Research by hurricane scientists may force the UN’s climate panel to reconsider its claims that greenhouse gas emissions have caused an increase in the number of tropical storms.
The benchmark report from the Intergovernmental Panel on Climate Change (IPCC) said that a worldwide increase in hurricane-force storms since 1970 was probably linked to global warming.
It followed some of the most damaging storms in history such as Hurricane Katrina, which hit New Orleans and Hurricane Dennis which hit Cuba, both in 2005.
The IPCC added that humanity could expect a big increase in such storms over the 21st century unless greenhouse gas emissions were controlled.
The warning helped turn hurricanes into one of the most iconic threats of global warming, with politicians including Ed Miliband, the energy secretary, and Al Gore citing them as a growing threat to humanity.
The cover of Gore’s newest book, Our Choice, even depicts an artist's impression of a world beset by a series of huge super-hurricanes as a warning of what might happen if carbon emissions continue to rise.
However, the latest research, just published in Nature Geoscience, paints a very different picture.
It suggests that the rise in hurricane frequency since 1995 was just part of a natural cycle, and that several similar previous increases have been recorded, each followed by a decline.
Looking to the future, it also draws on computer modelling to predict that the most likely impact of global warming will be to decrease the frequency of tropical storms, by up to 34% by 2100.
It does, however, suggest that when tropical storms do occur they could get slightly stronger, with average windspeeds rising by 2-11% by 2100. A storm is termed a hurricane when wind speeds exceed 74mph, but most are much stronger. A category 4 or 5 hurricane such as Katrina generates speeds in excess of 150mph.
“We have come to substantially different conclusions from the IPCC,” said Chris Landsea, a lead scientist at the American government’s National Hurricane Center, who co-authored the report.
He added: ”There are a lot of legitimate concerns about climate change but, in my opinion, hurricanes are not among them. We are looking at a decrease in frequency and a small increase in severity.” Landsea said he regarded the use of hurricane icons on the cover of Gore's book as "misleading".
Although the new report appears to criticise the IPCC it could mark a new start, showing that the beleagured body can recognise its mistakes and correct them as mistakes or new science emerge.
The Nature Geosciences study was actually commissioned by the World Meteorological Organisation (WMO), a UN agency which helps oversee the IPCC, in an attempt to resolve the bitter scientific row that had emerged over the relationship between global warming and tropical storms.
That row dates back to the hurricane season of 2004 when four major hurricanes hit north and central America.
It prompted senior IPCC scientists to give a press conference at Harvard University warning that global warming would cause many more such storms.
The claims attracted worldwide attention but Landsea pointed out there was no science so substantiate them and was so angry that he resigned his post as a senior IPCC author in January 2005, issuing a letter accusing the IPCC of having become “politicised”.
He added in the letter : “All previous and current research in the area of hurricane variability has shown no reliable, long-term trend up in the frequency or intensity of tropical cyclones.”
The following year seemed to have proved him wrong when North and Central America were hit by a series of tropical storms plus seven major hurricanes, including Katrina, which devastated New Orleans.
However he and other researchers have spent the years since then gathering historical evidence showing that hurricane frequency and intensity vary according to an entirely natural cycle, each lasting around 50-80 years.
The last such surge began around 1925 and lasted until about 1955. Conversely there were declines in frequency between both 1910-1925 and from 1955-1995.
Such findings have generated continuing tension among storm researchers and criticism of the IPCC’s stance, so the WMO brought together 10 leading scientists from all sides of the argument to try to resolve it.
Led by Thomas Knutson, a renowned hurricane researcher at Princeton University, the group also included Landsea and Kerry Emanuel, professor of meteorology at MIT. Kerry was a leading proponent of the idea that global warming meant more severe hurricanes.
Julian Heming, an expert in tropical storms at the Met Office, said: “Several of the authors have clashed in the past so the fact that they have co-authored this paper shows they have been prepared to adjust their stance on the basis of the recent research. ”
The IPCC’s reaction to the paper is uncertain but the organisation has confirmed it is reviewing several recent questions raised over its research and considering corrections where appropriate. One senior IPCC scientist, Professor Chris Field, has said he wants the IPCC to bring in new systems for checking and correcting its reports as important mistakes and new findings emerge.
Last Friday environment and climate ministers meeting in Bali also ordered a separate independent review of the IPCC’s leadership under Dr Rajendra Pachauri.
It followed articles in The Sunday Times highlighting the IPCC’s false claim that climate change could melt most Himalayan glaciers by 2035.
The ministers — led by Hilary Benn, the Environment Secretary, and his counterparts from Germany, Norway, Algeria and Antigua and Barbuda — said they were not questioning the basic science behind global warming.
Instead, they were concerned with the “aggressive” way in which Dr Pachauri had responded to criticism, including denouncing Indian research suggesting that the glaciers were not melting so rapidly as “voodoo science”.
A spokesman for Gore said the cover of Our Choice was not a scientific diagram but "an artist's rendering of an earth where unchecked global warming has wreaked havoc."