The 21-page study, due for release in November, lays out possible responses to global warming but cautions that some impacts are already inevitable, such as a gradual rise in sea levels that is set to last for centuries.

The report gives a first overview of 3,000 pages of research by the UN's climate panel already published in three instalments this year about the science, the likely impacts and the costs of slowing climate change.

The authoritative summary, obtained by Reuters and meant to guide governments in working out how to slow warming, reiterates that humans are to blame for climate change but that clean technologies are available to offset the most harmful emissions.

"Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic (from human activities) greenhouse gas concentrations," it says.

"Very likely" means at least 90 percent probability, up from 66 percent in a previous report by the UN's Intergovernmental Panel on Climate Change (IPCC) in 2001 when the link was only judged "likely". The IPCC draws on work by 2,500 scientists.

The report shows a table indicating worsening damage such as bleached corals, coastal flooding, increasing costs of treating disease, deaths from heatwaves and rising risks of extinctions of species of animals and plants.

But it says: "Many impacts can be avoided, reduced or delayed" by cutting greenhouse gas emissions.

FOSSIL FUELS

Among options to offset warming, blamed mainly on greenhouse gases from burning fossil fuels, are energy efficiency, wider use of renewable energies, carbon markets or burying carbon dioxide from coal-fired power plants.

The report indicates that the cost of such initiatives would be manageable for the world economy.

Global gross domestic product (GDP) in 2030 would be reduced by up to 3 percent in the most stringent case that would require emissions to peak within about 15 years. Other less tough goals would mean only a fractional loss of GDP by 2030.

The report will be issued in Valencia, Spain, on Nov. 17 after review by governments, along with an even shorter 5-page summary. The draft is dated May 15 -- an updated version has been written this month to take account of government suggestions, scientists said.

"Warming of the climate is now unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice and rising global mean sea level," the summary begins.

The report reiterates best estimates that temperatures will rise by 1.8 to 4.0 Celsius (3 to 7 Fahrenheit) this century and that sea levels will rise by between 18 and 59 centimetres.

But it says ocean levels are likely to keep rising "for many centuries" even if greenhouse gases are stabilised, because water expands as it heats up. The deep oceans will keep heating up as warmth filters down from the surface.

Under a range of scenarios, such thermal expansion of the oceans alone would bring sea level rises of 0.4 to 3.7 metres in coming centuries, without counting any melting of ice in glaciers or in the vast Greenland or Antarctic ice sheets.