Magazine article Science News

The Wind and the Fury: Has Climate Change Made Hurricanes Fiercer, or Are Such Claims Hot Air?

Magazine article Science News

The Wind and the Fury: Has Climate Change Made Hurricanes Fiercer, or Are Such Claims Hot Air?

Article excerpt

AS Hurricane Katrina steamed forward on Thursday, Aug. 25, residents of the southeastern U.S. shore breathed sighs of relief. The storm passed Miami as a weak hurricane, rating as only a category 1 storm on a scale from 1 to 5. But within days, relief turned to alarm, amid warnings from forecasters that the worst might be yet to come. The storm sucked energy from warm Gulf of Mexico waters as it moved west, swelling into a category 5 monster and then weakening only slightly before it slammed into the Mississippi shore as a category 4 hurricane. Abundant rain and a surge of ocean water overwhelmed flood-control measures and broke levees at nearby Lake Pontchartrain, deluging New Orleans with up to 20 feet of water and plunging the city into mayhem.

Katrina's ferocity left many people asking whether the monster storm came from mere chance or from something more long lasting--global warming. Although hurricane numbers and intensities are known to vary naturally, with some years producing many violent hurricanes and others hardly any, Hurricane Katrina isn't the only exceptionally destructive event in recent memory. In 1592, hurricane Andrew topped me charts as me most costly U.S. hurricane then on record, wreaking $25 million in damage in Florida--a record that Katrina will certainly break.

In the tropical Atlantic, moreover, hurricane numbers have been on the uptick since 1995, according to the National Oceanic and Atmospheric Administration (NOAA). In 2004, Florida suffered its worst hurricane season in 118 years, with nine hurricanes, five of which were classified as major. For 2005, NOAA's forecast predicted yet another above-average hurricane season for the region.

Scientists are divided on whether climate change, induced by industrial and automotive release of carbon dioxide and other greenhouse gases, is driving these statistics. Most climate scientists say that natural, cyclic phenomena that affect ocean currents and atmospheric temperature--such as El Nino in the Pacific Ocean and the North Atlantic Oscillation--yield decade-to-decade swings in total hurricane numbers that have nothing to do with global warming. Some researchers say that these phenomena are also responsible for all the observed changes in storm intensity.

But many other climate scientists are now pointing to global warming as the culprit for increasingly ferocious hurricanes worldwide. Both scientific theory and computer modeling predict that as human activities heat the world, warmer sea-surface temperatures will fuel hurricanes, increasing wind speeds and rainfall. Now, several new studies suggest that climate change has already made hurricanes grow stronger.

Many scientists predict that such an increase in storm violence will have consequences for coastal communities.

COOKING UP A STORM Hurricanes gain their destructive power from ocean moisture and heat. As the sea and atmosphere warm, more water evaporates from the ocean surface. When that moisture reaches the cool upper atmosphere, it condenses, releasing the energy that originally went into evaporating it. This "latent heat" powers the growing storm, says meteorologist Tom Knutson of NOAA's Geophysical Fluid Dynamics Laboratory in Princeton, N.J.

How warm the sea surface gets and how high into the atmosphere the evaporated water climbs set a speed limit on hurricane winds, says Kerry Emanuel of the Massachusetts Institute of Technology in Cambridge. In 1987, Emanuel predicted that with global warming, this speed limit would rise and that hurricanes would rev up their engines.

"If the climate warms, hurricanes have the potential to become substantially more intense," agrees Knutson. He and Robert E. Tuleya of Old Dominion University in Norfolk, Va., have used computer models to simulate how hurricanes would change in a warming world. If the atmospheric concentration of carbon dioxide, the greenhouse gas most responsible for global warming, doubles in the next 80 years, hurricanes' wind speeds will rise by about 5 percent, the researchers predicted in the Sept. …

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