* More special reports urged in shift from blockbusters
* Big studies time-consuming, often out of date
By Environment Correspondent Alister Doyle
OSLO, Sept 24 (Reuters) - Climate experts on a U.N. panel should focus more on shorter reports on specialist subjects such as extreme weather in a shift from sweeping overviews of the kind being prepared this week in Stockholm, many scientists and governments say.
The big studies about global warming, produced every six or seven years by the Intergovernmental Panel on Climate Change (IPCC), are authoritative but are time-consuming and in some cases are quickly out of date.
"A blockbuster every six years is no longer really helpful," said professor Myles Allen of Oxford University and among the authors who contributed to an IPCC summary of the findings that is due for presentation in Stockholm on Friday.
Many experts instead favour more frequent and targeted reports, for instance about droughts, floods and heatwaves in the preceding year, to see if climate change is influencing their frequency or severity.
A focus for special reports could be food production in a changing climate, the prospects for geoengineering - for instance, projects to dim sunlight - or the risks of irreversible changes such as a runaway melt of West Antarctica.
The IPCC is working on three overview reports totalling about 3,000 pages, starting with a 31-page draft summary of the science of climate change due to be released in Stockholm on Friday after four days of editing by governments and scientists.
A big strength of the IPCC is that its assessments of the climate are approved both by scientists and by governments - giving the findings broad acceptance in negotiations on a U.N. deal to fight climate change, due to be agreed by 2015. Possible reforms will be discussed at talks in Georgia in October.
"I support the global assessment cycle, but would strongly argue for the need to complement it with frequent updates," said Johan Rockstrom, director of the Stockholm Resilience Center.
Drafts of the Stockholm report show that the IPCC is set to raise the probability that most climate change since the 1950s is man-made to "extremely likely", or at least 95 percent, from "very likely" or 90 percent, in 2007.
Many nations including the United States, in submissions this year to the IPCC about reforms, also argue for more special reports. In recent years the IPCC has produced reports on extreme weather and on renewable energies.
Britain suggests using Web-based "wiki" type tools that could allow more frequent updates. Italy says that there is "no automatic need" for another blockbuster report about the science of climate change, like the one in Stockholm.
One problem is that IPCC assessments are quickly out of date. Scientists trying, for instance, to account for a "hiatus" in the pace of global warming this century are only allowed to consider peer-reviewed literature from before mid-March 2013.
Scientists who contribute to the IPCC work for free.
It means prestige - the IPCC shared the 2007 Nobel Peace Prize - but also criticism, for instance after the IPCC exaggerated the pace of the thaw of Himalayan glaciers in 2007 by projecting they might all vanish by 2035.
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