11:40 AM Sep. 15, 2004 PT
Hurricane Ivan is among the most powerful Atlantic storms in recent history, and more such storms are likely in the future due to global warming, say climate experts.
"Global warming is creating conditions that (are) more favorable for hurricanes to develop and be more severe," said Kevin Trenberth, head of the climate analysis section at the National Center for Atmospheric Research in Boulder, Colorado.
While few climate and hurricane experts are willing to go that far publicly, there is little debate that the Earth is retaining more of the sun's energy than in the past. Emissions of gases such as carbon dioxide act as an extra blanket that keeps some of the sun's energy from dissipating into space. The extra energy from this "greenhouse effect" has already warmed the Earth by about 1 degree Fahrenheit, according to the 2001 report by the Intergovernmental Panel on Climate Change. The report is based on evidence and research from more than 2,500 scientists from about 100 countries.
Hurricanes need warm water, and the oceans are heating up, as evidenced by the 1 1/4-inch rise in global sea levels over the past 10 years, said Trenberth. The additional heat is causing most of this sea level rise because of thermal expansion -- just as a very full pot of water heated on a stove will spill over. While the warming of the oceans isn't uniform -- the north Pacific and north Atlantic are a bit cooler over the past 10 years -- the hurricane-producing mid-Atlantic and Caribbean oceans are warmer and, most important, there is more water vapor in the air.
Water vapor or moisture in the air is the high-octane fuel of hurricanes. Oceans need to be 80 degrees Fahrenheit or more to produce enough water vapor for a hurricane to get started. New research by Trenberth's group has found that water-vapor levels are now 15 percent higher on average in the hurricane zone than they were 20 or 30 years ago.
Will that result in more Category 4 or 5 storms? "That's the logical conclusion, although it may be somewhat controversial," Trenberth said.
Before it struck Cuba a glancing blow, Ivan was a Category 5 on the Saffir-Simpson scale, which rates hurricanes from one to five according to wind speeds and destructive potential. Category 5 hurricanes have winds that blow continuously above 155 mph. Ivan's gusts topped 200 mph at times, and it is considered the sixth most powerful hurricane on record for the Atlantic Basin. Hurricanes need exactly the right conditions to form, and warm water and high water-vapor levels are just two of the ingredients, said David Battisti, an atmospheric scientist at the University of Washington.
However, global warming is greatly increasing the odds in favor of more intense and more frequent hurricanes and cyclones, Battisti said. Where these storms will appear is very difficult to predict. Traditional hurricane zones may not see any increase while countries that have never experienced them will, he said. Brazil was struck by the first-ever hurricane in the south Atlantic last March, while the Atlantic coast of Canada got smacked by the storm of the century, Hurricane Juan, late last year. While these may be flukes, the Canadian government suspects global warming and is worried about the future. "What is certain, the Earth is trapping more energy and that energy must be dissipated," Battisti said.
Largely unnoticed in the attention focused on hurricanes is the record number of tornadoes the United States has experienced this year. The National Oceanographic and Atmospheric Administration says a record 173 tornadoes were reported in the month of August, 47 more than the previous record, set in 1979. Iowa has already experienced a record high of 110 tornadoes this year, when its 30-year average is just 45.
As for thunderstorms, "the evidence is very strong that their frequency and intensity has increased in the U.S.," said Trenberth.
The changing ocean and atmospheric conditions due to global warming are also making historical weather cycles or patterns less useful in helping to do long-range climate forecasts, said Battisti. "In 50 years, conditions will be so different they will overwhelm these historical cycles."