Hurricanes in the Atlantic are increasing because of natural weather patterns rather than global warming, a study has concluded.

Growing numbers of hurricanes battering the United States and the Caribbean have made their presence felt in the past decade and are forecast to worsen. Global warming has been cited as a possible cause but researchers looking at sediment and coral deposits have now identified natural variations in their frequency.

Hurricane Katrina, which devastated New Orleans in 2005, was “unexceptional” when historic patterns of such stormy weather are analysed, they suggested.

Global warming may even have been responsible for unusually low levels of hurricanes in the 25 years before 1995 when the number began rising, according to the scientists, led by the Geological Survey of Sweden.

Using deposits trapped in sediment to indicate when hurricanes had taken place, the researchers built up a record detailing their number and frequency going back 270 years.

They found that the decline in hurricanes during the 1970s and 1980s was matched by similar declines in the past, indicating natural variations in the weather patterns.

“The record indicates that the average frequency of major hurricanes decreased gradually from the 1760s until the early 1960s, reaching anomalously low values during the 1970s and 1980s,” they reported in the journal Nature. “Furthermore, the phase of enhanced hurricane activity since 1995 is not unusual compared to other periods of high hurricane activity and appears to represent a recovery to normal hurricane activity.”

The findings are at variance with the conclusions in February of the Intergovernmental Panel on Climate Change, the United Nations organisation addressing global warming. The UN panel stopped short of blaming increased frequency of hurricanes on man-made temperature rises, but said it was “more likely than not” that greenhouse gas emissions had contributed to the greater intensity of cyclonic storms.

The Swedish-led research team suggested that hurricane levels were normal, though they accepted “a future possibility” of higher sea temperatures contributing to more intense hurricanes.The researchers were unable to identify any direct link between increased hurricanes and rising sea level temperatures, beyond the requirement for a minimum temperature of 27C (81F) to be reached before a hurricane developed.

From 1730-2005 there were on average 3-3.5 major hurricanes each year.

The researchers from Sweden, the US and Puerto Rico said that being able to calculate vertical wind shear – the differences in wind speeds at different heights – was crucial in determining the frequency of hurricanes.

Higher wind-shear levels disrupt developing hurricanes; low wind-shear levels fail to batter the storms sufficiently to prevent them developing.

The researchers suggested that higher air temperatures caused by global warming may have led to stronger vertical wind shear, which has destroyed developing hurricanes in the Atlantic before 1995, explaining the dearth.

Deposits of sediments accumulated from increased run-off from rainfall and plankton remains associated with increased levels of nutrients and provided clues to the scientists to historic vertical wind shear and hurricanes.

They were able to check their readings of the data by comparing their findings with documentation of hurricanes.

In the wind

–– From 1995 to 2005 there were an average of 4.1 major hurricanes (categories 3-5) in the Atlantic compared with an average of 1.5 from 1971-94

–– Five periods in the past 270 years were found to have had the same lack of hurricanes, combined with high wind shear, as 1971-94: 1730-36, 1793-99, 1827-30, 1852-66 and 1915-26

–– Six periods were identified as having the same high levels witnessed since 1995: 1756-74, 1780-85, 1801-12, 1840-50, 1873-90 and 1928-33

Source: Geological Survey of Sweden

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