Hurricane season

Raising the storm cones after Katrina

Perret lost everything when Hurricane Katrina hit. His house on the beachfront out on Highway 90 between Gulfport and Biloxi, Mississippi, was reduced to matchwood by 130-mile-an-hour winds, and sucked away by a 30-foot storm surge that washed up the beach and over the highway. "Nothing is left; it was totally destroyed," he told me weeks later. Out in the Gulf of Mexico, barrier islands that once provided protection against storms had also succumbed. Perret didn't know if hurricanes would be worse in the future, but without the islands, the effects would probably be worse anyway.

The houses along the section of Highway 90 where Perret lived, along with the hotels and resorts, had been built mostly between the 1970s and the 1990s, a period of quiet in the Gulf when there were few hurricanes. Hearing reports that no letup is likely anytime soon, some of his neighbors were going for good. They could see only more hurricanes and more havoc. They were off to Jackson or Dallas or Memphis, or anywhere inland. But when we spoke in late 2005, Perret still had his job as director of marine fisheries for the state of Mississippi, and was unsure what to do. He wanted to stay and rebuild, but was that wise?

The year 2005 had been an extraordinary one in the Atlantic. There were so many tropical storms that for the first time meteorologists ran out of names for them. Wilma became the most powerful Atlantic storm ever recorded. Katrina brought an entire U.S. city to its knees. It was the second hurricane year in a row to be described by meteorologists as "exceptional" and "unprecedented," and it came after a decade of rising hurricane activity that stretched the bounds of what had previously been regarded as natural. So what was going on? Are hurricanes becoming more destructive as global warming kicks in? Is there worse to come? The answer matters not just to the people in the firing line around the Gulf of Mexico and the Caribbean, or across the tropics in the Indian Ocean and the Pacific: if there's more severe disruption to oil production in the Gulf, or super-typhoons hit economic powerhouses like Shanghai or Tokyo at full force, we'll all feel the impact.

Until 2005, most of the world's leading hurricane experts were sanguine. The upsurge in the number of hurricanes in the North Atlantic in the previous decade had been just part of a normal cycle. Hurricanes had been strong before, from the 1940s through the 1960s. Climate models suggested that even a doubling of carbon dioxide levels in the atmosphere would increase hurricane intensity by only 10 percent or so. But that year the consensus was shattered. A flurry of papers claimed that hurricanes had grown more intense during the past thirty-year surge in global temperatures. Not more frequent, but more intense, with stronger winds, longer durations, more unrelenting rains, and even less predictable tracks. The trend was apparent in all the world's oceans, they said. From New Orleans to Tokyo, nobody was immune.

One of the authors, Kerry Emanuel, of the Massachusetts Institute of Technology, said: "My results suggest that future warming may lead to an upward trend in tropical cyclone destructive potential and—taking into account an increasing coastal population—a substantial increase in hurricane-related losses in the 21st century." Coming just weeks after the destruction of New Orleans, that sounded like a clear message to Corky Perret and the people of the Gulf Coast. No point in rebuilding, because the next super hurricane could be just around the corner. But the claims produced a schism among the high priests of hurricane forecasting. Many, like the veteran forecaster William Gray, of Colorado State University, said that they saw no upward trend and no human fingerprint. They accused the authors of the latest papers of bias and worse. So who was right?

Hurricanes are an established part of the climate system. There have always been hurricanes. They start off as clusters of thunderstorms that form as warm, humid air rises from the surface of the tropical ocean. As the air rises, the water vapor condenses, releasing energy that heats the air and makes it rise even higher. If enough storm clouds gather in close proximity, they can form what Emanuel calls a "pillar" of humid air, extending from the ocean surface for several miles into the troposphere. The low pressure at the base of the pillar sucks in more air, which picks up energy in the form of water vapor as it flows inward, and releases it as it rises. This lowers the pressure still further.

Meanwhile, the rotation of Earth, acting on the inward-flowing air, makes the pillar spin. If conditions are favorable, a tropical storm can rapidly turn into a hurricane as wind speeds pick up. Its power is staggering: Chris Landsea, of the National Oceanic and Atmospheric Administration, in Miami, has calculated that an average hurricane can release in a day as much energy as a million Hiroshima bombs. Luckily for all concerned, only a tiny fraction of this energy is converted into winds.

