Warming Was Top Factor in 2005 Hurricanes, New Data Says

Richard A. Lovett
for National Geographic News

June 28, 2006

Following the devastation caused by Hurricane
Katrina, climate scientists began a heated debate: Was last year's
superstrong hurricane season a result of global warming?

Several experts say no, pointing to a natural long-term fluctuation in sea-surface temperatures in the Atlantic Ocean called the Atlantic multidecadal oscillation (AMO).

Few scientists doubt that human-induced global warming is occurring. But some maintain that a natural cycle played a larger role in creating last year's bumper crop of storms.

Now a new study argues that global warming is probably the larger of the two factors.

Kevin Trenberth and Dennis Shea of the National Center for Atmospheric Research, in Boulder, Colorado, tried to untangle the competing factors by looking beyond the Atlantic records. Their research appears in this month's issue of the journal Geophysical Research Letters.

They collected 130 years of temperature records for other tropical and mid-latitude waters, which they used to determine what portion of the changes in the Atlantic were due to global temperature shifts and which were because of regional factors, such the natural sea-temperature cycle.

Warming Is the Major Player

Sea-surface temperatures are widely believed to play a major role in hurricane formation and growth by fueling the storms with extra heat and moisture (interactive feature: how hurricanes form).

The 2005 hurricane season saw record sea-surface temperatures. The thermometer readings were 1.6°F (0.9°C) higher than the average between 1901 and 1970 across the hurricane-generating belt of the tropical Atlantic.

Last year also saw a record 28 named storms, 4 of which—Emily, Katrina, Rita, and Wilma—reached Category Five hurricane strength. Such storms are the strongest on Earth, with winds exceeding 155 miles (249 kilometers) an hour.

Many experts believe that warm sea-surface temperatures from the AMO enhanced hurricane activity in the 1930s and '40s. When the cycle was at its minimum in the 1970s and '80s, hurricane activity was much lower.

This trend has led some climate scientists to speculate that the recent spate of hurricanes is simply a result of another AMO upswing.

The new study, however, argues that we're seeing several effects combined, and that the most significant is global warming. The AMO is actually the least significant factor, the study says.

According to Trenberth and Shea's data, global warming explains 0.8°F (0.45°C) of the change. Recent El Niños in the Pacific—which prevent the normal upwelling of cold, deep waters—added another 0.4°F (0.2°C).

That leaves only a 0.2°F (0.1°C) change attributable to the natural oscillation. The remainder of the change, they determined, is due to random weather-related factors.

"This gives a new picture of the AMO," Trenberth said in an email.

"It suggests that the AMO is now only slightly above the long-term average. In contrast, global warming has contributed about half of the record high [sea-surface temperature] values in the critical hurricane region."

Other factors will continue to fluctuate with time, he added, but "the global warming component continues, and provides an increasing base level, enhancing risk of more hurricane activity into the future."

Heated Debate

The findings are important, says J. Marshall Shepherd, a meteorologist and climatologist at the University of Georgia in Athens.

"Because the study considers both the AMO natural variability and human-induced forcing, there is a strong degree of credibility given to its finding that global warming may be the primary factor," he said.

"I don't think you can deny that there is some sort of influence from global warming."

But, he adds, the next few months will probably see continued debate of this issue in the academic journals.

One obvious problem with all such studies is that good data for sea-surface temperatures exist only for the past century or so.

"I think that will always be a problem," Shepherd said.

Chris Landsea, science and operations operator at the National Hurricane Center in Miami, Florida, sees the study as a "good attempt" to separate the effects of the AMO from global warming.

But the study is flawed, he says. The sea-temperature data it used to adjust Atlantic Ocean data for planet-wide trends included the Atlantic as part of the globe.

In other words, he said, it was effectively throwing away "some of the baby" along with the bathwater.

That "probably damps out the natural cycle somewhat," he said.

Landsea also believes that the study focuses too strongly on sea-surface temperatures.

The AMO involves changes in upper-level winds known as wind shear, he says, which play a bigger role in hurricane strength than sea-surface temperature.

"Wind shear may be a bigger driver for what's going on than the upgrade of ocean temperatures," he said, adding that global warming has no known effect on wind shear.

Nor are scientists convinced that today's hurricanes are actually more destructive than their long-ago predecessors.

Roger Pielke, Jr., director of the Center for Science and Technology Policy Research at the University of Colorado at Boulder, spoke about the issue at a meeting of the American Geophysical Union in December.

He said that the overwhelming reason why recent hurricanes have been so much more damaging than those in prior decades is that coastal development keeps upping the ante along hurricane-prone shores (read "Hurricanes vs. Homes: Should Building on U.S. Coasts Be Stopped?" [February 27, 2006]).

In 1926, a major hurricane struck a tract of lightly developed land in southern Florida—real estate that today includes downtown Miami.

Had that storm hit last year, Pielke estimated, the property damage would have exceeded 130 billion U.S. dollars, about 50 percent more than the direct damages attributed to Hurricane Katrina.

Study co-authors Trenberth and Shea add that that their findings don't mean that 2006 will set a new round of hurricane records.

Other factors, they agree, play a role in hurricane formation, and those variables made last year particularly favorable for strong storms.

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