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Launching a new field of science is always tricky, but starting up the study of hurricane behavior over the decades—in the wake of Katrina—has proved challenging indeed A Tempestuous Birth for Hurricane Climatology

MONTEREY, CALIFORNIA—It’s not every day warming, they concluded, it would noticeably warming, which explains why no one had been

one can witness the inception of a new field. amplify the destruction in coming decades. In searching for a trend in the ups and downs of the S E G

Researchers attending the 27th Conference on the wake of , those analyses storm record. Then, at an October 2004 press A M I

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Hurricanes and Tropical late last were enough to spark a highly public and some- conference, meteorologist Kevin Trenberth of T T

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month got their usual diet of potential vorticity times raucous new field: hurricane climatology. the National Center for Atmospheric Research G / C I

analyses and Madden-Julian oscillations. But (NCAR) in Boulder, Colorado, reacted to H P A

they also debated a newborn science created to For some, a turnabout claims that the particularly active 2004 season R G O

assess whether tropical —variously In theory, tropical cyclones aren’t supposed to in the Atlantic Ocean was just part of a natural E G

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called hurricanes, typhoons, or cyclones—have be noticeably stronger after a few decades of cycle that pumps up Atlantic storms every few A N O strengthened under global warming. decades. He and the others on the I T A

Science Nature N

Until and pub- panel “didn’t think that was right,” / lished two papers last year contend- says Trenberth. “We thought global T BEL ing that tropical cyclones around the warming was playing a role.” The world had strengthened, few scien- burst of hurricane activity since

tists paid much attention to long- 1995 just seemed too strong to be GRIFFITHS term variations in such storms. In entirely natural. those papers, a couple of academic Others thought Trenberth was ANNIE meteorologists took the long view of the one getting it wrong. “We were tropical records compiled rather skeptical” of Trenberth’s IMAGES; day-by-day by forecasters. remarks, says Peter Webster, who From those weather records, the specializes in monsoons and other meteorologists extracted storm cli- tropical phenomena. “We thought matologies, statistical histories of Kevin was sounding his trumpet a MUNTEFERING/GETTY

storm behavior that could be little bit.” So Webster, a meteorol- T R E

searched for any change over the ogist at the Georgia Institute of B O R

: decades. Unexpectedly, they found a Technology in Atlanta, and his ) M O

surge in intensity— colleagues looked at records of T T O

and if it continued under global maximum wind speed of storms B

O T around the world as gauged from * 27th Conference on Hurricanes and P O T

satellite images. ( Tropical Meteorology, 24–28 April, Mon- Blown away. Tropical storm power and destruction S T terey, California (sponsored by the Amer- go up with the cube of wind speed. In the 16 September 2005 issue I D E

ican Meteorological Society). Science R

of (p. 1844), less than 3 weeks C

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Rising threat? The number of Atlantic hurricanes tropical cyclone. But the Japanese group simu- minor factor” in “the most devastating hurri- (left) jumped in 1995, but globally the number of lated the present climate and the greenhouse- cane season the country has experienced in tropical cyclones is steady. The debate is over warmed climate near the end of the century at modern times.” The surge in whether storms have been getting stronger as the a resolution of just 20 kilometers, thanks to the activity since 1995 is the latest upswing in a world warms. Earth Simulator’s power. That was detailed natural cycle, the releases said. As the Atlantic enough for tropical cyclones to appear in the Ocean warms and wind patterns shift, hurri- after Hurricane Katrina ravaged New model, allowing the researchers to roughly canes increase for a decade or two until a lull Orleans, Webster and colleagues reported gauge their intensities. In the warmer world, sets in again. that in fact the abundance of tropical the total number of storms over the globe NOAA’s seemingly official pronounce- cyclones had not increased between 1970 and had actually decreased by 30%. But the num- ments sounded moderate next to that of 2004. But the number of the strongest ber of the rarer category 3 and 4 storms had William Gray of Colorado State University storms—those in categories 4 and 5—had increased substantially, not unlike Webster’s (CSU) in Fort Collins, an elder statesman of jumped 57% from the first half of the period observational results. the hurricane community. Well known for his to the second. That reinforced findings by Additional support for intensification also forecasts of the coming hurricane season, meteorologist and hurricane specialist Kerry came from a new, independent analysis of wind Gray pushes natural cycles with a vengeance. Emanuel of the Massachusetts Institute of data, mentioned in passing at the conference. “There’s definitely a big multidecadal cycle Technology in Cambridge. He had reported in the 4 August 2005 issue of Nature that the 15 total power released during the lives of Atlantic and western North Pacific storms Data Unreliable had risen between 40% and 50% from the first half of a 45-year record to the last half. Both Webster and Emanuel were taken ) aback by their own findings. “I changed my 2 ds 10

mind in a big way” about how much the on warming could be intensifying storms, says /sec 3

Emanuel. But it wasn’t just because of the s apparent upward trend of storm intensity. When Emanuel looked at how storm power and ocean temperature had varied, “what I found startled PDI (meter S

me,” he told the conference. In the area just / T 5 S

north of the equator in the Atlantic Ocean, S where most hurricanes get their start, the power Scaled August–October released during the lifetimes of storms is Sea-Surface Temperature “spectacularly well correlated with sea surface Adjusted Atlantic Storm temperature,” says Emanuel. Hurricane inten- Power Dissipation Index sity had risen along with temperature over the past half-century, even matching ups and 0 downs along the way. 1860 1880 1900 1920 1940 1960 1980 2000 2020 Year A The region where Atlantic hurricanes

NAS develop has in turn warmed in step with the Cause and effect? As late-summer water temperatures rose in the tropical Atlantic, where Northern Hemisphere for the past half-century, hurricanes get started, the power released by Atlantic hurricanes rose too. Emanuel noted. And that warming is widely held to be driven at least in part by rising green- Climate researchers Ryan Sriver and Matthew going on,” he said in his (BACKGROUND) house gases. Two studies may not be enough to Huber of Purdue University in West Lafayette, conference talk, not just in prove that Trenberth is right about greenhouse Indiana, will soon report in Geophysical the Atlantic Ocean but warming driving storm activity, but both Research Letters on how they measured the around the world. “This Emanuel and Webster now believe they see a power released by storms by using a compila- isn’t global-warming strengthening of tropical cyclones suspiciously tion of the world’s weather data developed at the induced; this is natural. in synchrony with global warming. European Centre for Medium-Range Weather We may go for a few PHOTOGRAPHY/CORBIS; Forecasting in Reading, U.K. They found a years, and then it’s going to Stormier models 25% increase in storm power between the first cool.” And with the cooling, hurricanes SCHEIN Newly minted hurricane climatologists at the half of the 45-year record and the second, con- will calm down again, he said, just as they

ALAN conference got some support from climate sistent with Emanuel’s analyses. have before. modelers. Kazuyoshi Oouchi of the Advanced The warming of recent decades and the jump (RIGHT) ; T

I Earth Science and Technology Organization in Even stormier objections in hurricanes are being driven by a surging M /

L Yokohama, Japan, and his colleagues pre- Intensifying tropical cyclones possibly driven ocean current that brings warm water northward E U

N sented results from highly detailed climate by the greenhouse might sound like one more into the Atlantic Ocean, Gray insisted. Climate A M

E simulations run on Japan’s Earth Simulator, the chapter in the familiar global warming story— models in which mounting greenhouse gases

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) world’s most powerful supercomputer devoted melting glaciers, rising seas, searing heat drive global warming have simply got it wrong, E C R

U to earth sciences. Global climate models typi- waves—but some read it differently. The U.S. he said; greenhouse gases are feeble agents of O S ( cally calculate climate at points 200 kilometers National Oceanic and Atmospheric Administra- warming. “I’m a great believer in computer : S T I or more apart. The resulting pictures of climate tion (NOAA) stated last year in press releases models,” he said. The audience laughed skep- D E R

C are too fuzzy to pick up anything as small as a that “longer-term appears to be a tically. No, he assured them, “I am—out to 10 or