Worldwide there are about eighty-five tropical cyclones each year, of which about sixty reach hurricane force. That figure has been fairly stable for as long as people have been counting hurricanes. But the distribution of the hurricanes varies a great deal from year to year. In 2005, for example, the Atlantic was battered but the Pacific was relatively peaceful. On the face of it, global warming is likely to make things worse. The initial pillar of humid air forms only when the temperature of the sea surface exceeds 78°F. AS the world's oceans warm, ever-larger areas of ocean exceed the threshold. There has been an average ocean warming in the tropics of 0.5 degrees already.

What is more, every degree above the threshold seems to encourage stronger hurricanes. When Katrina went from a category 1 to a category 5 hurricane back in August 2005, the surface of the Gulf of Mexico was around 86°F, which, so far as anyone knows, was a record. Whether or not climate change can be blamed for the record sea temperatures (and most would guess that it can), those temperatures certainly helped Katrina strengthen as it slipped across the Gulf from Florida toward the Louisiana coast.

This simple link between sea surface temperatures and hurricane formation and strength has encouraged the view that a warmer world will inevitably lead to more hurricanes, stronger hurricanes, and the formation of hurricanes in places formerly outside their range. But the world is not that simple, says William Gray. What actually drives the updrafts that create the storm clouds, he says, is not the absolute temperature at the sea's surface but the difference in temperature at the top of the storm. Climate models suggest that global warming will raise air temperatures aloft. So, if he is right, while the current sea surface temperatures necessary to create hurricanes may be 78°F or more, it could in future rise to 82° or more. In the final analysis, Gray argues, the hurricane-generating potential of the tropics may remain largely unchanged.

There are other limitations on hurricane formation. However hot the oceans get, air cannot rise everywhere. It has to fall in some places, too, whatever the ocean temperature. And many incipient hurricanes are defused by horizontal winds that lop off their tops. Climate models suggest that global warming will increase wind speeds at levels where they would disrupt hurricanes. Other disruptions include dust, which often blows across the Atlantic during dry years in the Sahara.

But some trends will make big storms more likely. Most tropical storms fizzle out because they lose contact with their fuel—the heat of warm ocean waters. This happens most obviously when a hurricane passes over land, but it also happens at sea. As the storm grows, its waves stir up the ocean, mixing the warm surface water with the generally cooler water beneath. The surface water cools, and that can be the end. In practice, a hurricane can grow only if the warmth extends for tens of yards or more below the surface. But with every year that passes, warm water is penetrating ever deeper into the world's oceans. That is clearly tied to global warming. And it is setting up ideal conditions for more violent thunderstorms. Katrina is again an object lesson here. It continued to strengthen as it headed toward New Orleans, because it moved over water in the Gulf of Mexico that was very warm, not just at the surface but to a depth of more than 300 feet.

The past decade in the North Atlantic has seen a string of records broken. The period from 1995 to 1998 experienced more Atlantic hurricanes than had ever before occurred in such a short time—a record broken only in 2004

and 2005. The 1998 season was the first in a 100-year record when, on September 25, four hurricanes were on weather charts of the North Atlantic at one time. And not long afterward came Hurricane Mitch, the most destructive storm in the Western Hemisphere for 200 years. Feeding on exceptionally warm waters in the Caribbean, it ripped through Central America in the final days of October 1998, its torrential rains bringing havoc to Honduras and Nicaragua and killing some 10,000 people in landslides and floods.

The Atlantic is also generating hurricanes in places where they have never been seen before. In March 2004, the first known hurricane in the South Atlantic formed, striking southern Brazil. That the hurricane, later named Catarina, even formed was startling enough. What caused the greatest shock was that it developed very close to a zone of ocean pinpointed a few years before by Britain's Hadley Centre modelers as a likely new focus for hurricane formation in a warmer greenhouse world. But they had predicted that the waters there wouldn't be up to the task till 2070. Many saw Catarina as a further sign that global warming was making its presence felt in the hurricane world rather ahead of schedule.

The billion-dollar question (literally so for insurance companies) is whether there is now a discernible climate change component at work in the frequency and intensity of hurricanes. Kerry Emanuel, for one, argues that whatever the natural variability, the "large upswing" in hurricanes in the North Atlantic in the past decade is "unprecedented, and probably reflects the effect of global warming." Jim Hansen weighed in at the end of 2005, insisting that climate change was the cause of a warmer tropical Atlantic and that "the contention that hurricane formation has nothing to do with global warming seems irrational and untenable."

