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Global Warming: The Culprit?

by: Jeffrey Kluger    26 September 2005

Nature doesn't always know when to quit--and nothing says that quite like a hurricane. The atmospheric convulsion that was Hurricane Katrina had barely left the Gulf Coast before its sister Rita was spinning to life out in the Atlantic. In the three weeks between them, five other named storms had lived and died in the warm Atlantic waters without making the same headlines their ferocious sisters did. With more than two months left in the official hurricane season, only Stan, Tammy, Vince and Wilma are still available on the National Hurricane Center's annual list of 21 storm names. If the next few weeks go like the past few, those names will be used up too, and the storms that follow will be identified simply by Greek letters. Never in the 52 years we have been naming storms has there been a Hurricane Alpha.

If 2005 goes down as the worst hurricane season on record in the North Atlantic, it will join 2004 as one of the most violent ever. And these two seasons are part of a trend of increasingly powerful and deadly hurricanes that has been playing out for more than 10 years. Says climatologist Judy Curry, chair of the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology: "The so-called once-in-a-lifetime storm isn't even once in a season anymore."

Head-snapping changes in the weather like this inevitably raise the question, Is global warming to blame? For years, environmentalists have warned that one of the first and most reliable signs of a climatological crash would be an upsurge in the most violent hurricanes, the kind that thrive in a suddenly warmer world. Scientists are quick to point out that changes in the weather and climate change are two different things. But now, after watching two Gulf Coast hurricanes reach Category 5 in the space of four weeks, even skeptical scientists are starting to wonder whether something serious might be going on.

"There is no doubt that climate is changing and humans are partly responsible," says Kevin Trenberth, head of the climate-analysis section at the National Center for Atmospheric Research (NCAR) in Boulder, Colo. "The odds have changed in favor of more intense storms and heavier rainfalls." Says NCAR meteorologist Greg Holland: "These are not small changes. We're talking about a very large change."

But do scientists really know for sure? Can man-made greenhouse gases really be blamed for the intensity of storms like Rita and Katrina? Or are there, as other experts insist, too many additional variables to say one way or the other?

That global warming ought to, in theory, exacerbate the problem of hurricanes is an easy conclusion to reach. Few scientists doubt that carbon dioxide and other greenhouse gases raise the temperature of Earth's atmosphere. Warmer air can easily translate into warmer oceans--and warm oceans are the jet fuel that drives the hurricane's turbine. When Katrina hit at the end of August, the Gulf of Mexico was a veritable hurricane refueling station, with water up to 5°F higher than normal. Rita too drew its killer strength from the Gulf, making its way past southern Florida as a Category 1 storm, then exploding into a Category 5 as it moved westward. "The Gulf is really warm this year, and it's just cooking those tropical storms," says Curry.

Local hot spots like this are not the same as global climate change, but they do appear to be part of a larger trend. Since 1970, mean ocean surface temperatures worldwide have risen about 1°F. Those numbers have moved in lockstep with global air temperatures, which have also inched up a degree. The warmest year ever recorded was 1998, with 2002, 2003 and 2004 close behind it.

So that ought to mean a lot more hurricanes, right? Actually, no--which is one of the reasons it's so hard to pin these trends down. The past 10 stormy years in the North Atlantic were preceded by many very quiet ones--all occurring at the same time that global temperatures were marching upward. Worldwide, there's a sort of equilibrium. When the number of storms in the North Atlantic increases, there is usually a corresponding fall in the number of storms in, say, the North Pacific. Over the course of a year, the variations tend to cancel one another out. "Globally," says atmospheric scientist Kerry Emanuel of the Massachusetts Institute of Technology, "we do not see any increase at all in the frequency of hurricanes."

But frequency is not the same as intensity, and two recent studies demonstrate that difference. Two weeks ago, a team of scientists that included Curry and Holland published a study in the journal Science that surveyed global hurricane frequency and intensity over the past 35 years. On the whole, they found, the number of Category 1, 2 and 3 storms has fallen slightly, while the number of Categories 4 and 5 storms--the most powerful ones--has climbed dramatically. In the 1970s, there were an average of 10 Category 4 and 5 hurricanes a year worldwide. Since 1990, the annual number has nearly doubled, to 18. Overall, the big storms have grown from just 20% of the global total to 35%. "We have a sustained increase [in hurricane intensity] over 30 years all over the globe," says Holland.

Emanuel came at the same question differently but got the same results. In a study published in the journal Nature last month, he surveyed roughly 4,800 hurricanes in the North Atlantic and North Pacific over the past 56 years. While he too found no increase in the total number of hurricanes, he found that their power--measured by wind speed and duration--had jumped 50% since the mid-1970s. "The storms are getting stronger," Emanuel says, "and they're lasting longer."

