Knutson: Climate Change to Decrease Atlantic Hurricane Frequency?



“… it is not the numbers that matter, it is 

also the intensity, duration and size,” he says.




18 May 2008




Climate change ‘to make Atlantic hurricanes rarer’


Increasing frequency of storms in past 25 years 

may not continue, although average severity may 



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 



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 

Geoscience 1, 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.

Storm warning


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 



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’.


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.


Size matters


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 <>



“In this business it is not the numbers that 

matter, it is also the intensity, duration and 

size,” he says.




 1. Knutson, T. R. et al. Nature Geosci. doi:10.1038/ngeo202 (2008).




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