The number of Category 4 or 5 hurricanes -- like the devastating Katrina--significantly increased in the last 35 years, fueled by hotter seas that have been linked to global warming, researchers reported.
Katrina hit the coast as a Category 4 storm, but its size, with hurricane winds extending 120 miles from its center, made it the most destructive ever to strike the United States, the National Oceanic and Atmospheric Administration said Thursday.
In 1970, the scientists found, these most powerful storms only made up about one-sixth of all hurricanes. In recent years, the proportion of major storms has risen to one-third of all hurricanes.
During the same time period the average temperature of the world's oceans has increased by about 1 degree Fahrenheit(0.5 degree Celsius).
"With some confidence, we can say that these two things are connected, and that there's probably a substantial contribution from greenhouse warming," said Judith Curry, a Georgia Institute of Technology researcher.
Water vapor that evaporates from the sea's surface into the atmosphere eventually condenses as rain, releasing heat and driving a tropical cyclone -- the swirling pattern that leads to a hurricane.
The warmer the sea surface, the greater amount of potential evaporation and the greater the fuel for a possible hurricane. And even small rises in sea surface temperature can cause rapid rises in evaporation.
There are good theoretical reasons, theregore, to believe that if global warming continues, and seas' temperature rises, hurricanes might become more violent.
The U.S. National Hurricane Center has forecast an extremely active Atlantic hurricane season, with nine to 11 hurricanes, including seven to nine major hurricanes, from July to November.