Analysis: Isabel a climate harbinger?

BOULDER, Colo., Sept. 17 (UPI) -- If Hurricane Isabel hits the East Coast with its predicted winds of up to 110 mile per hour, the buildings in its path will be subject to more damaging force than they faced during Hurricanes Fran or Floyd -- meaning the financial losses caused by the storm are likely to be considerably greater than those of its predecessors.

The earlier hurricanes were among the most disastrous in recent history. Fran, a category 3 storm, hit North Carolina in 1996 and caused $3.2 billion in damage. Floyd, at category 2, hit the Mid-Atlantic and Northeast regions in 1999, causing $4.5 billion in losses.

Isabel is a category 4 storm, with winds reported on Wednesday morning of about 110 miles per hour. It could even strengthen, however, as it passes over the warm waters of the Gulf Stream off the U.S. East Coast, meteorologists say.

Timothy Reinhold, a civil engineering professor at Clemson University in Clemson, S.C., studies the effects of hurricanes and high winds on low-rise structures -- such as homes. Though many of the most vulnerable buildings in the storm's path have been replaced since the last hurricanes hit, those constructed between the early 1970s and the mid-1990s remain at risk, he said.

"If you're seeing winds of 110-130 miles per hour, that's 20 percent more force than these houses have seen in Fran or Floyd," Reinhold told United Press International. "We may see more damage if it comes in at that intensity. A lot of the weaker homes have already been weeded out. A lot will depend on how damaging the winds ultimately are."

A critical factor in protecting the integrity of a low-rise structure is the strength of the roof.

"It varies a little bit by location and the age of the house you're talking about," Reinhold explained. Roofs made of planks with decking or sheathing on them, built mostly prior to the early 1970s, usually are attached about twice as strongly as houses built between that period and 1994-95.

In the period between the 1970s and the mid-'90s, plywood was the material of choice for roof bases. Plywood is more susceptible to wind damage. More recently, builders began using bigger nails with closer spacing, along with closer inspection of hurricane straps.

"Things built after 1994-95 -- since people have been more conscious about enforcing building codes -- those (methods) ought to perform pretty well, provided they have window protection," Reinhold said.

Meanwhile, scientists are pondering whether Isabel is the first harbinger of the expect effects of global warming on climate change. The storm has registered the highest winds ever recorded in an Atlantic hurricane.

Kerry Emanuel, a professor of tropical meteorology and climate at the Massachusetts Institute of Technology in Cambridge, Mass., said a probe dropped from a plane into Isabel's heart a few days ago recorded winds of 220 miles per hour.

"Whether that measurement will stand up to scrutiny, I don't know," he told UPI.

Emanuel studies the frequency and intensity of cyclones. He said the jury is still out on whether global warming is beginning to affect tropical storm strength and frequency.

"Both are quite sensitive to global climate," he said. "On the other hand, given what we think we know, and given how much warming the globe has experienced, we don't expect to be able to detect a signal." He noted hurricanes occur too infrequently to compile enough statistics within the short time frame of a few decades. "The error in estimating is too large, and swamps any signal," he said.

"We don't think we should be able to see any signal yet. We'd have to wait another 50 years," he added.

"We understand pretty well what controls the intensity of hurricanes in the present climate," Emanuel said. The main factors are the vertical profile of temperatures in the atmosphere and the sea surface temperature. Global warming is expected to alter those factors "in the direction of making hurricanes more intense," he explained. "For each degree Celsius (increase in) sea surface temperature, you expect the intensity of the winds to go up about 8 miles an hour."

The frequency issue is trickier, though. "We are just beginning to look at how the frequency changes globally to relate to suspected factors like sea surface temperature," Emanuel said. The data set for historical frequency is very incomplete. Before 1946, many storms at sea simply went undetected, forming over the ocean and decaying without comment. Not until the satellite era in the 1960s were many storms recorded at all. Even today, only a small fraction of storms are penetrated by aircraft.

Because of these gaps in the data, only a significant change in frequency would be noted, and scientists do not expect such a large change to have occurred yet. On the other hand, Emanuel noted hurricanes themselves appear to play a part in global climate change.

"There may be a non-trivial role," he said. "Hurricanes are very effective in stirring cold water up to the ocean surface -- they collectively cool the ocean. If you warm the climate, there are more hurricanes, and you cool the ocean more."

Dealing with the heat energy is the key, Emanuel explained. "The heat the tropical cyclones are getting rid of doesn't just vanish -- it goes up to the higher latitudes. It makes the higher latitudes get warmer than they otherwise would."