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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 (www.realclimate.org).
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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