Increasing frequency of storms in past 25 years may not continue, although average severity may grow.
Hurricanes may become rarer in the Atlantic throughout the 21st century if the world continues to warm, suggests a new study.
The research is the latest to address the question of how — and whether — global warming will affect the intensity and frequency of hurricanes.
Globally, the number of major hurricanes has shot up by 75% since 1970. And although rising ocean temperatures are generally accepted as the key culprit — hurricanes can only form where sea surface temperatures exceed 26ºC — the link to global warming has remained a contentious issue.
In the new study, published today in Nature Geoscience1, Thomas Knutson of the US National Oceanic and Atmospheric Administration (NOAA) and colleagues used a regional climate model of the Atlantic basin to simulate the observed increase in hurricane activity between 1980 and 2006, on the basis of observed sea-surface temperatures and atmospheric conditions.
“The study does not support the notion that rising greenhouse gases are causing an increase in tropical storm frequency,” says Knutson.
They then used two versions of the model, one assuming climate warming of 2.8ºC by 2100, and one without warming, to estimate whether hurricane activity will continue to increase in the region as a result of human-induced climate change.
Overall, the number of hurricanes will decrease, with weaker storms feeling the greatest impacts. Knutson and his team predict a 27% drop in tropical storms, 18% fewer hurricanes and 8% fewer 'major hurricanes'.
“We can’t simply extrapolate the trend from the last 25 years into the future. Isaac Held , NOAA”
So, despite the fact that hurricane activity has increased dramatically in the Atlantic over the past 25 years, this trend will not continue until the end of the century under warmer conditions. “We can’t simply extrapolate the trend from the past 25 years into the future,” says co-author Issac Held, also at NOAA.
The study focused primarily on changes in the number of hurricanes, but also projected a shift towards more intense storms and heavier rainfall events. This largely concurs with recent work by Kerry Emanuel, a hurricane expert at the Massachusetts Institute of Technology in Cambridge. Using a different type of model, Emanuel projected that global warming will result in fewer hurricanes globally, but that they will become more intense in some locations.
Kevin Trenberth, a climatologist at the National Center for Atmospheric Research in Boulder, Colorado, who was not involved in the study, agrees to an extent with the findings. “The results suggest fewer tropical storms in the Atlantic, and this seems reasonable given everything else we know”.
But he cautions that the authors may have underestimated increases in hurricanes and really severe storms, owing to the fact that their model was fairly low-resolution and could not account for changes in some of the largest of these events.
“In this business it is not the numbers that matter, it is also the intensity, duration and size,” he says.
Knutson, T. R. et al. Nature Geosci. doi:10.1038/ngeo202 (2008).