Warm Earth, bigger storms

Global warming partly blamed for extreme weather


This time it's Wilma, howling across the Gulf of Mexico toward a predicted landfall tomorrow on the southwest Florida coast, threatening residents still rebuilding from a battering by four hurricanes last year.

Wilma is this season's 21st named tropical storm and its 12th hurricane -- tying records on both counts. And No. 22 -- Tropical Storm Alpha -- formed yesterday afternoon.

Wilma is the third storm to hit the top of the hurricane intensity scale. Katrina, Rita and Wilma all reached Category 5 at sea, each with top sustained winds of 175 mph. That, too, appears to be an Atlantic basin record for one season. And it's not all hurricanes. Just this month, record early snows covered the Northern Plains, while a week of torrential rain flooded New Hampshire towns and threatened to burst a dam in Massachusetts.

Is this the stormy fate that global warming theorists have long warned about?

Most scientists say it seems to be. They can't say any particular storm is caused by global warming. But they are beginning to see its signature in a clear trend toward more extreme weather.

"Yes, global warming is happening, and the manifestations are now large enough that they're evident," said Kevin E. Trenberth, head of the climate analysis section at the National Center for Atmospheric Research (NCAR) in Boulder, Colo.

The toll of this year's barrage of violent storms from the tropics has been staggering.

Katrina was the costliest natural disaster in the nation's history, with some estimates exceeding $100 billion in damage. It killed more than 1,200, drowned New Orleans and erased entire Mississippi beach towns from the map. Rita killed more than 80 in Texas and Louisiana.

Hurricane Stan -- with only minimal hurricane winds -- packed rain that washed out entire hillsides in Central America, burying 2,000 Guatemalans in the mud.

Scientists say there's strong evidence that warming of the planet's oceans and atmosphere -- the result mainly of rising levels of manmade "greenhouse" gases -- increases the probability of extreme weather.

That means greater chances for heavy rain and snowstorms, more of the most intense hurricanes and more extreme heat waves and droughts.

That said, meteorologists never attribute a particular storm or heat wave to global warming. "For individual events, it will always be impossible to decide what it was due to," said Gabriele Hegerl, professor of earth and ocean sciences at Duke University.

She draws an analogy to smoking and lung cancer. Doctors can never say with certainty that an individual's cancer was caused by smoking. "But you can talk about what the probabilities are, and how they change. If somebody smokes, they are more likely to get lung cancer," she said.

Similarly, climatologists can only look at the data and say how much more likely an event such as the 2003 heat wave in France would be. That probability, Hegerl said, "has increased significantly due to global warming."

Trenberth warns that future weather looks worse than the present, so the public and policymakers must pay more attention to the storm warnings:

"A certain amount of institutional inertia [is] a key part of this problem. The key thing about global climate change is that the past is not as good a guide to the future as it has been."

The bottom line: Much of our national infrastructure -- levees, dams, bulkheads and drainage systems -- that was adequate in the past century might not be during this one.

Familiar mechanics

The basics of global warming theory are simple enough, and familiar by now:

The rise of industrialization and proliferation of automobiles were made possible by the combustion of fossil fuels -- mostly coal, oil and natural gas. The end product carbon dioxide (CO2) is wafted into the air from smokestacks and tailpipes.

That has changed the composition of the atmosphere, with CO2 levels up 32 percent over the past 250 years. Ominously, scientists say, about half of that increase has occurred since 1970.

The problem? Although CO2 allows solar energy to pass into the atmosphere, it blocks heat from escaping back into space -- much like the glass in a greenhouse. Hence the "greenhouse effect."

The result, Trenberth said, has been an increase of the global mean surface air temperature measured at about 1.5 degrees Fahrenheit since the start of the Industrial Age. Nearly a full degree of that has been since 1970.

One degree might not sound worth worrying about, but "unfortunately, it doesn't work that way," said Greg Holland, a senior scientist at NCAR. The one degree of warming is a global mean -- an average of all daily highs and lows everywhere.

"It turns out the extremes are what defines the temperature," he said. And the local extremes people experience might be dangerously higher. For example, the 100-degree summer day of 30 years ago could become a 110-degree day 50 years from now.

"A few degrees make a big difference in the impact on everything, including people," Holland said.

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