Storm season ends: Are potent hurricanes linked to global warming?
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Gray, for example, argues that Atlantic hurricane records show that storms are stronger and more frequent for several decades, then ease for several decades.Skip to next paragraph
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These changes correspond with what several scientists say are naturally occurring cycles in Atlantic sea-surface temperatures.
Yet in August and September, Dr. Emanuel and a team from the Georgia Institute of Technology published independent studies that pointed to an increase in tropical cyclone strength globally over the past 30 years. They noted that the increase coincides with rising average sea-surface temperatures in the tropics, which other researchers have linked to global warming.
One of the big challenges for everyone trying to sort out the issue is a paucity of good hurricane measurements before about 1950, Dr. Emanuel notes.
For an estimate of storms before then, researchers have to run a backward forecast, known as a hindcast. These hindcasts suggest that prior to the 1940s and '50s, the number of hurricanes eases through about 1900, then remains flat before 1900, Emanuel says.
Thus, he continues, at best the proponents of the natural-cycle notion have as few as two "peaks" and a "trough" to work with - not enough to firmly establish that it's a set of cycles at all.
Moreover, he adds, during much of the time that the oscillation was in cool phase, air pollution was high. This could mean that the cycle in sea-surface temperatures could be an artifact of air pollution, as it blows off North America toward Europe and cuts the amount of sunlight available to warm the sea surface.
"It's urgent to settle this debate," says Emanuel, who until publishing his study in August described himself as an agnostic on the question of global warming and tropical cyclones. "If it is a natural cycle, then we can expect to see a downturn" that will last for decades, he says.
It might also indicate that, at least for now, this natural cycle would overpower any signs of global warming on Atlantic tropical cyclones until roughly midcentury.
On the other hand, if recent Atlantic hurricane seasons are part of the broader global trends he and the Georgia Tech team say they've detected, "that's bad news. It means hurricane activity will keep going up."
• Dennis, then Emily, set records for the most intense hurricane before August.
• Katrina became the most destructive storm on record with an estimated $50 billion of insured damage, breaking the estimated $25 billion record (in 2005 dollars) set by Andrew in 1992.
• Wilma became the third Category 5 storm of the season - the first time three Category 5 storms have formed in one year.
• Alpha became the 22nd named storm of the 2005 season, breaking the record of 21 named storms in 1933.
• Beta became the 13th hurricane of the 2005 season, breaking the record of 12 hurricanes in 1969.
• Epsilon became the 26th named storm of the 2005 season, according to NOAA.
Sources: William Gray and Philip J. Klotzbach, Colorado State University and NOAA