Several factors help feed the trend. For example, when ocean temperatures rise, so does the amount of water vapor in the air. A moister atmosphere helps fuel storms by giving them more to spit out in the form of rain and by helping drive the convection that gives them their lethal spin. Warm oceans produce higher levels of vapor than cool oceans--at a rate of about 1.3% more per decade since 1988, according to one study--and nothing gets that process going better than greenhouse-heated air. "Water vapor increases the rainfall intensity," says Trenberth. "During Katrina, rainfall exceeded 12 inches near New Orleans."

It's not just warmer water on the surface that's powering the hurricanes; deeper warm water is too--at least in the Gulf of Mexico. Extending from the surface to a depth of 2,000 ft. or more is something scientists call the Loop Current, a U-shaped stream of warm water that flows from the Yucatán Straits to the Florida Straits and sometimes reaches as far north as the Mississippi River delta. Hurricanes that pass over the Loop typically get an energy boost, but the extra kick is brief, since they usually cross it and move on. But Rita and Katrina surfed it across the Gulf, picking up an even more powerful head of steam before slamming into the coastal states. Even if those unlucky beelines had been entirely random, the general trend toward warmer Gulf water may well have made the Loop even deadlier than usual.

"We don't know the temperature within the Loop Current," says Nan Walker, director of Louisiana State University's Earth Scan Laboratory. "It's possible that below the surface, it's warmer than normal. This needs to be investigated."

Other greenhouse-related variables may also be fueling the storms. Temperature-boosting carbon dioxide, for example, does not linger in the atmosphere forever. Some of it precipitates out in rain, settling partly on the oceans and sinking at least temporarily out of sight. But the violent frothing of the water caused by a hurricane can release some of that entrained CO2, sending it back into the sky, where it resumes its role in the warming cycle. During Hurricane Felix in 1995, measurements taken in one area the storm struck showed local CO2 levels spiking 100-fold.

So, are hurricanes actually speeding the effects of global warming and thus spawning even more violent storms? That's a matter of some dispute. While many scientists agree that this outgassing process goes on, not everyone agrees that it makes much of a difference. "The amount of CO2 given off is fairly insignificant in terms of the total CO2 in the atmosphere," says atmospheric scientist Chris Bretherton of the University of Washington in Seattle. "I am fairly confident in saying that there is no direct feedback from hurricanes."

Thus scientific uncertainty enters the debate--a debate already intensified by the political passions that surround any discussion of global warming. The fact is, there is plenty of room for doubt on both sides of the argument. Chris Landsea, a science and operations officer at the National Hurricane Center in Miami, is one of many experts who believe that global warming may be boosting the power of hurricanes--but only a bit, perhaps 1% to 5%. "A 100-mile-per-hour wind today would be a 105-mile-per-hour wind in a century," he says. "That is pretty tiny in comparison with the swings between hurricane cycles."

Skeptics are also troubled by what they see as a not inconsiderable bias in how hurricane researchers collect their data. Since most hurricanes spend the majority of their lives at sea--some never making land at all--it's impossible to measure rainfall precisely and therefore difficult to measure the true intensity of a storm.

What's more, historical studies of hurricanes like Emanuel's rely on measurements taken both before and during the era of satellites. Size up your storms in radically divergent ways, and you're likely to get radically divergent results. Even after satellites came into wide use--adding a significant measure of reliability to the data collected--the quality of the machines and the meteorologists who relied on them was often uneven. "The satellite technology available from 1970 to 1989 was not up to the job," says William Gray of Colorado State University. "And many people in non-U.S. areas were not trained well enough to determine the very fine differences between, say, the 130-m.p.h. wind speed of a Category 4 and, below that, a Category 3."

There's also some question as to whether there's a subtler, less scientific bias going on, one driven not by the raw power of the storms but by where they do their damage. Hurricanes that claw up empty coasts don't generate the same headlines as those that strike the places we like to live--and increasingly we like to live near the shore. The coastal population in the U.S. jumped 28% between 1980 and 2003. In Florida alone, the increase was a staggering 75%. Even the most objective scientists can be swayed when whole cities are being demolished by a hurricane.

"The storm activity this year is not necessarily higher than in previous high-activity years. It's just where they are going," says meteorologist Stan Goldenberg of the National Oceanic and Atmospheric Administration in Key Biscayne, Fla. "If you've got a guy shooting a machine gun but he's not shooting toward your neighborhood, it doesn't bother you."

