OSLO Global warming is already happening and people are very likely to blame. Impacts will range from disruptions to catastrophes, but the problem can still be fixed without derailing the world economy.
Those are likely to be the conclusions in a five-page U.N. summary in November boiling down more than 3,000 pages of science in three authoritative reports about global warming already issued this year, say experts involved.
The last report, on costs, was issued in Bangkok on Friday.
"It's difficult to put it all into one crisp sentence," said Coleen Vogel of the University of Witwatersrand in South Africa, one of 32 authors of the synthesis report to be issued in Valencia, Spain.
"Almost nobody will have read all three volumes," said Bettina Menne of the World Health Organisation, another author.
"So this is really where you put everything on the table," she said of a summary meant to tie together knowledge about how burning fossil fuels releases heat-trapping gases, feared impacts such as floods or droughts and costs of action.
The five-page report, and a longer 30-page summary, is likely to be the most read by government policy makers trying to work out ways to step up a fight against climate change beyond 2012 when a first period of the U.N.'s Kyoto Protocol runs out.
But deciding what stays and what gets cut out of the concise versions will be tough.
"The synthesis is meant to be the most policy-relevant document," said Carola Traverso Saibante, spokeswoman of the U.N.'s Intergovernmental Panel on Climate Change (IPCC).
"Integrating the three reports with the aim of giving a policy maker a good tool to act can be tricky," she said.
"Governments have asked for everything to be boiled down to a short summary of the main conclusions," said Peter Stott, of the British Meteorological Office's Hadley Centre and among authors.
The IPCC report issued in Bangkok on Friday said sweeping cuts in greenhouse gases are needed in the next 50 years to keep global warming in check. But it said it need cost only a fraction of world output.
And it said governments have a wide range of technologies available for braking global warming, ranging from solar energy to more controversial nuclear power. The IPCC reports are the first update of science since 2001.
The first report, in February, said it was at least 90 percent probable, or "very likely", that human activities are to blame for global warming, up from at least 66 percent or "likely" in the 2001 report.
The February report also said warming was under way and that temperatures were, at a best estimate, likely to rise by between 1.8 and 4.0 Celsius (3.2 and 7.2 Fahrenheit) this century.
A second report in April outlined impacts such as more hunger and water shortages in Africa and Asia, heatwaves in the United States and a rise in sea levels that could go on for centuries and drown coastal cities.
Menne, a doctor of medicine, said there would be many hard choices in a five-page document. She noted that global warming could have damaging impacts on health.
"For me a sentence like 'children, the aging, the socially deprived will be most at risk' is important. But the same might not be interesting for people looking at long-term trends in ecosystems, energy or economic perspectives," she said.