The strongest scientific warning to date that global warming is man-made may have a muted impact when it is released later this month with many governments more focused on nursing weak economies than on fixing the planet.
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Many are also still smarting from a failure to agree a global pact to fight climate change at a summit in Copenhagen in 2009 and wary of making bold promises under a new timetable meant to agree a global U.N. deal in 2015.
Even so, the planned release of a report by the U.N.'s Intergovernmental Panel on Climate Change (IPCC) in Stockholm on September 26 - the main guide for government action - may mark the start of a revival of attention on global warming.
"Climate politics has been in a coma since Copenhagen," said Yvo de Boer, former head of the U.N. Climate Change Secretariat, who led work at Copenhagen and now works at auditors KPMG. "I really hope the report can jolt us out of that coma."
A draft of the IPCC report seen by Reuters, which still has to be reviewed, raises the probability that climate change since the 1950s is mainly caused by human activities to "extremely likely", or at least 95 percent, from "very likely", or over 90 percent, in the last report in 2007.
Past IPCC reports have spurred U.N. talks with ever stronger warnings that greenhouse gases will cause more floods, droughts, heatwaves and rising seas. The impact may be less dramatic this time, partly because climate science is better known.
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"Climate change is not taking us by surprise any longer," said Achim Steiner, head of the U.N. Environment Programme. "The report as such will not be an awakening moment."
But he said the report, the first of four about the climate in the next year, would underscore risks of inaction.
"I think that will be the focus of the public debate. What are we going to do to catch up with the lost time?" he said.
Among past reports, a 1995 IPCC report that concluded it was more than 50 percent likely that mankind was to blame for climate change helped spur talks that led to the 1997 Kyoto Protocol for cutting emissions by developed nations.
The 2007 IPCC report spurred two years of negotiations that led to a summit in Copenhagen where world leaders failed to clinch a global deal. Governments agreed two years ago to have another try, giving themselves until 2015.
Todd Stern, U.S. President Barack Obama's climate envoy, said there was "substantial political will" around the world to reach a deal in 2015 for limiting emissions and that "whatever comes out of the IPCC will only accentuate that desire."
"It's too early to say," Stern told Reuters when asked if Washington would reaffirm a 2009 pathway for steep cuts in U.S. greenhouse gas emissions of 30 percent below 2005 levels by 2025 and 42 percent by 2030.
The IPCC will face extra scrutiny after errors were found in the 2007 report, led by an exaggeration of the melt of Himalayan glaciers. A review by outside experts found that the main conclusions were unaffected.
Richard Klein, a senior research fellow at the Stockholm Environment Institute, said the economic crisis would overshadow the report but remained hopeful that economic recovery could improve the prospects for a global deal in 2015.
Even so, austerity makes it hard for developed nations to keep a promise of increasing aid to developing countries to a promised $100 billion a year by 2020, from $10 billion in 2012.
"An age of austerity has slowed down the international sources of climate finance which are part of a grand bargain," said Rachel Kyte, vice president of the World Bank for sustainable development.
Even though climate change has taken a back seat to the financial crisis, opinion polls indicate it is a top concern.
In a poll by the Pew Research Center in June, 54 percent of almost 40,000 respondents worldwide rated climate change a "major threat", higher than percentages for other worries led by international financial instability and Islamist extremism.