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.

Perversely, although preventing "dangerous" climate change is the principal objective of the UN climate treaty signed in 1992, no-one has yet defined what dangerous actually is, and next week the British Government is hosting a major scientific conference to try to do so.

The report - based on an extensive review of the current scientific literature - will be widely noticed and its core message taken on board.

"What this underscores is that it's what we invest in now and in the next 20 years that will deliver a stable climate, not what we do in the middle of the century or later," said Tom Burke, a leading environmental adviser to business.

The report's influence will be enhanced further by the seniority of the authors, assembled by the Institute for Public Policy Research in Britain, the Centre for American Progress in the US, and the Australia Institute.

They included leaders from the political, business, academic and environmental communities in developed and developing countries.

Their chief scientific adviser was Dr Rakendra Pachauri, the chairman of the United Nations' Intergovernmental Panel on Climate Change, who in effect is the world's senior climate scientist.

It has been timed to coincide with British Prime Minister Tony Blair's promise to advance climate change policy this year, as chair of the G8 group and European Union.

The report starkly spells out the likely consequences of exceeding the global warming threshold:

"Beyond the 2 degrees C level, the risks to human societies and ecosystems grow significantly.

"It is likely, for example, that average-temperature increases larger than this will entail substantial agricultural losses, greatly increased numbers of people at risk of water shortages, and widespread adverse health impacts.

"[They] could also imperil a very high proportion of the world's coral reefs and cause irreversible damage to important terrestrial ecosystems, including the Amazon rainforest.

"Above the 2 degrees level, the risks of abrupt, accelerated, or runaway climate change also increase.

"The possibilities include the loss of the West Antarctic and Greenland ice sheets (which, between them, could raise sea level more than 10 metres), the shutdown of the Gulf Stream and the transformation of the planet's forests and soils from a net sink of carbon to a net source of carbon."