The warning was made by the Intergovernment Panel on Climate Change (IPCC), gathering climatologists, physicists, oceanographers, biologists and farming experts.

They issued it in a "Summary for Policymakers," a resume of the experts' latest assessment of the global warming phenomenon.

"Stabilization of CO2 (carbon dioxide) concentrations at any level requires eventual reduction of global CO2 net emissions to a small fraction of the current emission level," the document released here said.

"The lower the chosen level for stabilization, the sooner the decline in global net CO2 emissions needs to begin."

Carbon dioxide is the biggest culprit among six "greenhouse gases" blamed for causing a slow but possibly disastrous rise in the Earth's temperature.

Most of the gases are byproducts from burning carbon-based fossil fuels such as coal, natural gas and oil. They cause the Sun's heat to build up in the Earth's atmosphere, rather than let it radiate safely back into space.

CO2 levels in the atmosphere last year were estimated at 368 parts per million (ppm).

UN countries have adopted an agreement, the Kyoto Protocol, that aims to reduce global output of greenhouse gases by rich industrialized countries by 5.2 percent by a timeframe of 2008-2012 compared to their 1990 levels.

However, ecologists say the figure falls far short of what is needed, and many doubt whether it can be achieved at all.

Kyoto was weakened by a US pullout earlier this year, and its provisions were then diluted at the demand of Australia, Canada and Japan, which exacted concessions to help soften the cost of meeting their emissions targets.

The IPCC summary stood by estimates, made in a mammoth assessment report issued earlier this year, that the Earth has warmed between 0.4 to 0.8 Cto 1.4 F) since 1860, the highest increase in a millennium, with the last two decades being the warmest of the century.

It projects a further rise of between 1.4 and 5.8 C (2.5 to 10.4 F) from 1990-2100, causing a rise in sea levels through melting polar ice and thermal expansion of between eight and 88 cms (five and 55 inches).

The summary gloomily spelled out the inertial effect of global warming, saying that even if action began today on tackling greenhouse gases, climate change was inevitable.

"Reductions of greenhouse gas emissions, even stabilization of their concentration in the atmosphere at a low level, will neither altogether prevent climate change or sea-level rise, nor altogether prevent their impacts."

In northerly climates, warmer weather could help crop production, but in tropical latitudes, water stress could worsen and hit agriculture. Other likely casualties will be biodiversity, air and water quality, glaciers and the polar icecaps.

"This confirms the urgency of the threat and increased risks from climate change," said Bill Hare, Greenpeace International's climate policy campaign director.

"The only way we can avoid the scenarios predicted is for governments around the world to agree to substantially reduce their greenhouse gas emissions."

###