Is global warming causing bigger hurricanes?

The conventional wisdom debunked.
Oct. 13 2006 7:35 AM

Is Global Warming Causing Bigger Hurricanes?

The nasty scientific battle behind one of our most important weather mysteries.

Illustration by Robert Neubecker. Click image to see expanded view.

Does global warming cause bigger and/or more frequent hurricanes? This was an intriguing hypothetical question several years ago. Since 2005, which not only brought us Hurricane Katrina but set new records (28 named storms, four of which reached Category 5 strength), it has become a grippingly real one. Al Gore treats the idea that warming spawns worse storms as gospel in An Inconvenient Truth, the poster of which depicts the swirl of a hurricane rising out of a smokestack. "Temperature increases are taking place all over the world, and that's causing stronger storms," Gore declares. At one point in the film, the soundtrack turns ominous and Gore's profile is silhouetted against the glowing screen of his PowerBook. It shows a CNN feed of broken levees, flooded New Orleans streets, and Mayor Ray Nagin making his famous rant a few days after Katrina struck. It's a clear and powerful message: Global warming is not just a looming disaster we can palm off on future generations. It's here now, and people are dying because of it.

The latest big global-warming study has buttressed the link between global warming and hurricanes. The study, published in the Proceedings of the National Academy of Sciences, makes the case that man-made climate change is probably responsible for heating up tropical oceans—specifically, the two bands on either side of the equator where hurricanes form. The researchers used 22 computer models to demonstrate what many scientists already believed: that the atmosphere, warmed up by mankind's relentless pumping of carbon dioxide into the air, is also warming up the world's prime hurricane breeding grounds. (Tropical sea surface temperatures have risen by about 1 degree Fahrenheit since 1970, but a direct link to the warming atmosphere had not been so clearly established.) A media firm working for one of the study's sponsoring institutions turned the hype up a notch, billing the revelation as "the final piece of the puzzle" connecting an upsurge in powerful hurricanes to global warming.

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But that, like the alarmist certainty of An Inconvenient Truth, is a more troubling claim. The hurricane-warming link isn't settled at all. Rather, it's a very contentious debate between two groups of scientists—computer-modeling atmospheric scientists versus meteorologists—who have very different methods, ideas, and priorities. The debate has been raging for months, with attacks and counterattacks—albeit very polite ones—appearing regularly in top scientific journals. Because the issue has massive policy implications and the particulars are difficult to understand and explain, the competing groups have also resorted to dueling press releases and other forms of media outreach. Their disagreement over hurricanes isn't just an academic dispute, but a conflict that has very real consequences for how America addresses climate change.

Over the past year, four scientific papers have appeared in prominent journals suggesting that global warming is the major factor in a worldwide rise in hurricane activity. The drumbeat of studies comes from a group of atmospheric scientists whose principal tools are large-scale computer models. Their point man is Kerry Emanuel, a professor at the Massachusetts Institute of Technology who was among the first to suggest a possible warming-hurricane link in a 1987 paper. The idea makes sense intuitively—hurricanes are, in effect, enormous heat engines. In thermodynamics, a heat engine moves warmth around, releasing "work"—physical energy—in the process. A steam engine converts heat to spin gears or wheels. As hurricanes vent heat upward into the cooler reaches of the atmosphere, they convert some of it into high winds, tornadoes, and condensing falling rain. If the air and ocean temperature go up, it's logical to theorize that more and/or bigger storms will form to dissipate the extra heat and moisture. "When you can evaporate water faster into the atmosphere, you are essentially pumping heat into the engine at a great rate," Emanuel says.

In 1987, Emanuel could only theorize. But starting in 1995, hurricane activity in the Atlantic spiked upward alarmingly at the same time as global temperatures were steadily rising—and with them, ocean temperatures. In a field that trades in century-long projections and grand abstractions, Emanuel was handed a rare opportunity: The atmosphere, oceans, and storm activity were all changing before his eyes, fast enough, perhaps, to tease out connections as they happened. One question was how to measure hurricane activity. There are many ways to do it—numbers of storms, their relative strengths (a function of wind speed), atmospheric pressure in the eye, et al. Emanuel came up with something he dubbed the Power Dissipation Index, the annual sum, in a given region, of all storms' cumulative energy output. When he crunched the numbers, Emanuel found that around the world the power of hurricanes was spiking upward at rates that far exceeded anything that had come before. He also found a strong correlation between those increases and rising sea-surface temperatures. He published those results in a paper last fall, which was followed quickly by a related study led by scientists at the Georgia Institute of Technology. Their analysis of global data showed that the proportion of big storms—Category 4 and 5 on the Saffir-Simpson hurricane scale—had more than doubled since 1995. That suggests that global warming may not lead to greater numbers of hurricanes, but that it indeed may be pumping weak ones up into monster storms.

