May 18, 2008 / 5:17 PM / in 10 years

Atlantic cyclones may decrease as globe warms: study

MIAMI (Reuters) - Fewer but more intense hurricanes may form in the Atlantic Ocean as the globe warms toward the end of this century, according to a new study that counters predictions of more frequent cyclones due to climate change.

The study, published on Sunday in Nature Geoscience, adds fuel to a fierce scientific debate over whether human-produced greenhouse gases have contributed to a recent rise in hurricane activity in the Atlantic basin and whether tropical cyclones are becoming stronger.

A simulation of Atlantic hurricane activity for the final decades of the century projected an 18 percent decrease in hurricanes and a 27 percent decrease in tropical storms, researchers at the U.S. government’s Geophysical Fluid Dynamics Laboratory in New Jersey found.

“It does not support the notion that increasing greenhouse gases are causing a large increase in Atlantic hurricane or tropical storm frequency,” said Thomas Knutson, one of the study’s authors.

Recent studies have found links between rising sea surface temperatures in the tropical Atlantic Ocean, where cyclones form, and some measures of hurricane activity. Tropical storms and hurricanes draw their energy from warm sea water.

Around 1995, scientists believe, the Atlantic entered a period of higher hurricane activity that could last 25 to 40 years. Some researchers attribute the rise to natural cycles, while others believe global warming may be brewing more, and more intense, storms.

The frenzied hurricane seasons of 2004, when four strong hurricanes hit Florida, and 2005, when a record-breaking 28 storms formed, fired up the climate change debate.

The next six-month Atlantic hurricane season begins on June 1 and forecasters expect it to be a busy one, well above the long-term average season that produces about 10 tropical storms and six hurricanes.

A well-known forecasting team at Colorado State University predicts 15 storms, of which eight would become hurricanes.

Forecasters have been wrong in the last few years.

Global energy, commodities and insurance markets have paid close attention since 2004 and 2005, when the steady stream of Atlantic storms -- including $80 billion in damage from Hurricane Katrina -- marched through U.S. oil and gas fields in the Gulf of Mexico.


The study by the Geophysical Fluid Dynamics Laboratory used a global warming scenario for the rest of the century projected by the U.N. Intergovernmental Panel on Climate Change.

That included a rise of 1.7 degrees Celsius in Atlantic sea temperatures, higher vertical wind shear and lower low- to mid-tropospheric humidity across the Caribbean.

Researchers said water temperatures in the tropical Atlantic have risen more in recent decades than in other oceans but the models project more uniform warming in coming years.

“Now those models could be wrong ... (but) without that rapid increase in the Atlantic compared to the other tropical basins, our models do not expect a rapid increase in Atlantic storminess,” said researcher Isaac Held.

Vertical wind shear, a difference in wind speeds at varying altitudes that tends to tear apart nascent hurricanes, could be another contributor. Researchers said the environment may become more hostile to cyclones before the end of the century.

“We’ve had lower levels of vertical wind shear (in recent years), but the climate model projections are not indicating reduced wind shear in the Atlantic but rather ... increased vertical wind shear,” Knutson said.

The model projects more rainfall from hurricanes toward the end of the century and a modest increase in intensity.

Knutson said a past study forecast a 4 percent rise in intensity for every 1 degree C rise in sea temperature, but this latest study found a smaller rise of 1 to 2 percent.

The researchers said their findings for the Atlantic basin would not necessarily apply to other ocean basins, in part because wind shear was not expected to rise elsewhere.

Noting the debate among hurricane and climate researchers on the issue of global warming and hurricanes, Knutson said: “We don’t regard this as the last word on this topic.”

Editing by Michael Christie and John O'Callaghan

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