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As planet warms, storms grow stronger

Scientists see evidence that hurricanes and typhoons have intensified. Are new responses needed?

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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.

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Global warming vs. natural cycle

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 NiƱo, 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.

Measuring a typhoon's punch

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.