Enter the skeptics, led by Chris Landsea (yes, that's his real name), one of the National Oceanic and Atmospheric Association's top hurricane researchers. The group is made up of meteorologists—scientists who make their living forecasting the annual shifts in hurricane activity and the paths of individual storms. It includes Max Mayfield, the outgoing director of the National Hurricane Center, who famously warned President Bush that Katrina was likely to overwhelm the New Orleans levees. Most aren't global-warming skeptics, but they have attacked Emanuel's research, the Georgia Tech study, and others that suggest a hurricane-warming tie. Their favored technique is to comb the data—which, for meteorologists, is a stock in trade—and expose inconsistencies. They point out that the quality of hurricane measurements varies depending on where and when they were compiled. The technology and standards employed by developing nations in the Pacific basin have historically lagged behind the United States and other nations around the Atlantic basin. Infrared satellite photography of hurricanes—which allows easier assessment of size and strength—didn't begin in earnest until the 1980s. All of these problems, argue the skeptics, fatally compromise the global trend lines that show we're in an emerging era of souped-up megastorms. In a paper published in May, Philip Klotzbach of the University of Colorado—who issues well-known annual hurricane forecasts with William Gray—curtly dismissed the Georgia Tech study's conclusions about more big hurricanes: "Most of this increase is likely due to improved observational technology. These findings indicate that other important factors govern intensity and frequency of tropical cyclones besides SSTs (sea surface temperatures)."

Instead, Landsea and his colleagues have placed their chits—at least as far as the western Atlantic and neighboring eastern Pacific, the main focus of their own research—on a phenomenon called the Atlantic Multidecadal Oscillation. The AMO consists of decades-long ups and downs in sea-surface temperatures. Landsea and his colleagues say it is principally responsible for the sharp rise in hurricane activity since 1995. While a significant leap, they say, it doesn't diverge from dramatic shifts measured over the past century. (One reason storms seem more severe, they correctly note, is that there is simply more human settlement along the coastlines than in the previous up-cycle during the 1920s to the 1960s—ergo, more stuff gets destroyed now than used to.)

Landsea argues that if ocean temperatures are going up, and the worldwide trends toward bigger hurricanes are overstated or nonexistent (he and his allies tie the rise in the Atlantic basin to the AMO and dispute studies that say the same thing is happening elsewhere around the globe), then global warming can't be having much effect. Plus, many other things affect hurricane strength—for example, the shifting pattern in the prevailing winds in the upper atmosphere. Close to the tops of hurricanes, the vertical wind shear—varying wind speed at different altitudes—can make or break a storm. If wind shear is high, it can literally lop off the tops of thunderstorms, stifling nascent hurricanes. And if wind shear is low or nonexistent, hurricanes will flourish unimpeded. No one has yet tied changes in wind shear to global warming.

As it happens, vertical wind shear is particularly hard to incorporate into the global computer models used by scientists chasing the global warming link. And therein lies the chief difference between the two camps. It's not a difference of opinion, but of basic perspective, how different cliques of scientists view the world quite differently. The groups represent two distinct nerd archetypes. The meteorologists of the Landsea group are saying, we know better than anyone how the weather works, and you're oversimplifying it. They're more detail- and history-oriented, involved in short-term forecasting—workaday nerds, in other words, and temperamentally more conservative. "Hurricane forecasters are focused on what is the hurricane out there going to be doing in the next 24 hours," Emanuel says, "and that's not going to have much to do with global warming." And, indeed, their main objections are not a direct rebuttal of the theory on global warming and hurricanes, but a chipping away at the foundations, the data. They have kept their opponents busy rebutting their critiques, but have not really presented a clear argument why global warming should not be disrupting whatever natural cycles exist out there.

Emanuel and the climate scientists, by contrast, are more like prima donnas, looking for the big breakthrough, more willing to go out on a limb in their theories and speculations.

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