| Link to Reference: Jonathan M. Gitlin, 9/2/05 | Return to: watercenter.org sciencefaircenter.com watercenter.net  |
Highlights:
- what do we mean by a hurricane?
- Hurricanes are measured on the Saffir-Sampson scale, ranging from Category 1 (up to 95 mph) to Category 5 (above 155 mph).
- What has caused Katrina to be such a killer?
- The answers, it seems, are multiple. Geography has a large part to play. New Orleans is a city below sea level. This, in itself might not be so bad. Death Valley is also below sea level, but then Death Valley is pretty far from the Pacific coast. New Orleans is also built on the Mississippi delta, where the largest river in North America meets the ocean. Over the past hundred years, dikes and levees have been constructed along the banks of the Mississippi to contain its annual flood. Ironic, then, that this flood defense strategy must bear responsibility for the damage to the city. As the Mississippi cannot flood, all the silt and sediment that washes down river from as far north as Minnesota ends up in the Gulf of Mexico, rather than on the lower Mississippi flood plain. There are several consequences to this. New Orleans continues to sink, as the sediment it is built on settles and is not replenished, the surrounding coastal wetlands and bayou are lost—on the order of 30-40 square miles a year. The sediment that does run off into the Gulf, laden with fertilizer and agricultural run off, creates algal blooms that deplete the oxygen from large areas of ocean, causing large dead zones where marine life is wiped out.
The last consequence is merely disastrous for the marine inhabitants, and the fishing communities, but the first two have a direct effect on the damage we're seeing on TV. Obviously, New Orleans' low-lying nature means that, unlike other areas that experience flooding, there is nowhere for the water to drain. But what about the bayou? Every three or four miles of wetland between the coast and New Orleans will absorb around a foot of storm surge. Given that the coast is now 40 miles closer to the city than it was 60 years ago, you can see the impact this has had. Katrina’s storm surge was between 15 and 30 feet high. A longer journey from the sea would also have the added effect of reducing the intensity of the storm, as it would be robbed of warm water to fuel its winds.
Unless you've been in a submarine or hiking in the woods for the past week, it cannot have escaped your notice that Hurricane Katrina has devastated a large swath of Alabama, Louisiana and Mississippi, in what may well be one of the greatest natural disasters to hit the US. Therefore, this week Science.Ars is going to take a look at hurricanes—what they are, what made this one so bad, and what the future may have in store.
Firstly, what do we mean by a hurricane? Well, it's the name given to a severe tropical storm with windspeeds above 73 mph that originated in either the North Atlantic or North Pacific Oceans. Strong cyclonic storms that ravage other parts of the world go by different names, such as typhoon. When a tropical storm is over a region of warm water (above 79ºF, or 26ºC), a column of warm moist air will rise up through the atmosphere and condense. This forms a chimney of warm, rising air that heats up as it increases speed. This is the engine that drives the hurricane, and creates the eye of the storm.
Hurricanes are measured on the Saffir-Sampson scale, ranging from Category 1 (up to 95 mph) to Category 5 (above 155 mph). When Katrina first made landfall in Miami on August 25, she was a Category 1 storm. The storm then veered south into the Gulf of Mexico, where high sea temperatures helped fuel the intensity of the storm. Windspeed as high as 175 mph were recorded, and Katrina moved northwest towards the Gulf coast, weakening slightly once landfall was made.
Katrina in the Gulf of Mexico (image courtesy of NASA)
Hurricanes have hit the US before, and the damage and loss of life hasn't been on this scale, so what has caused Katrina to be such a killer?
The answers, it seems, are multiple. Geography has a large part to play. New Orleans is a city below sea level. This, in itself might not be so bad. Death Valley is also below sea level, but then Death Valley is pretty far from the Pacific coast. New Orleans is also built on the Mississippi delta, where the largest river in North America meets the ocean. Over the past hundred years, dikes and levees have been constructed along the banks of the Mississippi to contain its annual flood. Ironic, then, that this flood defense strategy must bear responsibility for the damage to the city. As the Mississippi cannot flood, all the silt and sediment that washes down river from as far north as Minnesota ends up in the Gulf of Mexico, rather than on the lower Mississippi flood plain. There are several consequences to this. New Orleans continues to sink, as the sediment it is built on settles and is not replenished, the surrounding coastal wetlands and bayou are lost—on the order of 30-40 square miles a year. The sediment that does run off into the Gulf, laden with fertilizer and agricultural run off, creates algal blooms that deplete the oxygen from large areas of ocean, causing large dead zones where marine life is wiped out.
The last consequence is merely disastrous for the marine inhabitants, and the fishing communities, but the first two have a direct effect on the damage we're seeing on TV. Obviously, New Orleans' low-lying nature means that, unlike other areas that experience flooding, there is nowhere for the water to drain. But what about the bayou? Every three or four miles of wetland between the coast and New Orleans will absorb around a foot of storm surge. Given that the coast is now 40 miles closer to the city than it was 60 years ago, you can see the impact this has had. Katrina’s storm surge was between 15 and 30 feet high. A longer journey from the sea would also have the added effect of reducing the intensity of the storm, as it would be robbed of warm water to fuel its winds.
There have also been a lot of reports in the media, mainly abroad, of the role of climate change in this disaster. I know what you're all thinking here—I'm about to go off on a rant and blame SUV drivers and fossil fuels for this tragedy. For once I'm glad to say that I don’t believe it to be the case, and I'm not alone. Dr. Kerry Emanuel, a world-leading hurricane specialist at MIT works on this very problem, and published a paper in Nature recently on the increasing intensity of hurricanes. For every 1ºC rise in sea temperature, there will be a corresponding 5 percent increase in peak wind velocity. But that doesn’t mean that climate change caused Katrina:
If you look at the Atlantic, it's perfectly fair to say that both the increase in ocean temperature in the last couple of decades and the upswing in hurricane activity is mostly natural. If there's a global warming signal in that, it's very hard to see. And that natural cycle, we don't fully understand it, by the way, I don't think anyone pretends that we do, but there have been in history, you know, periods of 20 or 30 years of inactivity followed by 20 or 30 years of activity. It's nothing new, in fact. Before the 1990s, a lot of hurricane specialists had forecasts that we were going to go back to an active period in the Atlantic, and again, this has nothing to do with global warming.
Whereas it might be a problem in coming years, Katrina is just par for the course, unfortunately. The cyclical (as opposed to cyclonic) nature of hurricanes is also a contributing factor, in a roundabout way. The quiescent period in the 1970s and 1980s lulled people into a false sense of security. Population in the area boomed, meaning many more potential victims.
Dr. Emanuel's work bodes ill for the future, however, with predictions of storms with greater intensity fueled by rising sea temperatures. Whether New Orleans and the surrounding areas are still there to receive them is another question. A plan to restore the wetlands was proposed several years ago, but rejected due to the high cost (US$14 billion). With damage from Katrina likely to be an order of magnitude more than that, it seems like a it might be a good idea.
I will continue to revist this story as we learn more about the after effects of the flooding. Predictions of possible disease epidemics, including cholera, typhoid, dysentery and even West Nile, are being made, and there are bound to be severe environmental contamination from refineries, industrial plants and other sources. Finally, my thoughts go out to everyone affected by Katrina, the thousands who’ve lost their homes, livelihoods and even their lives to the ferocity of our planet.