POSTED 8 SEPTEMBER 2005
Hurricanes: A global warming connection?
A series of horrific hurricanes in the Atlantic, capped by the cataclysmic Katrina, raises the question: Are hurricanes getting stronger now that global warming has started? Before we explore the relationship, let's recall some hurricane fundamentals:
Hurricanes are tropical cyclones -- rotating storms that develop in the tropics -- with winds exceeding 75 mph.
Tropical cyclones are driven by energy contained in water that evaporates from the warm sea surface.
Hurricanes don't develop when the sea-surface temperature is below about 26° C.
Given these facts, you might think that the warming ocean, heated by the global-warming effect of greenhouse gases, would lead to more hurricanes -- and more intense hurricanes. But while the greenhouse effect has warmed the globe by about 0.5°C, hurricanes (called typhoons in the Pacific) have not grown more common. Reflecting a consensus among climatologists, Kerry Emanuel, a professor of meteorology at the Massachusetts Institute of Technology, wrote: "There is no detectable trend in the global annual frequency of tropical cyclones" in historic data that would indicate that hurricanes are growing more common on the warming Earth (see "Increasing Destructiveness..." in the bibliography).
But what about hurricane intensity? When you turn the heat up under a stewpot of gumbo, the prawns, chilies and okra start sloshing around with extra vigor. If you turn up the planetary thermostat, does that happen with hurricanes?
Let's cleave the question: Have hurricanes gotten stronger now that greenhouse gases have warmed the planet? Will hurricanes get stronger as the globe warms further?
Warming and storming in the real world
Two months ago, the answer to the first question was, we expect stronger hurricanes, but the data doesn't show an increase in intensity. Then, about a month before Hurricane Katrina, Emanuel published a major study showing a significant increase in hurricane power over the past 30 years. Emanuel looked at records of wind speed and hurricane duration from 558 Atlantic hurricanes, and 1,557 Pacific hurricanes. To calculate energy production, he multiplied wind speed by itself three times, then factored in how many hours the hurricane lasted. (Wind speed cubed correlates well with hurricane damage.)
Overall, Emanuel found that by his measure, hurricane power had more than doubled in the past 30 years in the Atlantic, and almost as much in the Pacific. And when he graphed intensity against the temperature of the ocean surface, the lines were similar. "It shows a big upward trend, globally, beginning in the 1970s, that's very much in concert with tropical ocean temperature," Emanuel told us.
. Data from "Increasing Destructiveness ..." (see bibliography).
Tellingly, the results were similar in the tropical Atlantic, the Western Pacific and the Southern Hemisphere, he said. "What give us confidence is that, in the three major places where we have hurricanes... the trend is upward, and the same magnitude." Regarding the intensity index and sea-surface temperature, he added, "It would really be a coincidence if these happened to covary [move together] in all the places you look."
But could hurricanes get almost twice as strong from a sea-surface warming of just 0.5° Celsius? Apparently. ".5 degree doesn't sound like much," Emanuel said, "but given the actual heat content of the tropical ocean, that is an awful lot, a big change. It has produced a temperature in the tropical ocean that is warmer than in the last several thousand years."
And half a degree is a lot less than the 1.7° C rise in sea-surface temperature that several climate models have projected for late the 21st century, after atmospheric carbon-dioxide has doubled. (We'll get to the predictions about future hurricanes shortly.)
The results have been noticed in the hurricane biz. "I was initially surprised, a bit stunned, because of the magnitude of the change he was showing," says Thomas Knutson of the Geophysical Fluid Dynamics Laboratory. "A doubling of the power dissipation since the 1970s is a very large change, surprising. It's a very interesting paper." (Power dissipation is the amount of energy the hurricane extracts from the warm sea and uses to drive its winds.)
Emanuel's study merits scrutiny, says Galen McKinley, an assistant professor of atmospheric and oceanic science at University of Wisconsin-Madison. "Professor Emanuel is an expert on hurricanes, but these kind of things, when they are introduced, need some time to work through the scientific community and be generally accepted or refined." While most studies have looked at the number of hurricanes, or their Saffir-Simpson category, "This is another way of looking at it," she adds. "How do we want to define this index [of destructiveness]? There is always a debate about what is the proper index."
Experts are already prowling through the entrails of Emanuel's analysis. For one thing, Emanuel used the cube of wind speed, not wind speed itself, in his calculations, which had the effect of enhancing the impact of the worst storms. Although previous indexes of hurricane power have looked at velocity squared, Knutson observes that wind damage rises along with the cube of wind speed: "It's reasonable to look at velocity cubed."
Another question concerns the bugaboo of climate history: the veracity of old records. "One thing he wrestled with was the character of the historical record," says Kevin Trenberth of the National Center for Atmospheric Research. "There's an inconsistency in the earlier years between estimates of velocity and central pressure. We know there is a very strong relationship between those in the last 20 years, where we have good data, but in the early years, the relationship is not so good, so it depends on which you believe more."
Trenberth, who directs NCAR's climate analysis section, has been thinking about the relationship between global warming and hurricanes, and suggesting a shift in focus from counting hurricanes to assessing intensity.
By evaporating more water from the sea, he says, global warming could easily raise intensity, because a higher sea-surface temperature "increases the energy available for storms." Although there is more water vapor in the atmosphere, "it's not at all clear how much goes into individual thunderstorms and how many of these are organized into hurricanes," which are vast collections of thunderstorms.
Trenberth suggests that global warming has probably already made hurricanes more severe, even if it's hard to see the signal. But Katrina has also flushed out skeptical climate experts. The Detroit News , for example, quoted William Gray, a noted hurricane expert at Colorado State University: "There is absolutely no empirical evidence. The people who have a bias in favor of the argument that humans are making the globe warmer will push any data that suggests that humans are making hurricanes worse, but it just isn't so."
But Trenberth says Gray's off base. "Sea-surface temperature is rising because of global warming, and the role of humans is clearly established in that. That increase in sea level contributes to the storm surge. The increase in sea-surface temperature is increasing water vapor, and that is increasing rainfall, which raises the potential for flooding." More water vapor also adds energy to the storms, he adds. To "unequivocally state that global warming has nothing to do with what going on in Katrina, I think they are wrong. ... To say it has no role is totally irresponsible in my view."
We tried, but failed, to reach Gray for comment.
What do computer models say about hurricanes of the future?
Megan Anderson, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive