Storm season ends: Are potent hurricanes linked to global warming?

As the curtain drops on the 2005 hurricane season Wednesday, the collective sigh of relief from a storm-weary country is palpable.

The record-breaking year touched virtually every corner of the nation - from flattened, flooded homes in the Gulf to eye-popping fuel prices at the gasoline pump just about everywhere. Estimates of direct losses from hurricane Katrina alone range from $70 billion to $135 billion.

Indeed, "arguably this was the most devastating season in modern times," said Admiral Conrad Lautenbacher, who heads the National Oceanic and Atmospheric Administration (NOAA). "It's a relief to be almost at the end of the season," he added at a press briefing Tuesday.

One bright spot from this year's storm season is the return on the country's investment in hurricane-forecast research, according to Richard Anthes, president of the University Corporation for Atmospheric Research, which runs the National Center for Atmospheric Research in Boulder, Colo.

The storm-track forecasts "didn't just happen by luck," he says. The ability to weave field and satellite measurements with increasingly sophisticated computer models "were the result of years of hard work. It made a huge difference, saving thousands of lives." Mr. Lautenbacher suggests that the track forecast this year may have set records for accuracy.

Even so, it was a season intense enough to prompt even forecasters to try to ease the strain with rare attempts at irony, if not humor. As Vince made landfall as the first tropical cyclone to hit the Iberian peninsula, an otherwise terse notice from the National Hurricane Center in Miami carried a line that could have been read by Monty Python's John Cleese: "As the short, happy life of Vince is now over, this will be the last advisory."

When combined with last year's storms, the 2004 and 2005 seasons "have been incredibly active and destructive," note Colorado State University atmospheric scientists William Gray and Philip J. Klotzbach in their own wrap-up of the 2005 season.

One culprit in this two-year tale of destructive storms lies half a world away, they say. Unusually warm surface water in the central tropical Pacific eventually led to persistent wind patterns over the Atlantic basin that sped storms toward the west. The result: 54 percent of the two seasons' major hurricanes made landfall, compared with an average of only 9 percent during the 1995 to 2003 seasons.

Dr. Gray and Dr. Klotzbach say they expect to see the number of hurricanes making landfall drop as the unusual steering conditions ease.

Exhausted from back-to-back years of tropical-cyclone mayhem, many people are asking about future trends, notes Kerry Emanuel, an atmospheric scientist at the Massachusetts Institute of Technology, who focuses on weather and climate in the tropics.

He notes that within the climate-science community, tropical-cyclone specialists are embroiled in a debate over recent patterns in storm strength, intensity and the future.

Gray, for example, argues that Atlantic hurricane records show that storms are stronger and more frequent for several decades, then ease for several decades.

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