WASHINGTON, D.C. -- Research so far on global warming and Atlantic hurricanes indicates:

• It is premature to conclude that human activity--particularly greenhouse warming--has had a detectable impact on Atlantic hurricanes, and
• Model simulations indicate that 21st century greenhouse warming may lead to greater numbers of very intense Atlantic hurricanes and higher hurricane rainfall rates, but fewer hurricanes overall.

Century-long basin-wide observed records of very intense Atlantic hurricanes are considered unreliable, but tropical storm and hurricane counts have been used as long-term climate indicators. Unadjusted counts of tropical storms show a significant rise from the mid to late 1800s to present, while unadjusted hurricane counts do not, due to the large number of reported hurricanes in the late 1800s.

Analysis of historical ship track records suggests that reporting coverage was likely too sparse to detect all tropical storms, and after adjusting for this bias, tropical storm counts have no significant trend over 1878-2006.

A regional downscaling atmospheric model reproduces the observed rise and year-to-year variability in hurricane counts (1980-2006) remarkably well when forced with observed sea surface temperatures and large-scale atmospheric conditions. According to this model, the ensemble late 21st century climate change projected by current Intergovernmental Panel on Climate Change global models implies reduced numbers of Atlantic hurricanes and tropical storms, due mainly to increased vertical wind shear.

Downscaling experiments with several high resolution models indicate that despite a projected reduction in overall hurricane numbers, the number and intensity of the strongest hurricanes may increase

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