By Jared Chausow
By Katie Toth
By Elizabeth Flock
By Albert Samaha
By Anna Merlan
By Jon Campbell
By Jon Campbell
By Albert Samaha
"Hurricanes in the strongest categories (4 and 5) have almost doubled in number" from 1970 to 2004, reads a piece in the September 16 edition of Science. What's more, the journal says, strong hurricanes are making up a larger share of all hurricanes. A month earlier, in Nature, M.I.T.'s Kerry Emanuel created an index of how destructive hurricanes are and found "that this index has increased markedly since the mid 1970s . . . due to both longer storm lifetimes and greater storm intensities"and all during a span when the sea surface temperature rose half a degree Celsius.
The connection between heat and hurricanes makes sense because of the way those storms form. The hotter water is, the more quickly it evaporates. And it's through water evaporating, rising as vapor, condensing, and falling as rain that a hurricane gets the energy that powers it. "The hurricane is kind of like a giant heat engine," says Old Dominion University professor Robert Tuleya. "Basically the energy is transferred from the ocean to the atmosphere."
"The more heat that's transferred, the more intense the storm will be," adds Jay Gulledge of the Pew Center on Global Climate Change.
But is global warming really to blame for these trends? After all, there are cycles of high and low hurricane activity, and they last for decades. "We have good instrumental records of both water and surface temperatures extending well beyond the 30-year cycle of hurricanes, and there is a climate-warming signal in those data that is above and beyond the 30-year cycle," Gulledge says. And the greenhouse effect, he says, is "the most obvious mechanism" for causing that rise in temperatures.
Even the EPA acknowledges that "the Earth's surface temperature has risen by about 1 degree Fahrenheit in the past century, with accelerated warming during the past two decades" and notes that "new and stronger evidence" says "most of the warming over the last 50 years is attributable to human activities."
Scientists aren't trying to pin any particular storm on global warming; to do so, says Emanuel, "would be absurd." And while there's evidence that warming produces stronger storms, it's unclear whether it leads to more frequent hurricanes, because scientists don't yet have a good grasp on exactly what triggers storms.
So far, there's been no long-term increase in the frequency of hurricanes. But that could be changing. Since 1996, 21 hurricanes have hit the United States, making the last decade the most active since the 1940s. This year's storm activity has been furious; the Tropical Prediction Center has only four slots left on the list of names for the year (Stan, Tammy, Vince, and Wilma). "This one may end up being the most active of all time," says Gulledge of the 2005 season. "This year is phenomenal. It's truly phenomenal."
Also phenomenal is something that may have occurred during Katrina's ride to the Gulf Coast. It appears that the deadly storm tracked along a band of particularly deep, warm waterthat the heat not only fueled the hurricane but actually steered it. "It just looks like a red carpet going from the western Caribbean right up to the Gulf Coast," says Gulledge. Little is understood about what steers storms, Gulledge acknowledges, but if a hot track is indeed what directed Katrina, "it could be a kind of steering mechanism to bring hurricanes into the Gulf of Mexico more often."
Of course, there are skeptics about all this. Their denials are all over the map. Gary Sharp, an ocean researcher and consultant, claims "some of the worst hurricanes that we've ever had have been during cooling" and says, "There's no evidence that there's an enhanced or growing warm- water mass." And even if there is a shift in temperature, he adds, "climate transition is what life is all about"in other words, it's part of the Earth's cycles, not a result of human acts.
Anthony Lupo, a professor at the University of Missouri, says, "It's very difficult to even make a case that there's been a connection from the climate warming to intense hurricanes." The reason, he says, is that scientists' ability to measure the strength of hurricane winds has improved only recently, so it's possible some of the old storms were stronger than we think, and there's been no increase in intensity. (Other scientists counter that measurement of the central barometric pressure of storms, another indicator of intensity, has been consistent.)
Yet another argument, made in a recent letter to Senator John McCain that both Sharp and Lupo signed, is that if global warming does occur and makes the north and south poles warmer, that will actually decrease hurricanes, becausethey arguethe differences in temperature between the tropics and the poles are what causes storms.