However, the increase in ocean temperatures is unlikely to boost the average number of Atlantic hurricanes that form each year, they said.

Hurricane Ivan forced millions of people to evacuate a 400-mile stretch of the U.S. Gulf Coast. The storm is classified as a Category 4 hurricane with maximum sustained winds of 135 mph (215 km/h) and has been blamed for 68 deaths and extensive damage in the Caribbean.

Ivan will be the third major storm to batter the U.S. Gulf region during the past month. It is expected to make landfall early yesterday.

The Republican-led panel heard testimony from several scientists who said emissions known as greenhouse gases were gradually raising the earth's temperature and would contribute to more extreme weather including flooding, drought and changing storm patterns.

"Warmer water temperatures will promote more intense tropical storms, but not necessarily make the frequency of those storms greater," said Dan Cayan, a research meteorologist at the University of California in San Diego.

"An increase of even a degree or so in the right environment would cause intensities to increase," he said.

Some members of Congress, scientists and environmental groups contend that global warming is upsetting environmental balances by altering fragile weather patterns in the world.

However, 10 climatologists and scientists sent a letter to Sen. John McCain, the Arizona Republican who heads the committee, saying there is no scientific evidence of a link between severe weather - such as hurricanes, blizzards and heat waves - and global warming. They argued that warmer periods of temperatures have actually led to a decline in the number and severity of storms.

"We suggest that natural variability of storminess is the cause of Florida's recent hurricane disasters," they wrote. "In such times there is an emotional tendency to pin blame somewhere."

McCain said during the hearing that human activities are contributing to global warming and require "real reductions" in greenhouse gas emissions. The United States is the world's biggest producer of the gases, which come from automobiles, power plants and other sources.

Sen. Joseph Lieberman, a Connecticut Democrat, and McCain have introduced bipartisan legislation that would set nationwide emissions limits for transport, utilities and other sectors. The bill has been stalled in the Senate.

President Bush refused to join the 1997 Kyoto treaty on greenhouse gases, saying it would be too costly to the economy.