Hotter Oceans, Fiercer Storms

Is it fair to blame a single devastating storm like Katrina on global warming? "It's nonsense. It's statistical nonsense," says atmospheric scientist Kerry Emanuel. It's misguided to pinpoint global warming as the cause of specific storms. Yet both Emanuel and meteorologist Peter Webster and his team have gathered powerful evidence that rising sea surface temperatures have led, in general, to more intense storms over recent decades. Below, see what the scientists have to say and examine their data.—Susan K. Lewis

Note: Tropical cyclones are known around the world by different names—typhoons, hurricanes, cyclones. For simplicity, we refer here to all these storms as hurricanes.

Dr. Kerry Emanuel, MIT

Read the following excerpt from Kerry Emanuel's interview with NOVA scienceNOW. Then take a look at graphs he published in an August 2005 paper in Nature entitled "Increasing Destructiveness of Tropical Cyclones Over the Past 30 Years."

Emanuel: I was surprised. When I did this analysis in the Nature paper, I wasn't even looking for any kind of global trends. And global warming was far from my mind. I was looking for natural variability in the amount of energy expended by hurricanes. And you could see the natural variability, but on top of that, it's this trend that we couldn't really get rid of. It became worrying, and led to the Nature paper.

We find that in the Pacific, as well as in the Atlantic, there's this excellent correlation between this measure of hurricane energy that we developed and the temperature of the tropical ocean. It's very in concert on all kinds of different time scales. And the amount of energy expended by hurricanes has gone up in the last 50 years by somewhere between 50 and 80 percent.

If all we had to go on was the hurricane data, I don't think we would be terribly alarmed. We'd just say, well, it's been changing the last 25, 30 years, so what? It's the correlation with sea surface temperature and the fact that that trend is unprecedented for a long time that has us worried.

North Atlantic Storms
Emanuel devised a way to measure the amount of energy hurricanes release. He draws on wind speed data and factors in the number of days each storm lasts. Emanuel calls his measure PDI, or power dissipation index. The graph above shows the energy released annually by all storms in the North Atlantic compared to the September average sea surface temperature (SST) over a region known to generate storms. The correlation between the lines suggests that sea surface temperature influences how much energy storms release. Perhaps even more striking, the total Atlantic hurricane power dissipation has more than doubled in the past 30 years.

North Pacific Storms
As in the North Atlantic, there has been a remarkable upswing in the amount of energy released by hurricanes in the western North Pacific since around 1975. Over this time, power dissipation has increased by about 75 percent. The graph above shows the power dissipation index (PDI) compared to the July-November average sea surface temperature (SST) over a storm-generating region. Some of the upward and downward swings in PDI throughout the graph are associated with El Niños and other regional forces—part of what Emanuel calls "natural variability." Global warming, however, may be contributing to the sharp upswing in the last 30 years.

Combined Data
In an effort to verify that global warming trends are impacting hurricane energy, Emanuel compared the sum of the North Atlantic and western North Pacific PDI values to the average annual sea surface temperature (SST) in a wide swath around the equator. Once again, a significant rise in the amount of storm energy released in recent decades mirrors a rise in sea surface temperature. As this graph shows, the combined PDI has nearly doubled over the past 30 years.

Continue to see evidence from Peter Webster and his team.

© | Created January 2006