Hurricane intensity heats up debate on global warming

This year, the storm of the century has been arriving every few weeks.

When two hurricanes as powerful as Katrina and Rita pummel the Gulf Coast so close together, many Americans are understandably wondering if something in the air has changed.

Scientists are wondering the same thing.

The field's leading researchers say it is too early to reach unequivocal conclusions. But some of them see evidence that global warming may be increasing the share of hurricanes that reach the monster magnitude of Katrina, and Rita. It is difficult to imagine many problems Washington should be examining more urgently.

The issue is not whether global warming is increasing the total number of hurricanes. The U.S. has experienced a period of unusually high hurricane activity since 1995; this is one of the most active seasons ever, with 17 named tropical storms, of which nine have become hurricanes, according to the National Oceanic and Atmospheric Administration. But scientists generally agree that the number of storms fluctuates with natural climate cycles.

The 1920s and 1930s were active periods. So were the 1950s through the 1960s. Then things calmed down until the recent upsurge.

The scientific consensus is "that global warming does not have any impact on the frequency" of hurricanes, says Kerry Emanuel, professor of atmospheric science at the Massachusetts Institute of Technology and author of the new book "Divine Wind: The History and Science of Hurricanes."

But the potential relationship between global warming and hurricane intensity is causing more concern.

Hurricanes draw their energy from warm water. Ocean temperatures are rising (approximately 0.9 degree Fahrenheit since 1970), a phenomenon many experts link to global warming. No one attributes any individual hurricane to climate change. But in theory, warmer oceans should mean more mega-storms.

That's exactly the relationship found in two new studies. Last month in the journal Nature, Emanuel examined the intensity of hurricanes in the North Atlantic and western North Pacific oceans since the 1930s. The total amount of energy the hurricanes released — a figure calculated from their wind speed and duration — "has increased over the last 50 years by somewhere between 50 percent and 80 percent," he found. "That is a whopping big increase. And it is very well-correlated with tropical ocean temperatures."

This month, in the journal Science, Peter J. Webster of the Georgia Institute of Technology and three colleagues reached a similar conclusion with different data.

These researchers found that the share of hurricanes around the world reaching the most intense categories (4 or 5 on the Saffir-Simpson scale) was almost twice as large in the past 15 years as from 1975 through 1989. Only one-fifth of hurricanes reached those peak intensities in the earlier period, the researchers found, compared with 35 percent since 1990.

Just as important, the researchers concluded these changes had occurred "in all of the ocean basins."

These empirical observations generally track the most comprehensive simulation conducted on how global warming might affect hurricane intensity. Released last year by NOAA's Geophysical Fluid Dynamics Laboratory, the study concluded that "greenhouse-gas-induced warming may lead to a gradually increasing risk in the occurrence of highly destructive Category 5 storms."

In fact, the Webster and Emanuel studies found ocean temperatures turbocharged hurricanes far more than expected by the simulation, which forecast more gradual change, said Thomas R. Knutson, a NOAA research meteorologist who helped design the model.

Emanuel is quick to acknowledge that all of these results "are far too new to expect there to be a consensus about them."

Knutson agrees that "there is a lot more work to be done here." But he says the breadth of the recent results — finding common trends in oceans across the globe — argues against complacency. "All of these things taken alone have problems with them ... but the evidence is beginning to pile up," Knutson says. "If that's what the climate system is really doing, I find the implications to be rather alarming."

The implications are alarming enough that Washington should begin considering them before all the evidence is in. The most immediate priorities are more historical research (which NOAA has begun) and better measurements of contemporary storms.

Next, the administration and Congress need to explore a sensitive question: If hurricanes are intensifying, do we need to discourage development in coastal areas most vulnerable to damage?

Finally, the new research offers another powerful reason to resume serious discussion about global warming. In this year's energy bill, Congress and President Bush rejected every significant measure to reduce the release of greenhouse gases, including higher fuel economy standards for cars and caps on carbon emissions.

Imposing change today to combat tomorrow's problems is never easy, and no one should pretend that controlling greenhouse gases would be painless. But if these mammoth storms are a preview of a warmer world, future generations will surely wonder how we left them twisting in that wind.

Ronald Brownstein is a national political correspondent for the Los Angeles Times.