North Americans and Caribbean islanders have been having a vigorous hurricane season with more Atlantic tropical storms already named than have ever been listed before.
But what they've seen is mild compared with the storminess that dinosaurs may have faced when they faded out 65 million years ago. Superstorms stirred up by asteroids or intensive submarine volcanism may have nudged the beasts toward the exit.
That's the conclusion that emerges from computer simulations run by Richard Rotunno of the National Center for Atmospheric Research in Boulder, Colo., Kerry Emanuel of the Massachusetts Institute of Technology, and several colleagues. They've been playing an intellectual game that exercises both the computer model and their own understanding of how hurricanes work.
In a hurricane, billowing cumulus clouds pump heat and water vapor from the sea surface high into the atmosphere. Many hurricane studies focus on this action. But Dr. Rotunno explains that he and Dr. Emanuel think the interaction between the hurricane vortex and the sea surface is the key to how these great storms grow and intensify. Their computer simulations show that the warmer the sea surface is, the more intense are the storms that may develop.
Could this have a bearing on mass extinctions in the past? Some extinctions wiped out the majority of then-living species. For example, about 96 percent of all species in the fossil record around 245 million years ago vanished. Many scientists have suggested that massive volcanism or an incoming asteroid may have thrown up dust veils that blocked sunlight long enough to disrupt plant-based food chains.
Rotunno and Emanuel doubt such events could pollute the stratosphere long enough to affect global climate on the scale needed for mass extinctions. But the shock of a plunging asteroid or the heat of massive undersea eruptions could warm large patches of sea surface. For example, a swath several tens of kilometers wide and hundreds of kilometers long could reach 50 degrees C (122 degrees F).
That's much hotter than any sea surface the scientists have fed into their simulations before. But when they fed such temperatures into the computer model, it quickly generated tropical cyclones with central pressures as low as 20 percent of normal sea-level pressure. That's far below what the most intense hurricanes reach today. The computer storm also generated winds up to 300 meters a second (670 miles an hour) - nearly the speed of sound. Even after two days, when the storms settled into steady patterns, winds remained around 340 mph.
These ''hypercanes,'' as the scientists call them, could have thrown sheets of stratospheric clouds over large enough areas to seriously disrupt climate. They might also deplete the ozone layer and let in deadly amounts of ultraviolet radiation. Whether this ever happened, and on a scale large enough to cause mass extinctions, is just speculation.
Yet such modeling is instructive for our own time. The same kind of computer simulation indicates that even a few degrees warming of tropical seas could lead to significantly more violent hurricanes. And that is in the range of ocean warming that manmade global warming could produce. Whether or to what extent such warming will occur is still unclear. Nevertheless, these studies do warn of the possibility that more-violent hurricanes may increase in coming decades.