Global warming supplied about half the heat to the North Atlantic waters that spawned a record number of hurricanes in 2005, a study by the National Center for Atmospheric Research says.
The study, to be published next week in Geophysical Research Letters, says climate change and not just natural cycles was a major factor in the Atlantic hurricanes – including the Category 5 storms Katrina, Rita and Wilma.
The global warming “increases the risk of future enhancements in hurricane activity,” said Kevin Trenberth, one of the study’s authors and a scientist at Boulder-based NCAR.
The study was released on the same day the National Academy of Sciences reported that Earth is the hottest it’s been in at least 400 years. That panel of climate scientists told Congress that human activities are responsible for much of the recent warming.
Gases such as carbon dioxide from burning fossil fuels, which can trap heat, are building up the atmosphere. Carbon dioxide levels have risen about 19 percent in the past 48 years, according to federal data.
Global-warming skeptics challenged the new hurricane study, which found that during much of the 2005 hurricane season, sea-surface temperatures across the Atlantic were a record 1.7 degrees Fahrenheit above the 1901-1970 average.
Most researchers agree that the warm waters fueled hurricane intensity, but they split on the cause – greenhouse-gas fueled global warming or a natural decades-long cycle.
Earlier studies indicated the North Atlantic oscillation – a long-term ocean temperature and air pressure cycle – was to blame for the warming and cooling patterns associated with hurricane activity.
The NCAR study, which looked to global temperature patterns, said that global warming – not the oscillation – is responsible for the majority of the warming trend.
The NCAR study says the oscillation is actually weaker than previously thought.
William Gray, a longtime hurricane forecaster at Colorado State University, rejected the the NCAR contention that global warming was the main culprit for the hurricanes and that the Atlantic oscillation had weakened.
“That is not so – it’s just as strong as it was in the ’40s and ’50s when we had such a strong season,” Gray said.
In 2005 there were a record 28 tropical storms, 15 turning into hurricanes.
Gray said that other factors – such as upper level winds play a role in hurricane activity and should have been noted in the NCAR study.
The study’s authors said that warming sea-surface temperatures don’t mean that each year will set records for hurricanes.
“Each year will bring ups and downs in tropical Atlantic surface sea temperatures due to natural variations,” Trenberth said in a statement.
“However, the long-term ocean warming should raise the baseline of hurricane activity,” he said.
Staff writer Kim McGuire can be reached at 303-820-1240 or at kmcguire@denverpost.com.
