LONDON - The global warming danger threshold for the world has been clearly marked out for the first time in a report to be published tomorrow - and the bad news is, the world has nearly reached it already.

The climate can barely afford a 1C rise in average temperatures before massive climate changes hit the planet.

These could include widespread agricultural failure, major droughts, increased disease, sea-level rises and the death of forests, the melting of the Greenland ice sheet and West Antarctica and the switching-off of the North Atlantic Gulf Stream.

A task force of senior politicians, business leaders and academics spell out the warning in the report Meeting The Climate Change Challenge - and it is remarkably brief.

In as little as 10 years, the report says, the point of no return on global warming may have been reached.

This point will be 2C above the average world temperature prevailing in 1750 before the industrial revolution, when human activities - mainly the production of waste gases such as carbon dioxide (CO2) - first started to affect the climate.

But it points out that global average temperature has already risen by 0.8C since then, with more rises already in the pipeline - so the world has little more than a single degree of temperature latitude before the crucial point is reached.

More ominously still, the report says a 400 parts per million concentration of CO2 in the atmosphere will make that two-degree rise inevitable - and the level is already 379ppm and rising at 2ppm every year.

"There is an ecological time bomb ticking away," said Stephen Byers MP, the former British Transport Secretary who co-chaired the report with American Republican Senator Olympia Snowe.

The report makes clear that, although global warming's effects may seem distant, time is actually very short and it is action taken - or not taken - in the next few years which will be decisive for climate change this century and beyond.

The authors urge all countries in the G8 group of rich nations to generate a quarter of their electricity from renewable sources by 2025, and to double their research spending on low-carbon energy by 2010.

The study also calls on the G8 to form a climate group with leading developing nations such as China.

But its major impact will be in linking the twin climate danger thresholds of a 2C temperature rise and of the 400ppm concentration of carbon dioxide.