"The urgency of the situation means tough decisions need to be taken now to provide disincentives to carbon emissions," said Martin Rees, president of Britain's Royal Society.

"Much can then be done through positive changes such as developing new technology or making more efficient use of the systems already in place," he added.

Scientists said earlier this year average temperatures would rise by between 1.8 and 4.0 degrees Celsius this century due to burning fossil fuels for power and transport, causing floods, famines and storms and putting millions of lives at risk.

Germany, which this year has the presidency of the G8, is trying to push though a strongly-worded statement from the June 6-8 summit insisting that action must be taken to limit the rise this century to just two degrees above pre-industrial levels.

CARBON TRADING

It also wants the meeting to agree to cut climate-warming carbon emissions to 50 percent below 1990 levels by 2050 and promote carbon trading as a way of raising the price of emissions and thereby boosting clean technologies.

But the United States, supported by Canada, is adamant there should be neither targets nor timetables and rejects the call for global trading in carbon allowances as tacitly endorsing carbon emission limits.

The leaders of China, Mexico, India, Brazil and South Africa will also be at the meeting.

"Meeting the challenge of climate change demands cooperative action by all the G8 countries -- along with India, China, and other rapidly-developing nations," Rees said, noting that even two degrees would be a hard target to meet.

The Kyoto Protocol is the only global accord on cutting so-called greenhouse gas emissions, but it lapses after 2012 and was rejected by the United States in 2001 as economic suicide because it is not binding on booming emitters China and India.

China, expected to overtake the United States within a year as the world's biggest polluter, is building a coal-fired power station every four days on average to fuel its rapidly gowing economy.

Germany, backed by Britain, hopes the summit will give a boost to efforts to open serious negotiations on a successor to Kyoto -- expanding its scope and extending its life.