Discovery Communications, 1998
Traces of the biggest storms remain embedded, like calling cards, in certain lake and ocean sediments, and their chemical signature can be found deep within caves. Dan Schrag dives around coral reefs to find out about hurricanes from the past. He's among a pioneering group of researchers that have begun to look back in time through a brand-new discipline aptly called paleotempestology. A geochemist from Harvard University, Schrag has an enviable field duty: he collects cores of tropical coral reefs and examines their delicate skeletons for hurricane clues. He's not searching for direct evidence of storms, rather he analyzes the chemistry of corals for clues about the salinity and temperature of the water. "Corals grow like trees," he says, "leaving bands in their skeletons like the growth rings in trees. The warmer the water, the faster they grow and the more spread out their bands."
Schrag uses corals to reconstruct the fluctuations of El Niño and La Niña events in the tropics. El Niño, the periodic warming of the tropical Pacific Ocean, suppresses hurricanes in the Atlantic and boosts them in other parts of the world. La Niña, the cooling of the tropical Pacific, intensifies hurricanes in the Atlantic. Since the links between hurricane activity and El Niño are well known, charting the comings and goings of this periodic ocean warming and cooling gives researchers an indirect measure of the hurricane activity back in time. It might also resolve the question of whether global climate change is pushing El Niño and La Niña to extremes, possibly fueling processes that create monster storms. "Since 1976, we've had two of the largest El Niños this century," Schrag says. "We've also seen a rise in global temperatures. It would be incredible to me if they weren't connected in some way."
Other paleotempestologists are looking elsewhere for signs of hurricanes gone by. They are finding signs in the sediments of nearshore lakes and on the ocean floor. Storm surges have carried sand inland, leaving a layer of coarse sediment on the bottom of nearshore lakes. Sediment cores collected from Lake Shelby in coastal Alabama reveal that Category 4 and 5 hurricanes struck the area about every 600 years. Strong hurricanes also stir up sediments in shallow ocean water, creating little land slides on the ocean floor. By looking for these stirred up sediment layers, researchers can reconstruct hurricane activity over long periods of the geologic past.
In Bermuda, researchers have looked to cave stalagmites as keepers of storm records. The rain that falls in the cloud banks surrounding a hurricane has special chemical properties, a signature, that leaves its mark when hurricane rain flushes straight through the water table and into underground cave deposits. By analyzing and dating sections of stalagmites, scientists can tally the number of hurricanes that have swept through. For all their innovative pursuit of ancient whopper storms, the scientists caution that such direct storm records can only be analyzed so far. "It's a space-time problem," says Schrag. "Even if you could reconstruct all the storms in Louisiana over a certain time period, that's only a small part of the coast. It doesn't tell you the overall frequency of storms globally."
At a time when global climate change is hotly debated and explored, this raises the question of whether human activities are somehow adding to the ingredients for creating more monster storms in the future. On the surface it seems like a simple equation: More carbon dioxide and other greenhouse gases reduce the amount of heat escaping from the atmosphere. This leads to more surface warming and a hotter ocean and atmosphere, which leads to more evaporation and more fuel for developing hurricanes. So, if CO2 in the atmosphere doubles, more hurricanes will emerge, and they should be more intense, right? Unfortunately, nature isn't that simple.
Computer modelers who have looked at hurricane activity, attempting to duplicate the complex interactions between the ocean and atmosphere, have gotten mixed results. Some experiments suggest that the number of tropical storms will actually decrease in a greenhouse-warmed world. MIT professor Kerry Emanuel, who studies the past, present and future of hurricanes, says it's too soon to draw conclusions from these models. Instead of tackling the big picture, Emanuel works on part of the problem: Will greenhouse-enhanced hurricanes be more intense?
Emanuel estimates that if the tropical oceans warm up 3 degrees, wind speeds of the most intense hurricanes will increase by 10 percent. This isn't trivial: An increase of 15 mph on a Category 5 hurricane could more than double the already catastrophic damages a whopper cane can dish out. So, how accurate is Emanuel's, or anyone else's, crystal ball? "There's a simple solution," Emanuel says. "If you can establish whether warmer climates had more hurricanes historically, or in the geologic past, we can say with confidence what global warming will do to hurricane activity in the future."