The Coming Storms

Get ready for a couple of decades of ugly hurricanes. Here’s why.

Did that humongous wall of water in this summer’s hit movie The Perfect Storm blow your mind? Well, that’s entertainment. No wave that size was actually recorded during the real “perfect storm” in 1991.

Still, Atlantic storms can be fierce — and could be getting fiercer. Global climate change could soon whip up hurricanes unlike any ever seen before, say scientists.

MONSTER STORMS

The last several years have already been bad ones for Atlantic Ocean hurricanes. Two years ago this month, Hurricane Georges destroyed $2 billion in property in Puerto Rico. A month later, Hurricane Mitch killed 10,000 people in Central America. Then, last September, Hurricane Floyd trashed $6 billion in property on the U.S. eastern seaboard and prompted the largest evacuation (more than 3 million people) in the country’s history.

The recent upswing in storms is part of the natural up-and-down cycle of hurricane activity in the Atlantic Ocean, according to James Elsner of Florida State University. The number of big Atlantic hurricanes was relatively high form 1943 to 1964. In 1995, big hurricanes made a comeback that Elsner says could last through the next few decades.

Why does hurricane activity flip-flop? Elsner puts the blame on the thermal haline circulation, a circulation pattern that moves warm seawater from the Pacific Ocean by way of the Indian Ocean to the Atlantic Ocean, then back again.

During some periods, the thermalhaline circulation moves more quickly and makes the North Atlantic warmer than normal. Such periods prompt more hurricane activity in the Atlantic because warm water supplies the fuel–the heat energy — that keeps a hurricane going, said Elsner. (See “Anatomy of a Hurricane.”)

THE HEAT IS ON

The coming decades could be ones of not just more but also bigger ocean storms. Bigger storms could gain their extra power from global warming, the gradual rise in Earth’s surface temperature that many scientists believe has been caused by a buildup of carbon dioxide ([CO.sub.2]) in the air. Global warmlng is expected to continue well into the 21st century, making the world hotter than ever.

As the world gets warmer, so do the oceans. And warmer oceans hold more of the thermal (heat) energy that hurricanes feed on. According to Kerry Emanuel, an atmospheric scientist at the Massachusetts Institute of Technology, hurricane wind speeds increase by 5 miles per hour for every extra degree Fahrenheit of water temperature. A warmer ocean might push hurricane winds to speeds of 200 miles per hour or more. Only a few hurricanes have ever had wind speeds exceeding 155 miles per hour.

PACIFIC POOL

That’s one forecast. Some scientists say that further global warming might have an opposite effect and dampen hurricane activity. William Gray, a meteorologist at Colorado State University, says continued rises in global temperatures might produce more El Ninos. El Nino is huge pool of warm water that often develops in the Pacific Ocean. When an El Nino is in place, high-altitude winds regularly blow east to the Atlantic Ocean. Those winds can snuff out Atlantic hurricanes by essentially lopping off their tops.

Super hurricanes might fail to take shape for another reason. Scientists have noticed a tendency for Hurricanes to “commit suicide.” When a hurricane reaches a certain size, it starts stirring up cold water from deep in the ocean. Such an upwelling of cold water can rob a hurricane of its energy and kill it. Any supersize storms that arise due to global warming might also die out quickly from exposure to the cold.

STORM WARNINGS

Whatever global warming brings, scientists agree on the need for better hurricane forecasts. One scientist, Isaac Ginis of the University of Rhode Island, has devised a new way of assessing the strength of hurricanes. Ginis’s technique focuses on the thermocline, a narrow zone of water that underlies the ocean’s warm surface water. Beneath the thermocline, the ocean turns very cold.

As mentioned, hurricanes weaken and die when warm surface water mixes with the cold water below. That mixing is partly determined by the depth of the thermocline. The shallower the thermocline, the more likely the hurricane will stir cold water up to the surface.

Last year, Ginis and colleagues measured the depth of the thermocline over the entire North Atlantic. Their measurements helped them improve predictions of hurricane strength by about 30 percent, said Ginis.

Ginis’s forecasting method could prove to be a major lifesaver. If history is any guide, the current surge in hurricane activity could last at least 20 more years.

Anatomy of a Hurricane

  1. A hurricane forms where humid air flows over warm ocean water. The air, warmed by the ocean, begins to rise.
  2. When the rising humid air reaches a certain altitude, the water vapor in it condenses and forms walls of huge cumulonimbus clouds, called rainbands.
  3. The condensation of water vapor also releases a huge amount of heat energy. That energy fuels the hurricane, making it grow in size and shape.
  4. The spiraling motion of a hurricane is caused by the Coriolus effect, a turning of the wind produced by Earth’s rotation.

A tropical storm is classified as a hurricane when its winds reach 117 kilometers (73 miles) per hour. The average hurricane is 480 kilometers (300 miles) wide and lasts about ten days.

 

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