Hurricane Katrina has turned New Orleans into "a wilderness," said one public health official, who begged evacuated residents not to return to the city for at least a week. Rife with poisonous water moccasins and fire ants, downed trees and power lines, without fresh drinking water, power, gas or sewage, the storm has made the battered and flooded city uninhabitable.
Katrina is just the latest in a rash of powerful hurricanes that have been pummeling the Atlantic in recent years, including a record-breaking 33 between 1995 and 1999. It's made many wonder if global warming is bringing the wrath of the planet down upon all our heads. Kerry Emanuel, a professor of atmospheric science at Massachusetts Institute of Technology, who has studied historical records of hurricanes around the globe, said the answer is yes and no.
In a recent paper, "Increasing Destructiveness of Tropical Cyclones Over the Past 30 Years," published in the science journal Nature, Emanuel found that as sea temperatures rise, the duration and intensity of hurricanes are going up, too.
The reason for the correlation is pretty straightforward: "Hurricanes derive their energy from the evaporation of sea water," Emanuel explained in a phone interview. "When you evaporate water from the ocean you actually transfer heat from the ocean to the atmosphere. A similar effect happens when you come out of the shower in the morning. You feel cold because water is evaporating from your skin, and taking heat from your body. That heat energy doesn't disappear." Instead, it fuels the intensity of hurricanes.
So, as global warming increases, expect hurricanes to get stronger. However, that doesn't mean, as some perceive, that there are actually more of them lately. "When we looked at the historical record, we found that the frequency of storms globally hasn't really changed at all," Emanuel said. "It's about 90 per year, plus or minus 10. The frequency globally appears to be steady."
The recent hurricanes in the Atlantic, Emanuel explained, represent a natural fluctuation. Every 20 to 30 years, since records started being kept in the 19th century, there have been big shifts in the frequency of hurricanes in the Atlantic. "For example, in the 1940s and '50s, there were very busy years, whereas the 1970s and '80s were very quiet years," he said. "And we've had a big upswing in the Atlantic beginning in about 1995. That's all natural."
The reason violent Atlantic hurricanes like Katrina may strike people as unnatural, and cause them to blame the CO2 pouring out of their neighbors' Hummers, is not because of their frequency but their destruction to people and places.
"This natural fluctuation occurs in a social environment where there is a huge shift in demographic trends, and this makes a big difference in people's perception," Emanuel said. "In the 1940s and '50s, there were lots of hurricanes in Florida, but there weren't lots of people there. So now that we're having this upswing again, it's being perceived very differently" -- for the simple fact that there is a lot more stuff to be ruined.
Meteorologists performed admirably in alerting public officials to Katrina's rising destruction, allowing them to evacuate New Orleans and other Gulf Coast cities in plenty of time. But Emanuel said that other warnings by meteorologists have gone unheeded in past decades -- warnings to go easy on the housing and commercial development in areas like Florida that are highly at risk to Atlantic hurricanes.
"A lot of people in my business had been, even in the 1980s, warning anybody who would listen -- which was very few, it turned out -- that there was going to be this upswing in hurricanes," Emanuel said. "It's not rocket science. We've been building all this stuff in Florida during this lull that lasted 20 years. We built all this stuff, and it's waiting to get creamed. There's been a fantastic amount of construction. A lot of people have built homes on the water. And nobody really listened. And now all of those predictions are exactly coming true. But it doesn't have much to do with global warming."
To Emanuel, Katrina is not an unusual hurricane. "Not that many hurricanes get that powerful, but we've had hurricanes like Katrina before," he said. "Camille was about the same strength. Andrew was about the same strength. Katrina was just unfortunate, because it happened to hit a very densely populated area."
Ultimately, Emanuel said, it's not a vengeful Mother Nature but man's politics that are to blame for the destruction. As long as people insist on erecting homes and businesses, aided by low insurance rates and business lobbyists, in vulnerable areas like the Gulf Coast, there's little scientists can do to prevent the havoc. "I like to say that there is no such thing as a 100 percent natural disaster," Emanuel said. "We have to put stuff in harm's way for there to be a disaster, and we're very good at doing that, and subsidizing people who continue to do it."