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Socialism Today 95 - October 2005

Did global warming contribute to Katrina?

THE FINAL SCALE of the devastation in New Orleans and the Mississippi Delta is only slowly emerging. Hugh Kaufman, the head of the US Environmental Protection Agency Toxic Waste Unit, has said that it will take a decade to decontaminate New Orleans because waste storage facilities were breached, mixing their contents with flood water. Things could have been even worse because there is a nuclear power station in the area hit by the storm, raising the possibility that toxic nuclear waste could have been released – although there have been no press reports that this was the case.

What is clear is that government neglect of the flood defences and the local environment played a major role in the disaster. The flood barriers in New Orleans, called levees, were built to withstand only a level three hurricane. Katrina was a level four and the maximum strength is level five. The flood defences are supposed to provide protection from all but one-in-300-year storm events, but there have now been two catastrophic floods in the past 80 years, plus other slightly less serious situations. This puts into question the statistical basis of the calculations, with an implication that extra factors are at work, making the statistical likelihood of severe storms greater than that built into the original design calculations. It is possible that the effects of global warming could be one of these factors. (Consider, though, that the principal flood defence dykes in the Netherlands are designed to withstand one-in-10,000-year extreme weather events. If the Mississippi levees had been built to this standard there would have been no problem, regardless of possible global warming effects.)

No firm conclusions can be drawn about one individual storm being linked to the effects of global warming, but there is evidence that global warming will make, and possibly is making, hurricanes more destructive than they would otherwise have been, according to climatologists writing on the Real Climate website ( Their argument is that rises in sea surface temperatures are linked to the power of hurricanes, because warm water and the instability in the lower atmosphere created by it is the energy source of hurricanes. This is why hurricanes only arise in the tropics and during the time of year when sea temperatures are highest, which is June to November on the US Atlantic seaboard.

One cause of rising sea surface temperatures is the increased level of carbon dioxide in the atmosphere due to the burning of fossil fuels, ie global warming. (Apart from high sea temperatures, changes in wind strength and direction at different levels in the atmosphere, called wind shear, influences the formation of hurricanes by preventing the development of the configurations of wind patterns that are necessary for them to form. Changes in these wind shear effects have not been linked, so far, to global warming.)

Computer models, similar to the one used to predict Katrina’s path, that allow for the effects of climate change, have indicated that more intense, although not more frequent, hurricanes will occur. The models predict that the frequency of the strongest level five hurricanes triples when the effects of human-induced global warming are factored in. Although recent scientific papers have reported that there has been no increase in Atlantic hurricane activity in the past century, over a time when carbon dioxide emissions linked to global warming have been increasing rapidly, closer analysis reveals that this evidence does not contradict a possible global warming link. This is because only the frequency of all tropical storms was measured, not changes in their intensity. The computer models that predict a link to global warming only predict an increase in the frequency of the most severe category of storms, not in the frequency of hurricanes in general.

An article in July, in the prestigious science magazine Nature, has confirmed that there is a link between hurricane intensity and rising sea surface temperatures. The author, K Emanuel, concluded that as tropical sea temperatures have increased in the past decades so has the intrinsic destructive power of hurricanes (measured by the power dissipated by the storm, the so-called ‘power dissipation index’). The question remains disputed, though, whether the rise in sea surface temperatures is due to human induced global warming or to natural fluctuations. The National Hurricane Center in the US has asserted that the recent upturn in hurricane activity has been due to the latter. However, Emanuel, in the Nature article, argues against this being the only cause of increased sea temperatures. He measured the power dissipated by storms and the change in sea surface temperatures over the past 80 years, and found that the dramatic increase in both over the past ten years was well outside the fluctuations found in the previous 70 years. From this he concluded that, "the large upswing in the last decade is unprecedented, and probably reflects the effects of global warming".

The paper in Nature is recent and has not been verified or challenged yet by other evidence and analyses, so its results should be regarded as provisional. Other data and investigations show that natural variations in sea surface temperatures and global warming effects both contributed in roughly equal measure to the warming of the tropical Atlantic over the past few decades.

Coming back to hurricane Katrina, it began as a weak category one storm and only picked up in intensity as it crossed the warm waters over the Gulf of Mexico. This raises the questions: Why is the sea so hot in this area and can some of the heating be attributed to global warming? Although there is not the evidence yet to connect the sea temperature rise in the Gulf of Mexico to increased greenhouse gas concentrations that are linked to global warming, equally, there is no definite proof that natural fluctuations were the sole cause of the storm.

In the longer term, as greenhouse gas effects become stronger, ocean temperatures will increase further and the effects of natural variations will be completely overshadowed. For example, current predictions from the authoritative Hadley climate research centre in Britain show sea temperatures rising by several degrees due to global warming, not the fractions of a degree that are currently being seen. The implications of this for an increased incidence of extreme weather events in the future are serious, not just for the USA but much more significantly for areas like Bangladesh, where tens of millions live in highly vulnerable flood plains.

Even if decisive action was taken immediately to reduce greenhouse gases – and there is no sign of this, in fact emissions are accelerating – it would still take many decades, possibly over 100 years, for the symptoms of global warming to disappear. To avoid future catastrophes linked to extreme weather events urgent action needs to be taken now to strengthen coastal defences, and not just in the USA.

Pete Dickenson


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