Top climate scientists will blame mankind more clearly than ever for global warming next week but may struggle to drive home the message in a report that uses the term "uncertainty" 42 times. The 'language gap' between scientists and the policy makers, public and media they seek to alert is proving hard to bridge.
Scientists say uncertainty is inevitable at the frontiers of knowledge - in, for instance, calculating how much of Greenland will thaw or how fast temperatures will rise by 2100 - but that policymakers and the public often mistake it for ignorance.
That gap in semantics may complicate the message of greater overall understanding of global warming in the report by the U.N.'s Intergovernmental Panel on Climate Change (IPCC), due for release in Stockholm on Sept. 27 after a final round of editing.
A final draft summary raises the probability that most climate change since the 1950s has a human cause to at least 95 percent, from 90 in 2007 and 66 percent in 2001. Temperatures could rise by almost 5 degrees Celsius (9 F) by 2100, bringing enormous risks for society and nature.
Yet it also has the words "uncertainty" or "uncertainties" 42 times over 31 pages, according to a final draft obtained by Reuters, a comparable rate to 26 mentions in 18 pages in 2007.
Among the biggest uncertainties, it says, is how aerosols, such as air pollution, affect cloud formation. The white tops of low clouds can reflect sunlight and so cool the Earth's surface.
"When scientists are explicit about the underlying uncertainties an immediate response from decision-makers and the public is: 'Oh, scientists do not really know what they are talking about.'," said Ottmar Edenhofer of the Potsdam Institute for Climate Impact Research.
"This is actually an inappropriate response," he said. Edenhofer, who is a co-chair of a separate IPCC report looking at costs of fixing the problem due in 2014, and all other experts gave personal opinions and not details of the reports.
Society needs to understand uncertainty and risk, he said.
Unrelated to climate change, he noted that six scientists were sentenced to jail in Italy last year for manslaughter after wrongly reassuring people of low risks shortly before an earthquake killed more than 300 people in L'Aquila in 2009.
"We know more and more about the big picture" of climate change, said professor David Vaughan of the British Antarctic Survey and a lead author of the study to be issued in Stockholm.
"At exactly the same time we are getting more and more data about the little pictures which are much harder to explain."
Governments were no longer satisfied with estimates of global sea level rise, for instance, but wanted to know regional estimates, in places such as south England, to plan flood risks.
"Most people's view of science is that 'scientists know things'. But it's actually all about uncertainty," said James Painter, head of the Reuters Institute for the Study of Journalism at Oxford University.
A study he published on Wednesday found that media focused on disasters and uncertainty in covering climate change and that it might be better to stress risks and business opportunities.
Scientists reckon the focus on uncertainty, by governments and the media, may brake action to cut greenhouse gas emissions.
"There's a general frustration among scientists that we get more and more certain: why doesn't more happen?" said Cecilie Mauritzen, head of the Center for International Climate and Environmental Research in Oslo.
Scientists use a mixture of data and "expert judgment" to decide how likely it is that climate change is man-made and rule out other factors, such as changes in the sun's output.
The IPCC draft halves the likelihood that natural factors are to blame to 5 percent from 10, the flip side of raising the probability that climate change is man-made to 95 percent.
"It's based on a discussion among the authors...There must be multiple lines of evidence," said Eystein Jansen, of the Bjerknes Centre for Climate Research in Norway and one of the authors of the Stockholm draft.
A graph in the draft, reconstructing temperature rises in the 20th century, shows the trend cannot be explained without the warming effect of greenhouse gases spewed into the atmosphere from cars, factories and power plants.
Those sceptical over human contribution to warming often say more certainty is needed before acting, something proponents of action reject given risks of floods, heatwaves, and rising sea levels. Society may act on little certainty when risks are high.
Former U.S. Vice President Dick Cheney, for instance, once said that if there was a 1 percent chance that Pakistan was helping al Qaeda to develop nuclear weapons, then Washington had to treat it as a certainty in terms of its response.
Dan Kahan, a professor of Law and Psychology at Yale University, doubted that any change of certainty by the IPCC would have much impact on the public. Governments have not cut rising emissions even though repeated surveys show that 97-98 percent of climate scientists reckon warming is man-made.
"People fit evidence of what scientists believe - like all other sorts of evidence - to the position that affirms their cultural identity," he said. In the United States, Democrats are more likely to agree with climate science than Republicans.
And IPCC reports, stretching to about 3,000 pages, have had errors in the past, such as a mistake in 2007 that Himalayan glaciers might all melt by 2035, a big exaggeration of the thaw.
That has led to some criticisms that the IPCC stresses the negative effects of climate change. A review by outside experts in the InterAcademy Council in 2010 said that errors did not affect the IPCC's overall conclusions but that authors should do more to nail down the probabilities of their predictions.
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