OSLO Scientists meet on Monday to prepare the strongest warning yet that climate change is man-made and will cause more heatwaves, droughts and floods this century unless governments take action.
Officials from up to 195 governments and scientists will meet in Stockholm from September 23-26 to edit a 31-page draft that also tries to explain why the pace of warming has slowed this century despite rising human emissions of greenhouse gases.
The Intergovernmental Panel on Climate Change (IPCC) will present an edited, summary report on Friday as a main guide for governments, which have agreed to work out a United Nations deal by the end of 2015 to avert the worst impacts.
IPCC drafts seen by Reuters say that human activities, primarily the burning of fossil fuels, are "extremely likely" - at least a 95 percent probability - to be the main cause of warming since the 1950s. The likelihood is up from 90 percent in the last report in 2007 and 66 percent in 2001.
"There is high confidence that this has warmed the ocean, melted snow and ice, raised global mean sea level, and changed some climate extremes," the draft says of man-made warming.
Most impacts are projected to get worse unless governments sharply cut greenhouse gas emissions, it says. The report, by 259 authors in 39 countries, is the first of four due in the next year about climate change by the IPCC.
In itself, a shift from 90 to 95 percent "would not be a huge short of adrenalin" for spurring government and public awareness, said Alden Meyer, of the U.S.-based Union of Concerned Scientists.
But he said that extreme weather events, such as a 2010 drought in Russia that pushed up world grain prices, or last year's Superstorm Sandy in the United States, meant that "there is more of a visceral feel for climate change among the public."
Trying to boost weak global economic growth, governments have focused relatively little on climate change since failing to agree a U.N. deal at a summit in Copenhagen in 2009.
The draft says temperatures could rise by up to 4.8 degrees Celsius (8.6 Fahrenheit) this century, but could be held to a rise of 0.3C (0.5F) with deep cuts in greenhouse gas emissions. Governments have promised to limit a rise in temperatures to 2 degrees C (3.6 F) above pre-industrial times.
The range differs from scenarios of 1.1 to 6.4C (2.0-11.5F) gains by 2100 in 2007, largely because of new computer models.
The draft also says that sea levels, which rose 19 cm (7.5 inches) in the 20th century, could rise by an extra 26 to 81 cm towards the end of this century, in a threat to coasts.
That rise is more than was projected in 2007, although that report did not take full account of the risks of a melt in Greenland and Antarctica.
As the main guide for government action, the IPCC will face extra scrutiny after the 2007 report exaggerated the rate of melt of the Himalayan glaciers. A review of the IPCC said that the main conclusions were unaffected by the error.
The draft also seeks to explain why temperatures have not risen much this century. "Fifteen-year-long hiatus periods are common," in historical climate records, an accompanying 127-page technical summary says.
A combination of natural variations and other factors such as sun-dimming volcanic eruptions have caused the hiatus, it says, predicting a resumption of warming in coming years. The report also finds that the atmosphere may be slightly less sensitive to a build-up of carbon dioxide than expected.
(Editing by Robin Pomeroy)