The Atlantic Seaboard and the Caribbean face 10 to 40 years of stronger and more frequent hurricanes, a new analysis of weather data shows. As a result, crowded coastlines and islands are confronted with the greatest risk of devastation in a generation, said the scientists who conducted the study.
Many meteorologists said the new analysis provided the firmest evidence yet that cycles in ocean and atmospheric conditions that suppressed big storms from 1971 to 1994 had shifted into a storm-spawning state. The number and power of storms first jumped in 1995, but only now have enough years passed to measure a significant trend, the researchers said.
In a paper in todays issue of Science, they say the driving force is a periodic warming of the Atlantic north of the equator and a simultaneous drop in wind shear, the difference between trade winds near the surface and winds at higher altitudes. Tropical storms thrive on energy and moisture supplied by warmer water. In addition, the lack of clashing winds aloft allows the resulting cyclones to mushroom miles high, the researchers reported.
Conditions were similar from the 1920s through the 1960s, the study said. But coasts that were sparsely populated then are now dense with people, high-rise apartment buildings and summer homes, said Stanley B. Goldenberg, the lead author of the study and a meteorologist at the National Oceanographic and Atmospheric Administrations hurricane research center in Miami.
The greater frequency of stronger storms, combined with more people and buildings, spells a far greater risk of having a storm become a $100 billion disaster, Mr. Goldenberg said.
“Its like shooting arrows at a target,” he said. “The more arrows you shoot, the more likely you are to get a bulls-eye. The problem now is there are not only more arrows, but there are more targets because of the coastal buildup.”
In the Caribbean the biggest risk is of enormous loss of life. In the United States, it is enormous losses of property, Mr. Goldenberg said. “Its not only the number of new buildings, but their heights,” Mr. Goldenberg said. “Recent research here has shown that in strong storms, at around 300 to 500 feet you get tremendous peaks in winds” that can have twice the force of winds at the surface. Those potent gusts, he said, can easily rip the skin off a skyscraper.
In the study, the scientists said the return to cooler waters and more wind shear–and, thus, less stormy times–could happen in 10 years, but was
likely to persist much longer. They said no one knew what drove the cycle of warming and cooling in the North Atlantic, and the relationship between the water temperature and wind shifts was being studied.
Other scientists injected a note of caution, saying the recent string of stormy years was still too short to constitute a clear trend. In a separate analysis in Science, Dr. Lennart Bengtsson, a climatologist at the Max Planck Institute for Meteorology in Hamburg, Germany, said patterns of hurricane activity were nearly impossible to discern with only a few decades of data.
But he agreed that countries with vulnerable coasts could suffer huge
economic losses if they did not prepare for the worst and the worst
happened. “A change to what was typical, say, for 1920 to 1960 would create a potentially serious situation requiring most urgent attention,” he wrote in Science.
Dr. Bengtsson and the authors of the new study said the link between warmer seas and hurricane patterns did not necessarily mean that global warming would spawn more such storms. One reason given by Dr. Bengtsson was that, in a generally warmer world, the air above the ocean would warm as well as the water, reducing the contrast that drives the formation of storms.
The new study generated a picture of a tempestuous future by closely
tracking sea temperature and wind shear measurements over the last century–taken by ships and weather balloons –and comparing patterns with the frequency, severity and tracks of hurricanes from the 1940s until now. The team limited its full analysis to 1944 to 2000, a stretch with the best data. Graphs of hurricane frequency meshed well with graphs of water temperature, particularly in the region west of the bulge of Saharan Africa, where most tropical storms begin.
Although the relationship between the shifts in water temperatures and wind shear are still not understood, the pattern is consistent and clear, said Dr. Christopher W. Landsea, a colleague of Mr. Goldenberg at the hurricane center and a co-author of the paper. “The atmosphere and ocean are talking to each other,” Dr. Landsea said.
The research adds detail and new lines of evidence to earlier predictions of a stormier future by another of the co-authors, Dr. William M. Gray of Colorado State University, and to separate studies by Dr. James B. Elsner, a hurricane authority at Florida State University. There is still plenty of variability year to year, with the recent quiet stretch including, for example, the devastating Hurricane Andrew in 1992. So there is no reason to think that the 2001 hurricane season or any particular coming year will be exceptionally catastrophic, the scientists said.
And plenty of other weather factors and changes influence hurricanes. Most notable is El Niño, a condition that develops in the Pacific once or twice a decade and suppresses hurricanes by shifting eastward-flowing winds. But already, the researchers said, 1995 to 2000 had hurricanes form at a rate twice as great as seen in the preceding quiet period. Moreover, the frequency of strong storms, those with sustained winds of more than 110 miles per hour, is 2.5 times as high as it was.
They added that the recent stormy years have also seen a five-fold increase in hurricanes in the Caribbean, including Hurricane Mitch in October 1998, which killed more than 11,000 people.
The prospect of more exceptionally strong storms is particularly troubling because their destructive power rises enormously for even a small increase in wind speed. For example, Mr. Goldenberg said, winds of 130 m.p.h. have almost double the punch of winds of 100 m.p.h. “The strong storms produce more than 80 percent of all hurricane damage even though they only account for one-fifth of the storms that make landfall,” Dr. Landsea said.