisely known - but much scientific effort is being expended on finding it out.

Sir David is one of the world's most influential voices on climate change and his suggestion may provide a basis for eventual agreement on one of climate change's thorniest questions: exactly where must the rise in atmospheric C02 - which has gone from 315 parts per million in 1958 to 382ppm today - be halted?

The world community now agrees that the waste gas from motor vehicles and power stations is causing the atmosphere to warm rapidly. But it cannot agree on a precise figure which should be the absolute limit allowable to prevent global disaster.

Some commentators have suggested the C02 level must be halted at 400ppm - but that is now likely to be reached within 10 years and seems impossible to achieve in practice. Sir David has previously suggested 550ppm as "realistic" - but drew criticism for not being more ambitious.

His new suggestion, however, takes a different approach, pinpointing an undeniable disaster level and making that the target - whatever it turns out to be.

Greenland's "tipping point" is not yet known in terms of atmospheric C02 levels, although in terms of temperature it is assumed to be somewhere beyond a global rise of 3C above the level pertaining before the industrial revolution. (Global temperatures currently stand at about 0.7C above pre-industrial, and are steadily climbing). Scientists are seeking it with supercomputer mathematical models of the climate and of the ice mass.