Even correcting for our tendency to pay more attention to what is happening in our backyard, however, the global census of storms and the general measurement of their increasing power don't lie. And what those measurements tell scientists is that this already serious problem could grow a great deal worse--and do so very fast.

Some scientists are studying not just climate change but the even more alarming phenomenon of abrupt climate change. Complex systems like the atmosphere are known to move from one steady state to another with only very brief transitions in between. (Think of water, which when put over a flame becomes hotter and hotter until suddenly it turns into steam.) Ice cores taken from Greenland in the 1990s by geoscientist Richard Alley of Pennsylvania State University show that the last ice age came to an end not in the slow creep of geological time but in the quick pop of real time, with the entire planet abruptly warming in just three years.

"There are thresholds one crosses, and change runs a lot faster," Alley says. "Most of the time, climate responds as if it's being controlled by a dial, but occasionally it acts as if it's controlled by a switch." Adds Laurence Smith, an associate professor of geography at UCLA who has been studying fast climate change in the Arctic: "We face the possibility of abrupt changes that are economically and socially frightening."

Do we have the time to avert even a relatively slow climate change, or at least the nimbleness to survive it? That's what a lot of scientists are trying to determine. Japanese climatologists, for example, are using the Earth Simulator in Yokohama --one of the most powerful supercomputers in the world--to develop climate models that are more and more sophisticated. Scientists like geologist Claudia Mora of the University of Tennessee at Knoxville are going in another direction, studying isotopes locked in old tree rings to look for clues to past eras of heavy and light rainfall. Pair that information with global-temperature estimates for the same periods, and you can get a pretty good idea of how heat and hurricanes drive each other. "We've taken it back 100 years and didn't miss a storm," said Mora.

It's impossible to say whether any of that will convince the lingering global-warming skeptics. What does seem certain is that the ranks of those skeptics are growing thinner. In Washington successive administrations have ignored greenhouse warnings, piling up environmental debt the way we have been piling up fiscal debt. The problem is, when it comes to the atmosphere, there's no such thing as creative accounting. If we don't bring our climate ledgers back into balance, the climate will surely do it for us. --Reported by Mike Billips/Atlanta, Rita Healy/Denver, Kristin Kloberdanz/Chicago, Terry McCarthy/Los Angeles and Siobhan Morrissey/Miami

[This article contains a complex diagram. Â Please see hard copy or pdf.]



Over the past 35 years, the number of hurricanes each season has remained constant, but their average intensity has increased, with the number of Category 4 and 5 storms--the most powerful--nearly doubling. Given the swelling populations along the coasts, the danger from monster hurricanes like Rita and Katrina has risen dramatically

1. AREAS OF LOW PRESSURE over the ocean draw in air from surrounding, higher-pressure areas. The earth's rotation makes those winds spiral counterclockwise in the northern hemisphere

Low pressure area

High pressure areas

2. MOIST AIR WARMED by the heat of the ocean rises through the storm, intensifying the suction effect. Eventually the storm dumps some of its water as rain, which falls away and can then be pulled in again

Light winds

Warm ocean water

3. IF STRONG ATMOSPHERIC WINDS don't break this cycle, the storm becomes a hurricane when spiraling air speeds reach 74 m.p.h. (119 km/h), forming a vortex of rain-laden clouds that circle a calm eye


Storm surge

Warm, moist ascending air

Cool descending air


Area of heaviest rain and highest wind speeds

Spiral rain bands

Warm, moist air

Warm ocean water

Significant hurricane-force winds can extend 40 to 100 miles (64 to 161 km) from the eye

The longer a hurricane stays over warm waters like those in the Gulf of Mexico, the stronger it gets

For a hurricane to form, ocean water has to be at least 80 F (27 C) to a depth of 150 ft. (46 m)


Hurricanes are getting more powerful ...

Number of CATEGORY 4 AND 5 hurricanes in each
15-year span, by ocean

West Pacific 85 116

Indian 24 50

East Pacific 36 49

North Atlantic 16 25

1975-89 1990-2004


... and causing a lot more damage

10 COSTLIEST U.S. HURRICANES, 1906-2005 (in billions, 2004 dollars)

Katrina '05 $63*

Andrew '92 $43.7

Charley '04 15.0

Ivan '04 14.2

Hugo '89 12.3

Agnes '72 11.3

Betsy '65 10.8

Frances '04 8.9

Camille '69 8.9

Diane '55 7.0

*Experts think the figure could reach $200 billion

How much is global warming to blame?

Since ocean heat gives hurricanes their power, it's reasonable to conclude that global warming is at least indirectly responsible for the increase in powerful storms--but skeptics do remain

Source: National Oceanic and Atmospheric Administration