Study Finds Storm Cycles Etched in Lake BedsOctober 25, 2002 Study Finds Storm Cycles Etched in Lake Beds NY TIMES By ANDREW C. REVKIN Four times since the last ice age, at intervals roughly 3,000 years apart, the Northeast has been struck by cycles of storms far more powerful than any in recent times, according to a new study. The region appears to have entered a fifth era in which such superstorms are more likely, the researchers say. No one should necessarily start building dikes right away, say the researchers, who reported their work yesterday in the journal Nature. The stormy periods they identified each lasted a millennium or more, and giant floods occurred only sporadically in those stretches. Still, the work illustrates that natural extremes of weather — what one researcher, Paul R. Bierman, a geologist at the University of Vermont, called a "drumbeat of storminess" — are many times greater than those experienced in the modern era. The researchers spent several years extracting 12- to 20-foot-long cores of sediment that accumulated over 13,000 years in the beds of 2 lakes in eastern New York and 11 in Vermont. Buried in the muck were layer-cake patterns of sandy soil, each layer evidently formed when slopes crumbled under torrents of water and were washed into the lakes. Some of these layers are 10 times as thick as one apparently left by the greatest flood recorded in Vermont, which killed 84 people, drowned thousands of cows and demolished 1,200 bridges in November 1927. Layers that thick could be explained only by deluges far more potent than the storm of 1927, the scientists said. By helping to reveal elusive long-term patterns, the findings could eventually improve long-term climate forecasts and models, said Richard B. Alley, a Pennsylvania State University geologist who is an expert on post-ice-age conditions and was not involved with the new study. "This work shows that extremes are not just acts of God that happen to happen," Dr. Alley said. "They are linked to larger patterns in the climate system that may prove to be predictable." Experts in the emerging science of paleotempestology, which uses such buried clues to discern past patterns of destructive weather, called the work a significant advance. In particular, it is the first study to compile data from many separate lake beds, reducing the chance that the patterns resulted from fluky local conditions, said Kam-biu Liu, a geographer at Louisiana State University who has used the technique to study ancient hurricanes. Dr. Liu called the new work "a triumph." The clues from the lakes appear to mesh with evidence of other periods of stormy weather around the North Atlantic, including variations in traces of salt from sea spray locked in layers of Greenland glaciers, the authors said. They also appear synchronized with the occasional cold snaps in Europe that sent glaciers grinding forward down alpine valleys, the study says. The similar storm rhythms seen around the North Atlantic may mean that the overall pattern is driven by slow cycles in a pole-girdling wind and pressure pattern called the Arctic oscillation, which in turn could be caused by cycles of solar activity, they said. The lake records from the Northeast show that the region had much stormier eras that peaked 11,900, 9,100, 5,800 and 2,600 years ago. Then, about 600 years ago, another period of storminess appeared to begin and has been "ramping back up again," Dr. Bierman said. The current trend is so prolonged and diffuse that the century-plus history of recorded weather data is not long enough to pick up a pattern. But it is etched quite clearly in the lake beds, said another author, Eric J. Steig, a climatologist at the University of Washington. The scientists checked to see whether influences other than big storms might have made the surrounding earth more apt to crumble. They considered forest fires, but found no evidence of raised concentrations of charcoal in the lake bottom. The likeliest source of each layer is an intense burst of precipitation, perhaps on already soggy soil, over just a day or two, the researchers said. Given the much greater thickness of many of the ancient layers compared with those left by floods like the 1927 disaster in Vermont, they said, society should at least ponder the potential for much greater catastrophes. In an interview, the researchers emphasized that there was no way to quantify how severe the flooding might be, but they said rainfall could reach several inches an hour — easily enough to cause massive landslides, particularly if the soil was already soggy. "This shows that in human experience, at least historical human experience, we don't know what this climate system is capable of," Dr. Steig said. While revealing the rising potential for epic storms, the new findings are likely to confound efforts to discern whether human alterations of the atmosphere, particularly a buildup of heat-trapping greenhouse gases, are increasing the frequency of severe downpours, as many climate experts have predicted. But the research could indicate that engineers and planners, when considering the design of public works like bridges and reservoirs, should take into account the possibility of extremely rare, but extremely destructive, floods, said the study's lead author, Anders J. Noren, formerly of the University of Vermont and now at the Limnological Research Center of the University of Minnesota. "If this cycle continues," Mr. Noren said, "the frequency and severity of intense rainstorms that can cause massive flooding should continue to increase for the next several hundred years."