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October 2004

Hurricanes and climate change: Is there a connection?


An increase in severe hurricanes would make damage like this (Kitty Hawk, North Carolina) more common. (Photo courtesy of Carlye Calvin.)

It’s been an exceptional hurricane season in the Caribbean and U.S. Southeast, where a string of big storms left residents with hardly enough time to take the plywood off their windows after one hurricane before they had to hammer it up for the next. Millions went without power, at least 1,500 people died in Haiti alone, and the state of Florida is estimated to have $15 to $20 billion worth of damage.

And so it comes as no surprise that the relentless line-up of storms has ignited debate about the connection between hurricanes and global warming. The issue attracted a fair amount of media attention in September, when newspapers ran headlines saying “Global Warming May Spawn More Super-Storms” and “Ivan May Just Be a Messenger.”

From 1970 to 1994, hurricane activity in the Atlantic was fairly mild, generating half as many destructive storms as both the previous period, dating back to the 1920s, and the period since 1995. While the current period is the most active nine consecutive years on record and also contains some of the hottest years on record, climate scientists are divided on whether or not global warming affects hurricane activity.


Kevin Trenberth.

Kevin Trenberth (CGD) says that although it’s controversial, he thinks that global warming is in fact creating conditions that are favorable for hurricanes to be more severe. “Global climate change, and global warming in particular, create a different background environment in which the hurricanes are working,” he says. “The sea surface temperatures are a little warmer, the whole environment is a bit wetter, there’s more humidity, and that’s the main fuel for hurricanes.”

Warm water is the crucial fuel for hurricane formation. Water expands when heated, the way a full pot of water heated on the stove will overflow. The fact that global sea levels have risen 1.25 inches in the past ten years is evidence that oceans are getting warmer and expanding. When the sea surface temperature reaches 80°F or higher, it crosses a threshold for hurricane formation. Enough moisture evaporates into the atmosphere to trigger thunderstorms, which can in turn become tropical storms and hurricanes. The heat, released as water vapor, condenses in rainfall and fuels intensifying hurricanes.

Although the warming of the oceans isn’t uniform around the globe, the mid-Atlantic and Caribbean oceans have in fact warmed significantly. Kevin and CGD colleagues John Fasullo and Lesley Smith are drafting a paper to report that water vapor levels are perhaps 15% higher on average in the Atlantic hurricane zone than they were about 30 years ago. He says the logical conclusion is that this increase will result in more Category 4 and 5 hurricanes. “We’re not talking about changes in numbers of disturbances, only that the ones that do exist will be stronger and produce a lot more rain,” he says.

The hurricane season might also start earlier and last longer, Kevin says. Storms could strengthen in some parts of the world where they wouldn’t normally be vigorous enough to be considered hurricanes. Brazil was recently struck by the first hurricane ever recorded in the South Atlantic, and Hurricane Juan thrashed the coast of Canada last year.


Hurricane Frances over the Bahamas headed for the Florida coast on September 3, 2004.

Bob Gall, former MMM director and current lead scientist of the U.S. Weather Research Program, is less certain that global warming is fueling more severe hurricanes, for now at least. He says that at this point there is no evidence linking global warming to hurricanes. “Changes due to global warming, so far as I see it, are pretty small,” he says. “That doesn’t mean we aren’t on an upward trend, and I think we are, but I don’t know if the trend has been significant enough to provide a clear signal that anthropogenic warming is causing it.”

Bob says that in theory a warmer globe should increase hurricane frequency (this doesn’t conflict with Kevin’s research, which focuses on hurricane intensity), but adds that decade-long trends in hurricane frequency predate any global warming. “There is no proof that the high frequency of hurricanes this year was due to global warming,” he says. “You could explain it based on natural variation in the atmosphere.”

He also points out that under conditions of global warming, a phenomenon called polar amplification causes the planet’s polar regions to experience enhanced warming compared to other parts of the globe. Changes in the tropics, on the other hand, are relatively small.

In 2002, Junichi Tsutsui of the Central Research Institute of Electric Power Industry in Abiko, Japan, used NCAR’s Community Climate System Model version 2 to look at the implications of human-induced climate change on tropical cyclone activity. (The term tropical cyclone is used in the Indian Ocean to describe storms referred to as hurricanes or typhoons on other parts of the globe.) He found that the frequency of tropical cyclones remained unchanged in response to warming, but on a regional scale the storms in the western Pacific were more intense.

Junichi collaborates with NCAR scientists in running simulations on Japan’s Earth Simulator, the world’s most powerful supercomputer. He has used the computer for a study on the natural variability of tropical cyclones, including hurricanes. “This study is supposed to be a basis to understand possible impacts of global warming on tropical cyclones, and I want to advance this study in such a direction in near future,” he says.

• Nicole Gordon

 


Also in this issue...

Freezing drizzle: An aviation hazard that's no longer hard to see

Paul Swarztrauber looks back on 41 years at NCAR

Farewell to a well-loved tree

Digital Image Library now easily accessible

Delphi questions


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