BBC NEWS | Science/Nature | 'Fewer hurricanes' as world warms

Hurricanes and tropical storms will become less frequent by the end of the century as a result of climate change, US researchers have suggested.

But the scientists added their data also showed that there would be a "modest increase" in the intensity of these extreme weather events.

The findings are at odds with some other studies, which forecast a greater number of hurricanes in a warmer world.

The researchers' results appear in the journal Nature Geoscience.

The team from the National Oceanic and Atmospheric Administration's (Noaa) Geophysical Fluid Dynamics Laboratory (GFDL) said its findings did not support the notion that human-induced climate change was causing an increase in the number of hurricanes and tropical storms.

"There have been some studies published that have suggested that this is the case, but this modelling study does not support that idea," observed lead author Tom Knutson.

"Rather, we actually simulate a reduction in hurricane frequency in the Atlantic."

Eye of the storm

Although the study projected that there would be fewer extreme weather events in the future, Dr Knutson said that these storms were likely to be more powerful.

"The model is simulating increased intensity of the hurricanes that do occur, and also increased rainfall rates.

"This is something that has been seen in previous studies, and the IPCC use this [scenario] as a likely projection for future climate warming.

We do not regard this study as the last word on this topic Dr Tom Knutson, Noaa meteorologist

"These changes in intensity are still fairly modest in size."

A previous study by Noaa scientists showed a 4% increase in storm intensity for every 1C (1.8F) increase in sea surface temperature. Yet, he explained, this study suggested only a 1-2% increase.

A sea surface temperature (SST) above 26.5C (79.7F) is one of the key factors in the formation and feeding of a hurricane.

Over recent decades, the surfaces of most tropical oceans have warmed by up to 0.5C (0.9F), which the Intergovernmental Panel on Climate Change (IPCC) believes has been caused by an increase in greenhouse gas concentrations in the atmosphere.

In November 2006, the global community of tropical cyclone researchers gathered at a workshop organised by the World Meteorological Organization to consider the impact of human activity on the frequency and intensity of cyclones.

HOW HURRICANES FORM Sea surface temperatures above 26.5C (79.7F) A pre-existing weather disturbance Moisture in the atmosphere Favourable conditions, such as light winds or weak wind shear Animated guide: Hurricanes

In a concluding statement, the researchers said that although there was evidence both for and against the existence of a detectable anthropogenic signal in the tropical cyclone climate record, no firm conclusion could be made.

One reason for the uncertainty is the changes in observation methods used to record Atlantic hurricanes - a record that dates back to 1850.

From 1944, air reconnaissance flights were used to monitor tropical storms and hurricanes. This development allowed researchers to monitor a much greater area and not rely on ships' logs and storms reaching land.

And from the late 1960s, satellite technology has been used to monitor and track hurricanes.

Therefore, a reliable record of past hurricane activity only stretches back about 35 years.

Natural variations that affect SSTs - such as El Nino and La Nina episodes and the Atlantic Multi-decadal Oscillation - add to the difficulty of identifying the influence of human-induced climate change on the frequency and intensity of hurricanes.

Model mechanics

Dr Knutson's colleague and co-author, Isaac Held, said the team's model used a different approach to previous efforts, which gave them a high degree of confidence in their results.

The researchers project a "modest increase" in future storms' intensity

"Most of the literature to date on hurricanes and climate change has used statistical techniques," he said.

"You've had time series of hurricane activity and time series of sea surface temperatures, and people correlate them."

Because there was a high degree of confidence that the sea surface temperature trend was going to continue to rise, Dr Held explained, people had "tried to conclude that hurricane activity will increase rather dramatically in the future".

"We tried to simulate the fundamental fluid dynamics and thermodynamics that control hurricane genesis in the Atlantic in a numerical model to a very high resolution."

He added that the team ran data from the past 25 years through the model, and it returned results closely correlated to what actually occurred.

"It is interesting and important to understand why it is that this model is capable of simulating an increase in hurricane activity that we have seen in recent decades, yet it predicts a decrease in the future.

"This implies that we cannot simply extrapolate the past 25 years into the future."

Dr Knutson said that he did not expect the study's findings to end the scientific debate surrounding the impact of human-induced climate change on tropical storms.

"We do not regard this study as the last word on this topic," he told reporters.

"The main point that we want to emphasise is that there is no evidence in this study that we are seeing large greenhouse-gas-driven increases in Atlantic hurricane or tropical storm frequencies."

BBC NEWS | Science/Nature | 'Fewer hurricanes' as world warms

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