Long before 2005's Hurricane Katrina outdid all its American predecessors in damages, experts have warned that the United States is growing more vulnerable to extreme climate events. Part of planning for future disasters is finding out their patterns in the past. In the case of earthquakes, we have pursued this research—paleoseismology—for decades. For hurricanes and other storms, the corresponding research—paleotempestology—is just beginning.
The History of Hurricanes
Ancient Mayan records mention hurricanes, but the first one with a firm date is one Christopher Columbus weathered in 1502. Thus we have a 500-year record of these tropical storms. Naturally the record is poor for most of that time. Indeed, the basic nature of hurricanes was not appreciated until the 1800s. Before then, even large hurricanes could have gone unnoted among the scattered ships and settlements of the Caribbean and Gulf of Mexico. But there are old archives still unsearched and records left to correlate, so studies of historical documents can yet yield valuable data. And in China, good records of Pacific typhoons go back more than 1000 years.
Clearly a complete hurricane record is not possible, but then the majority of hurricanes are not significant for our purposes. For practical purposes—say, for insurance companies and government agencies—we want to know about the very largest storms. Those few hurricanes are better known, and their dates and trajectories can be checked against the geologic record where suitable data exists.
Isotopic Geology of Hurricanes
Hurricanes bring water with them that has a particular isotopic composition, but these storms are so temporary that they may leave no trace in the usual geologic records. An exciting study in Belize, reported in the February 2007 Geology, found clear signs of hurricanes in microsamples of cavestone based on its oxygen isotopes. This technique is promising, but still new.
There is cutting-edge research that may be able to detect signs of hurricane passage in corals, but for the most part biology-based studies, in tree rings and corals, can only tell us about seasonal climatic conditions on land and in the sea. These are better than nothing, but only sedimentary evidence can give us what we need.
Sedimentology of Hurricanes
Hurricanes are known for pushing huge piles of water, called storm surges, onto the land as they arrive. Surges carry beach sand with them and leave it blanketed on the shore and beyond. The best sites for preserving these overwash sand beds are coastal lakes, and a handful of researchers at Louisiana State University, University of Florida, University of South Carolina and elsewhere are digging through the mud up and down the coast.
Other natural events can create similar sand beds, most notably river floods, tsunamis and shifting rivers. Careful study can often determine which interpretation is correct: rivers bring terrestrial sediment, which can easily be distinguished from marine sediment; tsunamis affect much larger regions than hurricanes; and rivers shifting course (avulsion)do not build up the elevation of the land.
Promise and Hurdles
The sea level has been rising slowly for the last 10,000 years. This has caused the coasts to build upward in response, steadily burying a sediment record of hurricanes and other climatic information. In addition, the coasts have been sinking from the weight of the sediment deposited on them. New sediment keeps the coasts at sea level. Subsidence of the U.S. Gulf coast, one reason the region was so vulnerable to Hurricane Katrina, is what preserves traces of paleohurricanes.
Along the coast, upward and downward also means seaward and landward motion of the shoreline. So as the sea and land interact, advances and retreats of the ocean can wipe out sedimentary records. Moreover, storms can dig up ancient sediment and mix it with modern sand and mud, confusing the record for researchers.
These factors, subsidence aggradation and transport and contamination, make paleotempestology a complex field. But the need for the kind of information it promises only grows as time and civilization go on.
PS: Much of this research is funded not by government or universities, but by the insurance industry as part of the Risk Prediction Initiative. The industry has funded some earthquake-related research, too. Work like this is the foundation of a disaster resilient society.