Strong debate over fierce storms
By David E. Williams
The 2005 season broke records with 28 named storms, 15 hurricanes and four hurricanes reaching Category 5 status.
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(CNN) -- The debate over why hurricanes have grown more powerful and more frequent in recent years is swirling as the United States enters what is expected to be another active Atlantic storm season.
Some scientists say the increased activity is the result of an upswing in a natural cycle that regulates hurricane behavior, while others pin the blame on global warming.
Forecasters at the National Oceanic and Atmospheric Administration say they don't expect a repeat of last year, which saw a record 28 named storms and 15 hurricanes.
But they say warm Atlantic temperatures and favorable wind conditions could produce up to 10 hurricanes and up to six major hurricanes -- Category 3 or above -- with winds topping 111 mph. (Full story)
The Atlantic Basin has been in an active phase since 1995, with nine of the last 11 hurricane seasons above normal, according to the NOAA . National Hurricane Center director Max Mayfield said the trend could continue for years.
"We're in this very active period for major hurricanes that may last at least another 10 or 20 years," he said. "That's not good news, and the message is very clear that we have to be prepared."
NOAA is making changes in the wake of last year's devastation, said Don Sampson, deputy secretary of the U.S. Commerce Department:
Meanwhile, the debate over the causes behind the upswing in hurricane activity shows no signs of abating.
According to NOAA, the engine driving the hurricane cycles is called the Atlantic Multidecadal Oscillation, or AMO. The AMO is a series of long-term changes in North Atlantic sea surface temperatures that can cause droughts in the Western United States and intensify hurricanes in the Atlantic.
When the AMO is in a positive, or warm phase, "everything's just more favorable for an active hurricane season in the Atlantic," said Colorado State University researcher Philip Klotzbach.
He said the AMO led to calmer seasons from 1900 to 1925, followed by an active period from 1926 to the mid-1960s, and then another period of inactivity that lasted until about 1995.
But Kerry Emanuel, a professor of atmospheric studies at the Massachusetts Institute of Technology, blames the changes on specific factors, not a natural cycle.
After sea surface temperatures rose from the 1920s to the 1950s, he said, they were pushed down by a combination of volcanic eruptions, changes in solar activity, and pollution from sulfate aerosols, which can reflect solar radiation away from the Earth.
Since the 1980s, the temperatures have risen dramatically because of global warming, Emanuel said.
"The greenhouse gasses have caught up in recent years, so the upswing in the last decade or two seems to be a greenhouse-gas dominated effect," Emanuel said.
Since the 1980s, the total amount of energy generated by hurricanes over the lifetime of the storms has doubled, he said.
"There are more of them, they're not bigger necessarily in diameter, but they're stronger in terms of wind speed, and they last longer," Emanuel said.
NOAA meteorologist Thomas Knutson said there appear to be signs that greenhouse gases are contributing to the increase in hurricane intensity.
"Our view is that when looking at the climate behavior in the Atlantic, both of hurricanes and sea surface temperatures and so forth, that it's probably some mixture of internal climate variability, natural variability and (human activity)," he said.
Knutson said there is still an active debate about which factors have the most influence.
Florida State University professor James Brian Elsner said he also believes global warming is starting to influence Atlantic hurricanes.
"I'm not sure that hurricanes and typhoons worldwide are seeing an effect, but I think in the Atlantic the strong correlation between the global air temperature and the sea surface temperatures in the Atlantic -- which is really the fuel for hurricanes -- is very large," he said.
Klotzbach, who studied hurricane figures over the past 20 years, said the Atlantic increases do not indicate a global trend.
"The Atlantic has certainly gone up, but if you take the Atlantic and the Eastern Pacific basins and add them together, there is no trend," he said. "Virtually the entire increase in the Atlantic has been almost compensated for exactly by the decrease in the Eastern Pacific."
Klotzbach said previous studies that showed a global trend were based on data beginning in the 1970s. Since then, he said, forecasters have had access to better satellite data and have used more consistent methods of analyzing storm activity.
If there was a trend, he said, it would have shown up in his analysis since warming increased in the 1980s.
"It just basically raises questions about whether those trends are actually real or if it's more just driven by data from the 1970s being of suspect quality," he said.
Emanuel said seasonal forecasts are interesting to meteorologists but are not as important to the public because scientists cannot predict if a storm will make landfall until it's a just a few days from shore.
"Not that many people really care whether there's a hurricane, however intense it is, if it stays out in the ocean for its entire life," he said.
In 1995 there were "swarms and swarms of hurricanes and only one with any real intensity ever hit land," he said.
"And then on the opposite end you have one of the quietest years we ever had in the Atlantic, 1992, that was Andrew and it was a killer," Emanuel said, referring to the Category 5 storm that slammed into south Florida, causing $26.5 billion in damage and killing 26 people in the United States and Bahamas.
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