"Trends in human-influenced environmental changes are now evident in hurricane regions," says Trenberth. "These changes are expected to affect hurricane intensity and rainfall, but the effect on hurricane numbers remains unclear. The key scientific question is how hurricanes are changing."
The strongest links between hurricane intensity and climate change, according to Trenberth, are a long-term rise in ocean temperatures and an increase in atmospheric water vapor. Both processes are already under way and expected to continue, he says. The additional water vapor will tend to produce heavier rains within hurricanes and an increased risk of flooding at landfall, Trenberth notes.
Most hurricanes that strike the U.S. coastline are born in the tropical North Atlantic, where sea-surface temperatures over the last decade have been the warmest on record. Water vapor over oceans worldwide has increased by about 2% since 1988. The warmer sea surface and moister atmosphere furnish potential energy for the showers and thunderstorms that fuel hurricanes.
"Computer models also suggest a shift in hurricane intensities toward extreme hurricanes," says Trenberth.
Much more uncertain is the effect of human-induced climate change on hurricane numbers and landfalls. Models disagree on how global warming might affect the wind shear that can either support or discourage hurricane formation.
Globally, the number of hurricanes and typhoons tends to hold relatively steady from year to year. When activity increases in the Atlantic, it often decreases in the Pacific, and vice versa, based in part on El Ñino and La Ñina.
Trenberth points out that, because hurricane numbers vary so greatly on a regional level from year to year and decade to decade, it is difficult to use statistical techniques to extract longer-term trends in the number of hurricanes that form and where they move.
"There is no sound theoretical basis for drawing any conclusions about how anthropogenic climate change affects hurricane numbers or tracks, and thus how many hit land," Trenberth says.
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