Most researchers believe a large asteroid or comet smacked into Earth around 65 million years ago, killing off the dinosaurs and three- quarters of the other species. They even know where ground zero was: the northern coast of Mexico’s Yucatán Peninsula, where a large sediment-filled crater straddles the land and sea. What nobody is sure of, however, is how such an impact might have killed so widely. The conventional wisdom is that the impact--or volcanic eruptions triggered by the impact--launched a global pall of dust and gases that blocked out the sun or wiped out the ozone layer or both. But no one, says atmospheric scientist Kerry Emanuel of MIT, has been able to explain exactly how all that stuff got up into the stratosphere--10 to 35 miles above the surface--which it would have to do if it were to spread all over the planet. Now Emanuel and a team of colleagues think they’ve found the answer. It’s still purely hypothetical, and that is fortunate. A hypercane, as they call their creation, would be a hurricane we’d never want to see.
We were trying to predict the maximum intensity that ordinary hurricanes could reach, says Emanuel. And we noticed that if we made the ocean too warm and the atmosphere too cold, the equation didn’t yield any sensible solution--it kind of blew up. Unable to solve the problem with pencil and paper, Emanuel and his colleagues ran a computer simulation of a hurricane over a pool of hot ocean. The computer spat out a phenomenal storm--20 miles high, with winds approaching 500 miles an hour.
The water that created this hypercane was so hot--120 degrees at the center of the pool--that the team knew such a storm couldn’t occur in the present climate or in any climates that Earth had experienced, except maybe near its origin, Emanuel says. But we thought that under extraordinary circumstances one might have observed such a storm. A large asteroid or comet slamming into the ocean floor, for instance, would release a lot of heat. If an area of ocean at least 30 miles across were heated to around 120 degrees, Emanuel and his colleagues have found, the result would be a hypercane.
An ordinary hurricane forms over a much larger region of the ocean, and one that has been baked to a much lower temperature by the sun. The warm water heats the air above it, and as that warm air begins to rise, it creates a low-pressure zone that draws in air from all sides. The wind causes more water to evaporate, which transfers more heat to the air, which accelerates the nascent storm--and since Earth is spinning, the storm spins, too.
In an ordinary hurricane, friction exerted by the sea on the swirling winds limits their speed to 200 miles an hour or less. But in a hypercane, that control is overwhelmed by the tremendous heat of the impact, which keeps pumping energy into the storm. The heat engine of the hurricane just runs away, Emanuel says. Friction can’t keep up with it. The winds accelerate to 500 miles an hour. Because the angular momentum of the storm must stay the same, it shrinks to a tight knot just 10 miles across--around a sixth of the diameter of an ordinary hurricane.
Meanwhile it is growing to twice the height, 20 miles high or so, because the air in its center is so hot; that air must rise until it has cooled to the temperature of the air around it. The result is a storm tall enough and strong enough to catapult a huge amount of material--water vapor, sea salt, and maybe dust, if the crater happened to nick a coastline, as it did in the Yucatán--well into the stratosphere.
The water vapor or chlorine released by the sea salt could have triggered reactions that destroyed the ozone layer, exposing the animals below to withering ultraviolet radiation; the dust could have frozen and starved them by blocking out the sun. Emanuel and his colleagues aren’t sure which disaster befell the dinosaurs. But a single hypercane, they know, wouldn’t have done the trick--it would have taken a series. A hypercane could only exist over very hot water, Emanuel explains, and as soon as it moved off that hot water--carried by the background flow of the atmosphere--it would die, or turn into a normal hurricane. So you could get a sequence of these things--one would develop and move off downwind, and then another would develop over the hot water. If that sequence lasted more than a week, it could have dramatic global effects.