Two Such Powerful Storms a Rarity, Their Cause a Mystery

Although the Gulf of Mexico has seen two Category 5 hurricanes this summer -- with sustained wind speeds in excess of 155 mph -- such extremely powerful storms are rare beasts that often fade before reaching land.

Since 1928, only 28 Category 5 hurricanes have formed in the Atlantic Ocean, and of those, eight have struck land, three of them in the United States.

Those three were an unnamed storm that hit the Florida Keys in 1935; Camille, which hit Mississippi in 1969; and Andrew, which struck southern Florida in 1992.

Hurricane Katrina weakened to Category 4, with winds of 131 mph to 155 mph, before it struck the Gulf Coast last month. Rita is expected to do the same before it encounters the Texas coast Friday night or Saturday morning.

The occurrence of two such massive storms within a month has prompted a rash of speculation about the causes, whether global warming or simply cyclical changes in ocean temperature.

The bottom line is, no one knows.

One thing most scientists agree on is that higher ocean temperatures lead to more intense storms.

Hurricanes cannot form unless the ocean is at least 80 degrees Fahrenheit for at least 150 feet below the surface. Otherwise, water does not evaporate from the surface rapidly enough to sustain the huge energy required for a hurricane.

The higher the temperature, the more water vapor and heat energy are released into the air, fueling the storm.

Other factors, such as winds at certain altitudes, can interfere with the formation and stability of a storm, but water temperature is the ultimate driving force.

Currently, waters in the Gulf of Mexico average about 84 degrees, providing powerful fuel for Rita, according to meteorologist T.N. Krishnamurti of Florida State University.

Waters in the tropical Atlantic are about one degree warmer than they were in the 1970s, hurricane specialist Kerry Emanuel of the Massachusetts Institute of Technology said.

In a report published in the journal Nature at the end of July, Emanuel concluded that the destructive power of Atlantic hurricanes had nearly doubled during that period, even though the average number of tropical cyclones worldwide had remained stable at about 90 per year.

Suzana Camargo, a cyclone specialist at Columbia University, supported Emanuel’s conclusion based on her studies of typhoons in the Pacific.

When water temperatures rise by a few degrees as the result of cyclical El Nino events, she said, the number of typhoons does not increase, but their intensity does.

Those findings leave unanswered the question of what is causing the temperature rise in the Atlantic.

Emanuel argues it is human activity -- global warming caused by the release of carbon dioxide and other gases related to industrial activity that trap the sun’s heat, raising Earth’s temperature.

But George Taylor, the state climatologist of Oregon, counters that records of past hurricanes reflect a cyclical heating and cooling of ocean waters.

Taylor also notes that global warming models predict increases in ocean temperatures mainly at the most northern and most southern latitudes, not in the mid-ocean regions where hurricanes are spawned.