The countdown to climate-change catastrophe is spelt out by a task force of senior politicians, business leaders and academics from around the world - and it is remarkably brief. In as little as 10 years, or even less, their report indicates, the point of no return with global warming may have been reached.

The report, Meeting The Climate Challenge, is aimed at policymakers in every country, from national leaders down. It has been timed to coincide with Tony Blair's promised efforts to advance climate change policy in 2005 as chairman of both the G8 group of rich countries and the European Union.

"There is an ecological timebomb ticking away," said Stephen Byers, the former [UK] transport secretary, who co-chaired the task force that produced the report with the US Republican senator Olympia Snowe. It was assembled by the Institute for Public Policy Research in the UK, the Centre for American Progress in the US, and The Australia Institute.The group's chief scientific adviser is Dr Rakendra Pachauri, chairman of the UN's Intergovernmental Panel on Climate Change.

The report starkly spells out the likely consequences of exceeding the threshold. "Beyond the 2 degrees C level, the risks to human societies and ecosystems grow significantly," it says.

"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."

It goes on: "Above the 2 degrees level, the risks of abrupt, accelerated, or runaway climate change also increase. The possibilities include reaching climatic tipping points leading, for example, to the loss of the West Antarctic and Greenland ice sheets (which, between them, could raise sea level more than 10 metres over the space of a few centuries), the shutdown of the thermohaline ocean circulation (and, with it, 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."

The report urges all the G8 countries to agree to generate a quarter of their electricity from renewable sources by 2025, and to double their research spending on low-carbon energy technologies by 2010. It also calls on the G8 to form a climate group with leading developing nations such as India and China, which have big and growing CO2 emissions.