Hurricanes Grow More Powerful

The occurrence of the most powerful hurricanes is significantly increasing, according to two recent studies. The finding should inform decisions about dealing with communities threatened or damaged by hurricanes.

In September, Kerry Emanuel, professor of atmospheric science at Massachusetts Institute of Technology, reported that over the past 30 years, hurricanes in the North Atlantic and North Pacific Basins have nearly doubled in destructive power, based on wind speed and duration (Nature 2005, 436, 686). And in October, Judith A. Curry and Peter J. Webster of Georgia Institute of Technology reported an analysis of satellite data that shows that the total number of Category 4 and 5 cyclones in all tropical ocean basins increased 80% since 1970 (Science 2005, 309, 1844).

Between 1970 and 2004, tropical sea surface temperatures rose an average of 0.5 C (0.9 F), Curry says. Much theoretical work had predicted that hurricane intensity would rise with increasing sea surface temperatures; the new research provides the first empirical test of the theory. Global warming is the only logical explanation for the increase in sea surface temperatures in every ocean basin, she says. If it were a cyclical phenomenon, ocean surface temperatures would rise in one basin while falling in another.

What does the new research imply for the rest of the century? Because average global air temperatures are predicted to rise 1.4 to 5.8 C (2.5 to 10.4 F) by 2100, ocean surface temperatures will become even warmer, spawning even more hurricanes with the strength of Rita and Katrina, Curry says.

In the Atlantic, about 15% of hurricanes make landfall, she says. In any particular year, only a few might strike the U.S. But over the years, more of those that do make landfall are likely to be extremely powerful.

Furthermore, the number of hurricanes expected in the Atlantic over the next few decades is likely to be greater than it was from 1970 to 1995. This increase is a cyclical phenomenon that is repeated every 25 to 30 years. We anticipate more intense and more frequent hurricanes in the Atlantic Basin, Webster says.

A further worrisome trend is growing evidence that the global rise of sea level is accelerating. Over the past decade, Emanuel says, sea level rose 1.4 inches. About one-third of that is due to melting glaciers and the remainder to thermal expansion of the oceans. Observations suggest that the massive ice sheets in Greenland and Antarctica are deteriorating much faster than can be accounted for by current theories, says Michael Oppenheimer, a physicist at Princeton University.

A United Nations report released in 2003 projected that sea level would rise 0.5 to 3 feet by 2100. Many scientists now expect a rise at the upper end of this range, with a much larger contribution from melting glaciers than we have heretofore expected, Oppenheimer says. If we, in fact, get more intense hurricanes, along with the sea level rise, there is a real potential for repeated disaster along the U.S. coasts.

The prospects for New Orleans are even more dire. By 2100, New Orleans is expected to subside 3 feet, as it did in the 20th century, so the relative sea level rise it experiences will be greater than for most other cities, says Nicholas Pinter, a geologist at Southern Illinois University, Carbondale. Any city built on sediments sinks as the organic matter in the sediments decays.

Every place has natural hazards of one kind or another, Pinter explains. But there are spots where it is reasonable to have a high-density, high-population center, and there are other places where you are going to be fighting Mother Nature every inch of the way. New Orleans has to be at the top of that list.

Given the clamor for spending up to $300 billion to rebuild New Orleans, now is a good time to ask if New Orleans is a favorable setting for a U.S. city, Pinter says. Given that the bottom is dropping out, given that New Orleans has had three glancing blows or near-misses from hurricanes in the past seven years, we need to be asking whether we want to be paying this price tag, not only now, but again in 10 or 20 years down the line when the next hurricane strikes.

Because New Orleans is largely below sea level and nearly surrounded by water, we need to think carefully about whether the city can be protected for more than a few years. Restoring the wetlands and rebuilding the levees to withstand a Category 5 hurricane can help a great deal, but the question is, Can it be done fast enough at reasonable cost to battle the three enemies: subsidence, more frequent and intensifying hurricanes, and sea level rise?

Political leaders need to reconsider development policies along the entire Gulf and East Coasts. Maine has a policy of strategic retreat from the coastline, a policy that says if you are flooded once, you cant rebuild in the same place, says Paul H. Kirshen, civil and environmental engineering professor at Tufts University, Medford, Mass.

Better protection from hurricanes also will require much more research on how to predict storm intensity. Meteorologists are adept at tracking storms but still poor at forecasting intensity as storms over water move toward shore. Intensity forecasts would allow people to make much more rational decisions about whether to evacuate a city.