Name: Stronger Hurricanes
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<div class="rich-text">(This video is no longer available for streaming.) Are hurricanes becoming more intense? As M.I.T. atmospheric scientist Kerry Emanuel relates in this video, over the past half century the average strength and duration of hurricanes in the tropical regions of both the Atlantic and Pacific oceans has doubled. Sea surface temperatures have also been on the rise, likely exacerbating the situation. Will this sobering trend continue?
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Is global warming making hurricanes more intense?

<div class="rich-text">Is global warming making hurricanes more intense?</div>

Stronger Hurricanes

Planet Earth

<div class="rich-text"><h3>
 STRONGER HURRICANES
</h3>

 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.
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