The matter of North Atlantic hurricane trends is likely to be debated for many years yet. The "signal" of climate change will be difficult to disentangle from the "noise" of natural variability. But while it is easy to become obsessed with hurricanes in the North Atlantic, they amount to only around a tenth of the global total—and a rather smaller proportion of those that make landfall in a typical year. The biggest source of hurricanes is, and is likely to remain, in the western Pacific, where they terrorize vulnerable and densely populated nations like the Philippines, Vietnam, and China. So it is the global picture that both matters most and is most likely to resolve the issue of the impact of climate change.

Several research groups have been scouring records of past hurricanes worldwide to see if there is any evidence of a trend as the world has warmed. Emanuel has concluded that, on average, storms are lasting 60 percent longer and generating wind speeds 15 percent higher than they did back in the 1950s. The damage done by a hurricane is proportional not to the wind speed but to the wind speed cubed. And Emanuel's results suggest that the destructive power of a typical hurricane has increased by an alarming 70 percent. "Global tropical cyclone activity is responding in a rather large way to global warming," he says.

Others are coming to agree. Only weeks after Emanuel's paper appeared, in the autumn of 2005, three other leading hurricane researchers published a similarly alarming conclusion. Peter Webster and Judy Curry, of the Georgia Institute of Technology, and Greg Holland, of NCAR, concluded that while there had been no overall increase in the number of hurricanes worldwide, the frequency of the strongest storms—categories 4 and 5—had almost doubled since the early 1970s. They now made up 35 percent of the total, compared with 20 percent just three decades before. The trend, the researchers said, was global, and they agreed with Emanuel that it was clearly connected to the worldwide rise in sea surface temperatures. That made it extremely unlikely that natural cycles, which are relatively short-term and confined to single ocean basins, were causing the trend. "We can say with confidence that the trends in sea surface temperatures and hurricane intensity are connected to climate change," Curry declared.

William Gray and some other traditional hurricane forecasters have contested the findings, claiming that some of the data, particularly old estimates of wind speed from the Pacific in the 1970s, are flawed. In an increasingly vitriolic exchange, Gray argued that the papers simply could not be true. Emanuel and Webster agree that the data are not as good as they might like. But "Gray has not brought to my attention any difficulties with the data [of] which I was not already aware," Emanuel said, with some irritation. Webster says Gray is "grasping at thin air."

So where does that leave us? There is as yet nothing unique about recent individual hurricanes, though Katrina, Wilma, and Mitch clearly stretch the bounds of what can be regarded as normal. The largest and most powerful hurricane ever recorded, Typhoon Tip, with wind speeds of more than 1 80 miles per hour, grazed Japan a quarter of a century ago, in 1979. The storm that hit Galveston in 1900 killed 10,000 people, many more than Katrina. Both pale compared with a hurricane in 1970 that may have killed half a million people in what is now Bangladesh.

But even if we don't yet see "superhurricanes," evidence is emerging of a human fingerprint in the rising number of stronger, longer-lasting hurricanes. It is not yet proof of a long-term global trend tied to global warming, but the striking finding from both Emanuel and Webster that there is a consistent, global connection between rising sea surface temperature and rising storm strength is strong evidence of such a link. Whatever the theoretical concerns, for now it seems that, as the climatologist Kevin Trenberth, of the National Oceanic and Atmospheric Administration, puts it: "High sea surface temperatures make for more intense storms." In a paper published in June 2006, Trenberth calculated that about half of the extra warmth in the waters of the tropical North Atlantic in 2005 could be attributed to global warming. This warming, he said, "provides a new background level that increases the risks of future enhancements in hurricane activity."

One puzzling question is how scientists have until now failed to spot the sharply increased destructiveness of modern hurricanes. There is no dispute that, taken together, hurricanes have been doing a lot more damage in recent years. In badly organized countries, such as many in Central America, that has often meant a heavy loss of life. Elsewhere, if evacuation systems work, it has simply meant a huge loss of property. Insurance claims for hurricane disasters have been soaring for some years.

The prevailing view has, until recently, been that the problem is one of bad planning, rising populations, and more people putting themselves in harm's way. The beach resorts along Highway 90 and the large squatter colonies spreading along low-lying coastal land in Asia give some support to that view. But the new data suggest that there is more to it than that. A lot more. And that most of the extra damage is being caused by the storms themselves becoming more intense. The trend seems set to continue.

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