Warming Increasing Hurricane Frequency: NCAR Study

The Atlantic Ocean is getting stormier, and climate change is responsible, according to research by a Colorado meteorologist and colleagues. About twice as many hurricanes are forming in the Atlantic as were occurring 100 years ago.

“These numbers are a strong indication that climate change is a major factor,” says Greg Holland of Boulder’s National Center for Atmospheric Research.

Holland’s paper will probably prove to be very important as well as controversial. Most of the talk recently about climate change has been about how expensive it will be to address the issue. Holland’s paper implies that real economic damage may be occurring at least partly as a result of  more frequent storms caused by the changing climate.

A close-up view of the eye of Hurricane Isabel, the deadliest and costliest storm of the 2003 season. Photo credit: NASA
The changing strength and frequency of hurricanes has been a controversial issue in climate research. Even a few years ago, climatologists were reluctant to speculate about even whether more storms were occurring. The data for the period prior to 1945 was incomplete because there was neither storm-observing planes nor satellite observation.

As recently as November of 2006, a consensus statement by researchers from the International Workshop on Tropical Cyclones concluded:

Currently there is large overall uncertainty in future changes in tropical cyclone frequency as projected by climate models with future greenhouse gases. The most recent results obtained from medium and high resolution GCM (general climate models) indicate a consistent signal of fewer tropical cyclones globally in a warmer climate.

According to University of Colorado Prof. Roger Pielke, Jr., hurricane damage in the U.S. totaled more than $150 billion in 2004 and 2005. The image of climate-driven catastrophe was brought home to many people by the devastation of Hurricane Katrina, which former Vice President Al Gore used to dramatic effect in the movie An Inconvenient Truth.

Scientists are reluctant to attribute any single event to global warming, but Holland’s research found three pronounced periods of increased hurricane activity since about 1900.

“The analysis identifies three periods since 1900, separated by sharp transitions, during which the average number of hurricanes and tropical storms increased dramatically and then remained elevated and relatively steady. The first period, between 1900 and 1930, saw an average of six Atlantic tropical cyclones (or major storms), of which four were hurricanes and two were tropical storms. From 1930 to 1940, the annual average increased to 10, consisting of five hurricanes and five tropical storms. In the final study period, from 1995 to 2005, the average reached 15, of which eight were hurricanes and seven were tropical storms.”

Holland said in an interview that global warming has caused sea surface temperatures to increase in the Atlantic. Ocean temperatures provide the engine for development and strength of hurricanes. But the SSTs don’t in themselves increase frequency.

“It’s not a direct relationship,” Holland says. “The only time series we have are the hurricanes and the sea surface temperatures” But there is a statistical relationship between SSts and frequency. The physical mechanism is “the sea surface temperatures change, and they  change the atmospheric circulation … It’s not that the SSTs went up and magically we had more hurricanes. Changes to the wind field change the tropical cyclones.”

CU’s Pielke says, however, that hurricane and tropical storm frequency is not what is driving the increase the damage from these phenomena. The main problem is simply that more people live on the coasts.

In an email (a generous response, since he’s on vacation in England), Pielke said:

The bottom line of my analysis is that whatever scenario one envisions for the future the societal factors overwhelmingly dominate the growth of future losses, regardless of what the climate does.  This DOES NOT mean that we can ignore climate mitigation, which makes sense for many reasons.  It does mean that reducing hurricane losses should not be a primary justification for changes to energy policies.

According to one estimate Pielke provided, one scenario of population trends indicates “combined global population and wealth in locations exposed to tropical cyclones will be 7 times greater than today” by 2050. This growth of wealth and population along the coasts is more important in causing the severe economic losses from hurricanes and tropical storms than any increase in storm frequency, Pielke says.

Holland says, though, “It’s something that’s serious, and it’s something we need to pay very serious attention to. The certainty level has risen to the point where we just simply need to start taking policy and political decisions which take into account the likelihood of something being not so good in the future.”

The new hurricane study by Greg Holland and Peter Webster is appearing in Philosophical Transactions of the Royal Society of London.