A United Nations panel of experts met on Monday to review a draft report that raises the probability that climate change is man-made to 95 percent and warns of ever more extreme weather unless governments take strong action.
Scientists and officials from more than 110 governments began a four-day meeting in Stockholm to edit and approve the 31-page draft that also tries to explain a "hiatus" in the pace of global warming this century despite rising greenhouse gas emissions.
The Intergovernmental Panel on Climate Change (IPCC) will go through the document line by line and present it on Friday as a main guide for governments, which have agreed to work out a United Nations deal by the end of 2015 to fight global warming.
"I expect the world will understand the simplicity and the gravity of the message that we provide," Rajendra Pachauri, chairman of the IPCC, said after the opening session.
Climate change "will transform our lives, our economies and indeed the way our planet will function in the future," Achim Steiner, head of the U.N. Environment Programme, told delegates.
A shift towards a greener economy, based on renewable energies, would hold multiple benefits for society, he said.
Human role more certain
IPCC drafts seen by Reuters say human activities, primarily burning fossil fuels, are "extremely likely" — at least a 95 percent probability — to be the main cause of global warming since the 1950s.
That is up from "very likely", or at least a 90 percent probability, in the last report in 2007 and 66 percent in 2001, draining hopes that natural variations in the climate might be the cause.
"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 cut greenhouse gas emissions sharply, 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.
Temporary warming slowdown
One of the hardest issues for the IPCC may be accounting for why temperatures have not risen much since the year 2000. "Fifteen-year-long hiatus periods are common," in historical climate records, an accompanying 127-page technical summary says.
A combination of natural variations, including a cyclical dip in energy emitted by the sun, and factors such as volcanic eruptions — which send ash into the atmosphere and help block sunlight — have caused the hiatus, it says, predicting a resumption of faster 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.
Thomas Stocker, a scientist from the University of Bern who is co-chair of the U.N. panel, urged delegates to produce a clear document "with no compromises to scientific accuracy."
The draft says temperatures could rise by up to 4.8 degrees Celsius this century, but could be held to a rise of 0.3C with deep cuts in greenhouse gas emissions. Governments have promised to limit a rise in temperatures to 2 degrees C above pre-industrial times.
The range differs from scenarios of 1.1 to 6.4 C gains by 2100 in 2007, largely because of new computer models.
Higher sea level predictions
The draft also says sea levels, which rose 19 cm in the 20th century, could rise by an extra 26 to 81 cm towards the end of this century, threatening coasts.
That rise is more than was projected in 2007, although that report did not take full account of melting ice in Greenland and Antarctica.
The report is the main guide for governments planning action against global warming and steps to mitigate its effects. It 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.
Environmentalists called for quick action. Greenpeace said governments should heed the report and shift to clean energies. The WWF's Samantha Smith said: "The natural world is sending a distress signal and we're ignoring it at our own peril."