The world may have little more than a decade to avert catastrophic climate change, politicians and scientists say.
A report by the International Climate Change Taskforce says it is vital that global temperatures do not rise by more than 2C above pre-industrial levels.
Atmospheric carbon dioxide levels that would trigger this rise could possibly be reached in about 10 years or so.
A leading climate scientist has told the BBC he thinks temperatures may be higher than 2C some time this century.
Rapid risk increase
The taskforce was set up by the Institute for Public Policy Research, the Centre for American Progress and the Australia Institute.
"We might end up in the middle of that temperature range, and if we do that wouldn't make very good news" Dr Rajendra Pachauri, IPCC
One of its co-chairs is the UK politican Stephen Byers MP, a former transport secretary.
In its report, Meeting the Climate Challenge, the taskforce urges governments to agree to a long-term objective of preventing global average temperatures exceeding the levels before the Industrial Revolution by more than 2C.
It says: "Beyond the 2C level, the risks to human societies and ecosystems grow significantly."
It says they would involve substantial agricultural losses, widespread adverse health effects and greatly increased risks of water shortage.
Many coral reefs and even the Amazon rainforest could suffer irreversible damage, the report says.
Point of no return
It says: "Above the 2C 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 levels more than 10 metres over the space of a few centuries."
It says the circulation of water in the North Atlantic could also shut down, altering the Gulf Stream which warms north-west Europe.
The report says limiting temperature rise to 2C is likely to mean making sure atmospheric CO2 concentrations do not rise above about 400 parts per million (ppm).
They have already reached about 380 ppm, and have been rising recently at more than 2 ppm annually, meaning the taskforce's threshold could be crossed by about 2015.
Stephen Byers said: "Our planet is at risk. With climate change, there is an ecological time-bomb ticking away, and people are becoming increasingly concerned by the changes and extreme weather events they are already seeing."
Large rise possible
The taskforce's scientific adviser is Dr Rajendra Pachauri, the chairman of the Intergovernmental Panel on Climate Change.
The IPCC predicts that on present trends global sea levels will probably have risen by 9 to 88cm by 2100 and average temperatures will be between 1.5 and 5.5C higher than now. The last Ice Age was only 4-5C colder than today.
Dr Pachauri told the BBC News website: "I think in the last few years the increase in emissions does cause concern.
"It gives you the feeling we might end up in the middle of that temperature range, and if we do that wouldn't make very good news."
The taskforce's other recommendations include:
- the G8 and other major economies, including from the developing world, form a G8+ Climate Group
- G8 governments generate at least 25% of electricity from renewable energy sources by 2025
- governments remove barriers to and increase investment in renewable energy and energy-efficient technologies and practices by taking steps including the phase-out of fossil fuel subsidies.