WBZ Weather Almanac

Is Our Weather Getting Worse?

(WBZ) David Laskin, Weather Writer, Seattle, WA

These days, when a spell of nasty weather hits, the press inevitably rumbles about the role of global warming. Hurricanes Katrina, Rita and Wilma, and the four powerful tropical systems that hit Florida in 2004, have given added focus to the debate. As journalist Ross Gelbspan wrote, because of global warming "we have already crossed into a new weather regime marked by extremes of all kinds."

How much of this is hype and how much is hard science? Is global warming really pushing us into a new and stormier meteorological order or are recent bouts of violent weather simply natural anomalies? Research meteorologists and climatologists, though more tentative than journalists, are taking an increasingly clear position at least some aspects of the world's climate are in fact becoming more extreme and global warming is likely the cause.

For starters, a warmer world will feature more heat waves, but this is not as circular as it sounds. Obviously, with global warming mean temperatures will creep up but factored into this slow rise will be intense localized flare-ups of hot weather. And as it gets hotter, it will also get wetter. Physics dictates that the warmer the atmosphere, the more water vapor it can hold. To be precise, air's capacity to hold water increases 7% with every 1ºC hike in temperature. With hotter temperatures evaporating more moisture into the atmosphere, and a warming atmosphere holding increasing amounts of moisture, more precipitation is a pretty sure bet.

Thomas R. Karl, director of the National Climatic Data Center, has been monitoring swings in precipitation. In 1998, Karl and his colleague Richard W. Knight published a paper documenting a significant increase in the amount of heavy rain (defined as the upper 10% of precipitation events) falling on the United States between 1910 and 1995. Critics
were quick to point out that this translated into an average of 2" of rain in the course of 24 hours little more than a healthy cloudburst in Texas, but hardly an extreme event. In a report published last year, Karl and colleagues raised the bar to consider only "very heavy" rain, which they defined as the top 0.3% of daily precipitation events (generally 4-5 inches in the course of a day). Again, they found a significant increase in these events in much of the U.S. particularly the center of the country, which saw a 20% increase in very heavy precipitation events from 1893 to 2002.

The forecast is far less clear when it comes to other kinds of bad weather hurricanes, snowstorms and severe thunderstorms though most climatologists agree that as the world warms, tropical storms will rev up in intensity if not frequency. Warm water is hurricane fuel, notes Dr. Kevin E. Trenberth, head of the Climate Analysis Section at the National Center of Atmospheric Research, so as oceans warm, hurricanes will become "stronger and produce a lot more rain."

The high-profile bad weather movie The Day After Tomorrow warned of an impending climate calamity compliments of Hollywood. But unlike the character played in the film by Dennis Quaid, real climatologists scoff at the notion that global warming might one day flip a switch that would unleash a barrage of hellish storms potent enough to lay waste to the Northern Hemisphere. Yet today, there is an ominous tone to scientists' forecasts for the future you did not hear a few years ago. "Climate change is changing the odds, changing the environment for extreme weather," says Trenberth. "What used to be 100 year storms are now 30 years storms."

Bad weather is hard to define but we know it when we see it. And from the look of things, it is likely we will see more of it in the decades to come.

Seattle-based writer and weather enthusiast David Laskin has written several books about weather and history including "The Children's Blizzard" and "Braving the Elements: The Stormy History of American Weather."

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