By Jonathan Leake, Science and Environment Editor
THE United Nations climate science panel faces new controversy for wrongly linking global warming to an increase in the number and severity of natural disasters such as hurricanes and floods.
It based the claims on an unpublished report that had not been subjected to routine scientific scrutiny — and ignored warnings from scientific advisers that the evidence supporting the link too weak. The report's own authors later withdrew the claim because they felt the evidence was not strong enough.
The claim by the Intergovernmental Panel on Climate Change (IPCC), that global warming is already affecting the severity and frequency of global disasters, has since become embedded in political and public debate. It was central to discussions at last month's Copenhagen climate summit, including a demand by developing countries for compensation of $100 billion (£62 billion) from the rich nations blamed for creating the most emissions.
Ed Miliband, the energy and climate change minister, has suggested British and overseas floods — such as those in Bangladesh in 2007 — could be linked to global warming. Barack Obama, the US president, said last autumn: "More powerful storms and floods threaten every continent."
Last month Gordon Brown, the prime minister, told the Commons that the financial agreement at Copenhagen "must address the great injustice that . . . those hit first and hardest by climate change are those that have done least harm".
The latest criticism of the IPCC comes a week after reports in The Sunday Times forced it to retract claims in its benchmark 2007 report that the Himalayan glaciers would be largely melted by 2035. It turned out that the bogus claim had been lifted from a news report published in 1999 by New Scientist magazine.
The new controversy also goes back to the IPCC's 2007 report in which a separate section warned that the world had "suffered rapidly rising costs due to extreme weather-related events since the 1970s".
It suggested a part of this increase was due to global warming and cited the unpublished report, saying: "One study has found that while the dominant signal remains that of the significant increases in the values of exposure at risk, once losses are normalised for exposure, there still remains an underlying rising trend."
The Sunday Times has since found that the scientific paper on which the IPCC based its claim had not been peer reviewed, nor published, at the time the climate body issued its report.
When the paper was eventually published, in 2008, it had a new caveat. It said: "We find insufficient evidence to claim a statistical relationship between global temperature increase and catastrophe losses."
Despite this change the IPCC did not issue a clarification ahead of the Copenhagen climate summit last month. It has also emerged that at least two scientific reviewers who checked drafts of the IPCC report urged greater caution in proposing a link between climate change and disaster impacts — but were ignored.
The claim will now be re-examined and could be withdrawn. Professor Jean-Pascal van Ypersele, a climatologist at the Universite Catholique de Louvain in Belgium, who is vice-chair of the IPCC, said: "We are reassessing the evidence and will publish a report on natural disasters and extreme weather with the latest findings. Despite recent events the IPCC process is still very rigorous and scientific."
The academic paper at the centre of the latest questions was written in 2006 by Robert Muir-Wood, head of research at Risk Management Solutions, a London consultancy, who later became a contributing author to the section of the IPCC's 2007 report dealing with climate change impacts. He is widely respected as an expert on disaster impacts.
Muir-Wood wanted to find out if the 8% year-on-year increase in global losses caused by weather-related disasters since the 1960s was larger than could be explained by the impact of social changes like growth in population and infrastructure.
Such an increase, coinciding with rising temperatures, might suggest that global warming was to blame. If proven this would be highly significant, both politically and scientifically, because it would confirm the many predictions that global warming will increase the frequency and severity of natural hazards.
In the research Muir-Wood looked at a wide range of hazards, including tropical cyclones, thunder and hail storms, and wildfires as well as floods and hurricanes.
He found from 1950 to 2005 there was no increase in the impact of disasters once growth was accounted for. For 1970-2005, however, he found a 2% annual increase which "corresponded with a period of rising global temperatures,"
Muir-Wood was, however, careful to point out that almost all this increase could be accounted for by the exceptionally strong hurricane seasons in 2004 and 2005. There were also other more technical factors that could cause bias, such as exchange rates which meant that disasters hitting the US would appear to cost proportionately more in insurance payouts.
Despite such caveats, the IPCC report used the study in its section on disasters and hazards, but cited only the 1970-2005 results.
The IPCC report said: "Once the data were normalised, a small statistically significant trend was found for an increase in annual catastrophe loss since 1970 of 2% a year." It added: "Once losses are normalised for exposure, there still remains an underlying rising trend."
Muir-Wood's paper was originally commissioned by Roger Pielke, professor of environmental studies at Colorado University, also an expert on disaster impacts, for a workshop on disaster losses in 2006. The researchers who attended that workshop published a statement agreeing that so far there was no evidence to link global warming with any increase in the severity or frequency of disasters. Pielke has also told the IPCC that citing one section of Muir-Wood's paper in preference to the rest of his work, and all the other peer-reviewed literature, was wrong.
He said: "All the literature published before and since the IPCC report shows that rising disaster losses can be explained entirely by social change. People have looked hard for evidence that global warming plays a part but can't find it. Muir-Wood's study actually confirmed that."
Mike Hulme, professor of climate change at the Tyndall Centre, which advises the UK government on global warming, said there was no real evidence that natural disasters were already being made worse by climate change. He said: “A proper analysis shows that these claims are usually superficial”
Such warnings may prove uncomfortable for Miliband whose recent speeches have often linked climate change with disasters such as the floods that recently hit Bangladesh and Cumbria. Last month he said: “We must not let the sceptics pass off political opinion as scientific fact. Events in Cumbria give a foretaste of the kind of weather runaway climate change could bring. Abroad, the melting of the Himalayan glaciers that feed the great rivers of South Asia could put hundreds of millions of people at risk of drought. Our security is at stake.”
Muir-Wood himself is more cautious. He said: "The idea that catastrophes are rising in cost partly because of climate change is completely misleading. "We could not tell if it was just an association or cause and effect. Also, our study included 2004 and 2005 which was when there were some major hurricanes. If you took those years away then the significance of climate change vanished."
Some researchers have argued that it is unfair to attack the IPCC too strongly, pointing out that some errors are inevitable in a report as long and technical as the IPCC's round-up of climate science. "Part of the problem could simply be that expectations are too high," said one researcher. "We have been seen as a scientific gold standard and that's hard to live up to."
Professor Christopher Field,director of the Department of Global Ecology at the Carnegie Institution in California, who is the new co-chairman of the IPCC working group overseeing the climate impacts report, said the 2007 report had been broadly accurate at the time it was written.
He said: “The 2007 study should be seen as “a snapshot of what was known then. Science is progressive. If something turns out to be wrong we can fix it next time around.” However he confirmed he would be introducing rigorous new review procedures for future reports to ensure errors were kept to a minimum.