— Global warming is likely to affect cyclones and hurricanes, concludes a new statement from 125 experts, but they say the evidence for this to date is inconclusive.
"There could be an effect but it's impossible to say for sure," says Julian Heming of the UK Met Office. The statement was issued at the end of a workshop organised by the World Meteorological Organization (WMO).
The workshop concluded that the increasing economic damage caused by tropical cyclones is to a large extent the result of "increasing coastal populations [ ] and, perhaps, a rising sensitivity of modern societies to disruptions of infrastructure".
The influence of climate change on tropical cyclones a term used to describe hurricanes, typhoons and cyclones is a hotly contested debate. It gained public attention after 2004 and 2005 brought an unusual number of high-impact storms to Japan, China and the US (see State of denial). The events culminated in 1300 deaths, mainly through flooding in New Orleans, when Hurricane Katrina hit the US coast.
Temperature records and computer models suggest the warming climate will generate more frequent and powerful storms. But differences of opinion over the data and methods divide the scientific community.
For and against
In 2005, two groups suggested that tropical cyclones have become more powerful and more numerous in recent years. Kerry Emmanuel of MIT in the US used historical data to show that tropical cyclones in the West Pacific and the Atlantic have become "substantially" more powerful over last 50 years.
Similarly, Peter Webster at the Georgia Institute of Technology, and colleagues, suggested there have been more severe tropical cyclones and fewer weaker storms in the last 30 years, worldwide (see Warming world blamed for more strong hurricanes).
Then, in 2006, Kazuyoshi Oouchi of the Advanced Earth Science and Technology Organization in Japan, used some of the highest-resolution models available to show that a warmer climate should reduce the number of tropical cyclones globally, but that some regions should see more of them. The same models show some increase in the intensity of the storms as the climate warms.
But both observational studies and the models are debated. Scientists are aware that they have to be careful not to confuse changes in the observational methods used with real changes in climate. They also acknowledge that the use of models involves uncertainties.
However, the North Atlantic, where the hurricanes that hit the US gather strength, is an exception. Measurements of tropical cyclones there have been of higher quality for longer than elsewhere. Hurricanes in the North Atlantic are also more sensitive to changes in sea-surface temperatures, a factor which is known to be critical in the formation of tropical cyclones.
This is because tropical cyclones are born when sea-surface temperatures rise above 26°C to 27°C. The surface waters in most tropical oceans remain above the 27°C threshold all year. But in the North Atlantic, temperatures move above and below this range, making it a switch for the formation of hurricanes.
But here again, consensus breaks down. One group led by Stanley Goldenberg, at the US National Oceanic and Atmospheric Administration's Hurricane division, says changes in sea-surface temperatures are the result of natural variations. Another group, led by Michael Mann, at Pennsylvania State University, and Kerry Emmanuel, maintain they are due to human-induced climate change.
Still, in spite of the controversy, the 125 experts gathered by the WMO did agree that "given the consistency between high resolution global models, regional hurricane models and 'maximum potential intensity' theories, it is likely that some increase in tropical cyclone intensity will occur if the climate continues to warm".
It all comes down to determining the measure of "some", says Heming.
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