The International Climate Change Taskforce said damage to the planet could increase at an accelerating rate unless governments set a goal of limiting the rise in temperature since pre-industrial times to 2.0 Celsius (3.6F), including gains of 0.6C so far.
"Our planet is at risk. With climate change there is an ecological time bomb ticking away," said Stephen Byers, a British member of parliament for the ruling Labour party and co-chair of the report with US Republican Sen. Olympia Snowe.
"Urgent action is required," he said.
A rise in temperatures above 2C, likely in coming decades, could be a threshold that triggers "substantial agricultural losses, greatly increased numbers of people at risk of water shortages, and widespread adverse health impacts," the report said.
It urged British Prime Minister Tony Blair, holding the presidency of the Group of Eight industrial nations in 2005, to set up a climate group to encourage both rich and poor nations to do much more to limit greenhouse gas emissions.
The G8 climate panel "should engage the United States and major developing countries in action to reduce greenhouse gas emissions," it said. The report was due for publication in full on Tuesday.
In 2001, US President George W. Bush pulled out of the Kyoto protocol, the main UN scheme for reining in greenhouse gases. He said it was fatally flawed, would cost US jobs and wrongly excluded developing nations from a first round.
US G8 OUTSIDER ON KYOTO
All other G8 members have signed up for Kyoto, which will enter into force on Feb. 16. The United States, the main source of greenhouse gases, is focusing on braking a rise in emissions but stops short of Kyoto-style caps.
The climate change taskforce urged G8 nations to set a goal of generating at least 25 percent of their electricity from renewable sources by 2025 and to set up markets to trade in greenhouse gas emissions, as the European Union is doing.
It also said that rich nations should shift farm subsidies from food crops to biofuels.
Most scientists say that human burning of fossil fuels in cars, factories and power plants is releasing carbon dioxide and other gases into the atmosphere where they trap heat.
They say that rising temperatures may cause more floods, heatwaves, droughts and a rise in sea levels. A minority of scientists say natural factors like variations in solar radiation may explain global warming.
A tiny step to limit rising temperatures, the Kyoto protocol aims to cut emissions of carbon dioxide by developed states by 5.2 percent below 1990 levels by 2008-12. The report said Washington should be encouraged to rejoin Kyoto after 2012.
The report also said a target of limiting the rise in temperatures to 2C corresponded to carbon dioxide concentrations of about 400 parts per million (ppm), compared with 379 ppm in early 2004 and 280 ppm in 1750 before the Industrial Revolution.
"Concentrations of carbon dioxide alone are likely to rise above 400 ppm in coming decades," it said.
The report was compiled by the British Institute for Public Policy Research, the Center for American Progress and the Australia Institute. Rajendra Pachauri, the head of the UN climate panel, was scientific adviser.