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12 days. But when you get to the Storm central. Planes were able climate scale, you get into a can to penetrate Katrina’s eye wall of worms. Any climate person (left) and drop probes through the who believes in a model should storm (lower left), greatly improving have their head examined. They storm-intensity measurements. all stink.” “This was highly entertain- either. By the time Hurricane ing,” observed meteorologist Carol hit in 1954, Landsea Gregory Holland of NCAR noted, forecasters were flying from the audience, “but unfor- into storms—if they weren’t too tunately you obfuscate the real strong—and judging wind issue.” Holland, a graduate speed by looking down at waves student of Gray’s at CSU in the on the ocean. Sometimes they early 1980s, is second author on would estimate maximum Webster’s Science paper. He winds by making their best went on to staunchly defend climate models. guess of how winds would change from their “Right now, they’re providing a very good, aircraft’s altitude to the near-surface. Such rational picture of the situation,” he concluded. sampling wouldn’t have caught Hurricane Supportive applause rose from the audience. Wilma’s 1-day leap last fall from minimal hurri- Later, during a panel discussion, Emanuel cane to category 5, he said. That took eight types questioned even the existence of a natural of measurements made 280 times over 12 days, cycle of Atlantic warming and cooling, at and some of those measurements required least one that influences the development of air-dropped instrumentation and satellites. hurricanes. He believes a misstep in a classic Even today, says Landsea, “there are big analysis—a 2001 Science paper on which Gray discrepancies about how strong a storm was.” was an author—tended to create a cycle where The U.S. and Japanese typhoon warning centers none exists. By subtracting a linear trend from for the Pacific Ocean—where no aircraft recon- a record of rising temperature that actually naissance is done now—at times differ in their contained the nonlinear, upward-curved trend estimates by as much as two categories. of global warming, the analysis created an At the conference, a half-dozen speakers oscillation, he said. In the end, Emanuel finds documented the sad state of tropical cyclone “no evidence for natural cycles in late summer intensity measurements. Two groups—led by tropical Atlantic sea surface on these time John Knaff of CSU and by Bruce Harper of scales,” which for him leaves global warming Systems Engineering Australia Proprietary as the leading candidate for a driver. Limited in Brisbane—attempted to correct intensity records from parts of the Pacific A haphazard record Ocean for now-obvious errors. Both reanalyses Emanuel’s line of argument caught critics by reduced the upward trend of storm intensity. surprise, and his challenge to a purely natural Knaff ’s work, in particular, suggests that driver for hurricane activity went largely unan- Emanuel’s and Webster’s studies “may have swered at the conference. NOAA scientists such dees found considerable grounds for agreement: been premature,” says Landsea. “The database

as meteorologist Christopher Landsea of the The record is far, far from perfect. Hurricane wasn’t up to it.” C O A

National Hurricane Center in Miami, Florida— climate researchers have the same problem as On the other hand, the reanalyses did not A A O

another former Gray student—claimed that climate researchers had when they began eliminate the trend. “The data’s not very N

; ) NOAA public affairs staff members writing the searching for signs of global warming. Weather good,” agreed Webster. “However, to say it’s C O A (

press releases had overstated the case for a natu- forecasters created the surface temperature all artificial is an exaggeration. We would R E T

ral cycle. And the warming may well be largely record as they went about their business predict- have had to have misidentified 160 to 180 cat- N E C

human-induced, said Landsea. The question, he ing the next day’s weather. They never planned egory 4’s and 5’s.” He doubts they were off S N O argued, is not what’s causing tropical warming, to string their twice-daily measurements into a by that much. I T but how much of an impact does that warming century-long record, so they had no qualms about The discussions at the conference,

have on hurricane intensity? Not much, he sus- moving their thermometers from place to place, although informative, did not change many OPERA pects. According to theory and computer mod- and they paid no mind when heat-retaining cities minds. “There are persuasive arguments on eling, by now the intensification should be only both sides,” says Hugh Willoughby of Florida

grew up around them. AIRCRAFT a sixth of what Webster and Emanuel have Likewise, forecasters’ observations of International University in Miami and for- reported, he noted. Rather than a real strength- tropical cyclones “weren’t designed to be cli- mer director of NOAA’s Hurricane Research ening, Landsea said, Emanuel and Webster may mate records,” notes Landsea. And to make Division. “Honestly, we don’t know” who’s SILAH/NOAA well be seeing a fictitious one created by a matters even worse for hurricane climatolo- right, he says. “That’s the real story.” Tropical deeply flawed hurricane record. gists, hurricane forecasters almost never cyclones probably intensify under warmer LCDR Emanuel disagrees. “They tend to count [the directly measure storm intensity—the maxi- climates, he says, but most likely not as much

anomalous strengthening] against the observa- mum wind speed 10 meters above the surface as Webster and Emanuel believe. “We don’t BOTTOM):

tions,” he says. “I count it against the theory, averaged over 1 minute. Instead, they usually know where we are in the middle.” That’s TO

although I helped develop the theory.” gauge maximum wind speed indirectly. what the new field of hurricane climatology (TOP That moved the debate to the observational Indirect measurements of storm intensity hopes to find out.

record of tropical cyclone intensity, where atten- haven’t always been done well or frequently, –RICHARD A. KERR CREDITS

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