They are called hundred-year storms because they strike with a fury so enormous that meteorologists figure they can`t come around more than once a century.
Why have the past three years seen both Hurricane Andrew and Hurricane Hugo, which smacked into South Carolina in 1989 and was rated a 4 on the 5- point scale measuring storm intensity? And what about Gilbert, which ravaged Jamaica and Mexico`s Yucatan Peninsula in 1988 and was rated a 5?
It might be a horrible coincidence. Or it might be a harbinger. One predicted consequence of the greenhouse effect -- a global warming caused by the release into the atmosphere of such heat-trapping gases as carbon dioxide and methane -- is that we will have more severe storms.
If the climatologists` computer models are right, a hurricane that would otherwise have rated a 3 would be whipped up to an Andrew-size 5. ``(We could see) a 50 percent increase in the destructive potential`` of the most powerful tropical storms, says meteorologist Kerry Emanuel of the Massachusetts Institute of Technology.
Extra heat turns up the intensity of a storm by strengthening all the forces that shape it. Water vapor, rising and cooling and releasing heat into the air, is the engine that drives a hurricane, says climatologist Jerry Mahlman of the National Oceanic and Atmospheric Administration.
The warmer the ocean, the more water evaporates; the warmer the air, the more water vapor it can hold. ``(More moisture) would feed energy into the storms, and they would increase`` in size and severity, says atmospheric physicist Michael Oppenheimer of the Environmental Defense Fund.
And since the greenhouse effect is expected to warm the oceans by at least 2 or 3 degrees, the area of tropical ocean that is warm enough to spawn a hurricane ``will almost certainly spread out,`` says Mahlman. Places that hurricanes seldom hit may become targets.
Might a warmer world also increase the frequency, and not only the severity, of storms? That is, will the same forces that turn a Level 2 hurricane into a Level 5 turn a not-quite hurricane -- a mere tropical depression -- into a full-blown hurricane? NOAA`s Mahlman thinks they may.
But researchers do not know exactly what turns a storm into a hurricane, says MIT`s Emanuel, ``so we are virtually helpless to say anything about how the frequency of hurricanes will change with climate.`` In fact, in some models a warmer world could even nip some hurricanes in the bud.
Since a warmer atmosphere would produce wind patterns that are different from today`s, and since these lateral currents affect a storm`s ability to form and grow, they might allow fewer hurricanes to develop.
The likelihood of worse storms completely changes the calculus of everything from building codes to zoning laws. Now structures such as homes and sea walls are built to withstand storms of a certain severity. If they are battered by stronger winds, they are simply going to be destroyed.
And if insurers write their actuarial tables based on a Hugo or an Andrew occurring only once a century, they may be seriously underestimating the odds on disaster.
Tracking global warming remains an uncertain science, of course. Not only can scientists not be sure it is happening already (although the 1980s saw six of the 10 hottest years on record) but also they cannot say unequivocally how much current emissions of carbon dioxide will heat things up in the next 50 years.
So policymakers ask scientists to estimate both the costs of mitigating the greenhouse effect -- by energy efficiency, say, or by switching from coal to hydropower -- and the costs of inhabiting a greenhouse world.
None of those lists includes an entry along the lines of ``Cost to clean up half a state after a major hurricane: $20 billion.``
Andrew reminds them that maybe there should be. When it comes to evaluating the economic impact of global warming, ``we haven`t done our homework,`` says William Sprigg, director of the Board on Atmospheric Sciences and Climate at the National Academy of Sciences.
It is impossible to blame any particular storm on global warming, just as it is impossible to blame a single heat wave on it. All climatologists can say is that a warmer world increases the likelihood of killer storms.
Many scientists are also confident enough to say: Look at Andrew; that may be what a greenhouse world would be like.