Stronger Hurricanes

STRONGER HURRICANES

PBS Airdate: January 10, 2006

After Hurricane Katrina a lot of folks were thinking about our planet and were asking, "Are killer storms on Earth becoming more frequent? And if they are, why?" We've always known that hurricanes on our planet come in cycles, but reporter Chad Cohen looked back at the past season and wondered if maybe there's something else going on here.

CHAD COHEN (Correspondent): You're looking into the eye of the most intense Atlantic storm ever measured. It's not Katrina, the hurricane that devastated New Orleans. It's Wilma, the third Category 5 storm to form in the Atlantic in the 2005 season. Wilma set a new record, suggesting that more intense hurricanes may be in store.

And if there's a reason why, it's out here. Research ships like this one, run by N.O.A.A., take the ocean's temperature every three hours, 24 hours a day. As the crew reels in this elaborate probe, they bring back evidence that the oceans are heating up.

These numbers have presented us with one inescapable fact, the surface temperatures of the world's oceans has gone up a half degree Celsius in just the last 35 years. And there's nothing hurricanes like more than warm water. The more warm seawater a storm can churn into vapor, the more heat is released into the upper atmosphere. That lowers the pressure and causes winds at the ocean surface to spiral inward and pick up speed. And some scientists, like M.I.T. atmospherics professor Kerry Emanuel, believe that the rise in ocean temperatures is the result of global warming, the heating of the Earth caused by human activity.

Can we say for sure that we're causing that warming?

KERRY EMANUEL (Massachusetts Institute of Technology): The last few decades, the temperature has gone up so quickly and so far out of what we've seen in the last thousand years or so, that virtually everybody in the business now believes we're seeing a manmade signal in the global temperatures.

CHAD COHEN: And we know that hurricanes like warm water. So it's very easy to just kind of say, "There it is. We're causing hurricanes."

KERRY EMANUEL: We can't say we're causing more hurricanes.

CHAD COHEN: We can't say we're causing more hurricanes, because the total number of hurricanes worldwide hasn't changed at all. For reasons no one can explain, it always seems to hover right around 90 per year. So if global warming hasn't caused more hurricanes, has it made them stronger?

To find out, Kerry Emanuel took advantage of five decades worth of data collected by aircraft flying directly into the paths of hurricanes. He concluded that over this time, the average strength and duration of hurricanes in the tropical regions of both the Atlantic and Pacific oceans has doubled. But even more sobering is how closely this increase in storm power matches the rise in ocean temperature.

It's pretty striking, actually. You look at the graph and the sea surface temperatures go up like this, and the intensity of hurricanes goes up, it follows perfectly.

KERRY EMANUEL: Yes, that's right. I think you can say, fairly unequivocally, that half a degree rise in ocean temperature will cause hurricanes to be more intense.

PETER WEBSTER (Georgia Institute of Technology): This is the equator, here.

CHAD COHEN: At Georgia Tech, Peter Webster examined a completely different set of hurricane measurements, 30 years of global satellite observations. His conclusions mirror those of Kerry Emanuel's.

PETER WEBSTER: We find a consistency between the increase of surface temperature in all of the oceans and a change in intensity to more intense storms.

CHAD COHEN: But while both scientists agree that warmer oceans have contributed to more intense storms, they caution that formation of hurricanes is complex, to say the least, and involves many other factors, including just plain chance.

KERRY EMANUEL: Hurricanes are like any phenomenon in the atmosphere, creatures of chance. If you're interested in any given storm, it is the roll of the die. And the whole question is whether that die is weighted.

CHAD COHEN: The problem with hurricanes is that we can never say for sure what exactly happens. There's always an element of chance. So we take the temperature of the water, measure the amount of moisture in the atmosphere, we look at what the winds are doing, and depending on how all those factors come together, we get a probability that a hurricane will become either a Category 1, a 2, a Category 3, 4 or 5.

KERRY EMANUEL: I think there's little doubt that we are loading the die. We're causing global warming. The tropical temperatures are going up.

CHAD COHEN: But whether global warming is having an impact on hurricanes, well, not every scientist agrees, even those on the front lines of storm forecasting.

CHRIS LANDSEA (National Hurricane Center): A hurricane was predicted to come ashore.

CHAD COHEN: Last October, as Hurricane Wilma was poised to hit Florida, Chris Landsea, at the National Hurricane Center in Miami, helped track the powerful storm.

CHRIS LANDSEA: There's distinct cycles of hurricane activity in the Atlantic, that it tends to go 25 or 40 years very busy, and 25 to 40 years fairly quiet.

KERRY EMANUEL: 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, there...you know, 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.

CHRIS LANDSEA: We're not seeing unprecedented. We're seeing the same type of hurricane activity that we saw in the middle part of the 20th century. And it actually may have been busier back then, than it is now.

KERRY EMANUEL: A lot of things in science, sometimes there isn't a smoking gun. There isn't one piece of evidence that everybody looks at and says, "Yeah, you know, that proves that global warming is affecting hurricanes." What you have are multiple pieces of evidence which all point in the same direction.

CHAD COHEN: This high stakes debate will only be resolved with more data, and that won't be a problem. Hurricane season is never too far away.

Stronger Hurricanes

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This material is based upon work supported by the National Science Foundation under Grant No. 0229297. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

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Image credit: (Hurricane Katrina) Courtesy NASA

Chad Cohen

Correspondent

Kerry Emmanuel

Professor of Tropical Meteorology and Climate, Massachusetts Institute of Technology

Christopher Landsea

Science and Operations officer, National Hurricane Center

Peter Webster

Professor of Earth and Atmospheric Sciences, Georgia Institute of Technology