For years, hurricanes and typhoons have served as poster children for the hazards of global warming.
When simulated tropical storms churn inside the silicon universe of researchers' computers, such cyclones grow in power, and sometimes in number as well, as tropical temperatures increase. But when researchers have looked for global warming's fingerprints on real tropical cyclones, the evidence often has been inconclusive.
Now, one of the top researchers in the field reports that worldwide, these storms are nearly twice as powerful today as they were 30 years ago. Global warming has intensified the trend, exerting an influence stronger than he would have believed even a few months ago, he says.
"I'd been thinking of a very modest response" of tropical cyclones to climate change, "and what we're seeing is not so modest," says Kerry Emanuel, a professor of atmospheric science at the Massachusetts Institute of Technology in Cambridge, Mass.
The upshot: The 21st century could be a rough one for people who settle in hurricane or typhoon-prone areas.
As a result, more communities should be drawing on the experience of states such as Florida in devising building and zoning codes that can reduce damage and fatalities, analysts say. For people who insist on building on vulnerable barrier islands or along fragile coasts, insurance companies should be given a freer hand in deciding who they will cover and what they will charge for hurricane insurance, researchers and policy analysts say.
Dr. Emanuel's results are appearing at a time when residents along the US Gulf Coast and throughout the Caribbean are still recovering from what forecasters are calling the most active start to the hurricane season on record. Since June 1, six storms grew strong enough to merit names - from Arlene to Cindy to Franklin. Three became hurricanes. Two reached a potent category four out of five. According to forecasters at the National Hurricane Center in Miami, hurricane Dennis, which reached category four on July 7, ranks as the earliest Caribbean storm on record to reach that strength.
Some researchers argue that in practical terms, the allure to live near the sea will do far more to boost society's risk from such storms over the next several decades than any effect global warming could have on the storms themselves.
Until he concluded this study, Emanuel says he was among that group. Now, he says, global warming's impact on the storms may play a bigger a role than previously believed in putting societies at risk, particularly in less-developed countries. Dr. Emanuel's research, published Sunday on the journal Nature's website, adds a fresh perspective to the discussion about the effects of global warming on tropical cyclones, says Kevin Trenberth, a senior scientist at the National Center for Atmospheric Research in Boulder, Colo.
Early on, concerns about the future of these storms arose based on computer forecasts and basic theory. "Given the information we had at the time, the results were overhyped a bit," Dr. Trenberth acknowledges. He notes that the study doesn't have much comment on the effects of storm surges and torrential rainfall that accompany land-falling hurricanes - factors far more destructive than winds.
Still, Emanuel's approach "adds a new element," says Trenberth. It shows a strong real-world correlation between the oceans' current warming trend - which scientists have linked to the heating- trapping effect of industrial carbon dioxide and other "greenhouse gases" - and the increasing power of tropical cyclones.
Other researchers have noted that this is more likely a natural period of intense activity for Atlantic hurricanes. For example, William Gray, a specialist in tropical meteorology at Colorado State University who pioneered seasonal hurricane forecasts, notes that the region goes through swings in activity that can span decades. He and his colleagues have noted that the US and its southern neighbors have faced above-average hurricane seasons for the past decade and is likely to do so for some time to come.
Emanuel acknowledges that such cycles are important. Depending on the region under scrutiny, the impact of natural cycles such as El Niño, or the multidecade cycles Dr. Gray observes, can swamp any global-warming signal the storms may carry. But viewed worldwide, the signal starts to appear.
His latest finding, he says, grew out of attempts to answer a broader question: Do hurricanes help drive large-scale ocean currents? These currents carry tropical waters toward the poles, bringing warmth to middle and high latitudes.
Initial calculations suggested that hurricane activity could account for up to half or more of the driving force behind these currents. If so, a significant long-term rise in tropical cyclones could push warmer water toward higher latitudes. This could lead to warmer average temperatures at middle and high latitudes than climate models currently project.
To answer the question, however, Emanuel needed to gauge a hurricane's or typhoon's punch. So he built a measure based on sustained wind speeds over the life of each storm and on each storm's duration. Combined, they reflect a storm's total power output. Since the mid-70s, storm power fluctuated with well-known natural cycles. But through this natural "noise," global warming's signal emerged as an increase in strength that tracked rising temperatures in the tropical oceans' surface waters.
The work certainly will not be the last word on the subject. Some researchers are already raising questions about Emanuel's approach.
In one sense, however, there is broad agreement, notes Roger Pielky Jr., director of the Center for Science and Technology Policy Research at the University of Colorado at Boulder. Whether scientists attribute the increased tropical cyclone intensity to global warming or natural cycles, the trend is likely to hold for at least a decade.
Looking at the costs to society from these storms, for every dollar in damage from tropical cyclones the Intergovernmental Panel on Climate Change anticipates by 2050, the IPCC's demographic numbers suggest that societal changes will add another $22 to $60 in impact. "If you're a planner, you're saying: We'd better get ready," Dr. Pielke observes.