Seeking to Link Climate Change and Hurricanes

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MELISSA BLOCK, host:

Hurricanes draw their strength from the ocean, and scientists are finding a connection between global warming and hurricane intensity around the world. Kerry Emanuel is a professor of atmospheric science at MIT and author of the book "Divine Wind: The History and Science of Hurricanes."

Welcome to the program.

Professor KERRY EMANUEL (MIT): It's nice to be here.

BLOCK: And, Dr. Emanuel, explain for us, if you would please, how hurricanes form and how global warming would affect that.

Prof. EMANUEL: Well, there are about 90 hurricanes that form around the world every year for which about 10 form in the Atlantic, and the process of formation is called, not surprisingly, genesis. But once they form, they develop and they operate by sucking heat energy out of the ocean through the evaporation of seawater and they take the heat energy and convert it into the energy of the winds. And we understand that process well enough that we can say with some certainty how strong a hurricane could get given the ocean temperature and the profile of the temperature in the atmospheric environment of the storm.

BLOCK: So the notion then would be that the intensity of a hurricane would directly depend on the temperature of the ocean?

Prof. EMANUEL: That's one of the most important things it depends on. And, in particular, the ocean temperature provides a limiting intensity for the hurricane. So if we know the ocean temperature, we can say with great accuracy that a particular hurricane can't get more than intense than some particular wind speed that depends on that temperature.

BLOCK: And just to be clear here, ocean temperature would not mean that hurricanes are becoming more frequent, just that they're becoming more intense, stronger.

Prof. EMANUEL: That's right. If you look at the record over the last 30 to 50 years of storms in the Atlantic, Pacific and Indian oceans, you see that there's been really no change whatsoever in the frequency. But on the other hand, if you look at the intensity, in the global data set, there does appear to be an upward trend actually both in the intensity and the duration of storms. And this upward trend is very nicely synchronized with an upward trend in the temperature of the tropical oceans.

BLOCK: What do the numbers show on that in terms of this upward trend that you're talking about?

Prof. EMANUEL: What the data actually show is that if you look at a measure of the total amount of power generated by hurricanes globally over the last 30 to 50 years, it seems to have increased by somewhere on the order of 70 percent to 80 percent, a really big increase. Now that does not mean that the wind speeds have gone up 70 to 80 percent; we think the wind speeds have gone up something more like 10 percent or so.

BLOCK: And that correlates to what change in the ocean temperature over that same period?

Prof. EMANUEL: What we see when we look at the data is that tropical ocean temperatures have increased by about a half a degree Centigrade, or one degree Fahrenheit, over the last 30 to 50 years. Now that might not sound like very much, but the ocean has an enormous heat capacity and this represents a really big increase in the heat content of the oceans. You see particularly in the last 15 years or so an upswing that you really can't find an analog to in the record going back hundreds of years.

BLOCK: You also talk about a human element of all of this. You say this is being made much worse by human choices, people rushing to populate tropical coastlines.

Prof. EMANUEL: In fact, the people working in hurricanes have for a long time believed that the really big problem is the demographic problem. We've seen huge increases in the population of coastal regions which are vulnerable to hurricanes. And at the same time, there's been a big increase in the infrastructure that's in harm's way. That is the immediate pressing problem, I think. If you were looking out a hundred years or more in the United States, you'd start to worry about global warming effects on hurricanes. But for the next 20 to 50 years, it's the demographics: What do we do about increasing coastal population? How do we change our policy with respect to things like insurance to either slow this down or make it more likely that people will build much stronger structures in coastal regions?

BLOCK: Dr. Emanuel, thanks very much for talking with us.

Prof. EMANUEL: You're welcome.

BLOCK: Kerry Emanuel is professor of atmospheric science at MIT and author of the book "Divine Wind: The History and Science of Hurricanes."

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