From the very day after Katrina hit the gulf coast,
environmentalists and other leftists seemed almost delighted to tell us that
this was the result of global warming, even to the point of claiming Katrina’s
name actually was “global warming”. The
Spiegel
Online produced a summary of claims from various environmental groups, of
which this by Jürgen Trittin, a Green Party member, was a sample:
The Bush
government rejects international climate protection goals by insisting that
imposing them would negatively impact the American economy. The American
president is closing his eyes to the economic and human costs his land and the
world economy are suffering under natural catastrophes like Katrina and because
of neglected environmental policies.
I’m no Bush supporter but this seemed a little
presumptuous to me. It seemed a little
too soon to be determining the cause of such a monumental disaster affecting so
many people.
But the opposite side sprang pretty quickly to debunk the
claims, notably this from the blog EU
Rota that produced some handy graphs from the NOAA showing that the number
of hurricanes was not increasing. No
increase in the number of hurricanes = Katrina not caused by global warming,
yes? Case closed. Well, I’m not a climate scientist but this
also seemed a little glib. What if the number of hurricanes stayed the same but
their intensity had increased?
Well, the guys at Real Climate are climate scientists and they have an
interesting article about it. And yes,
they talk about the increase in intensity of the storms as well as in the
simple number:
Some past
studies (e.g. Goldenberg et al, 2001) assert that there is no evidence of any
long-term increase in statistical measures of tropical Atlantic hurricane
activity, despite the ongoing global warming. These studies, however, have focused on the frequency of all tropical
storms and hurricanes (lumping the weak ones in with the strong ones) rather than a measure of changes in the
intensity of the storms. As we have discussed elsewhere on this site,
statistical measures that focus on trends in the strongest category storms,
maximum hurricane winds, and changes in minimum central pressures, suggest a systematic increase in the
intensities of those storms that form. This finding is consistent with the
model simulations.
(My bold.)
Hum, so the intensity of storm has increased. But is this due to global warming?
The correct
answer… is that there is no way to prove that Katrina either was, or was not,
affected by global warming. For a single event, regardless of how extreme, such
attribution is fundamentally impossible. We only have one Earth, and it will
follow only one of an infinite number of possible weather sequences. It is
impossible to know whether or not this event would have taken place if we had
not increased the concentration of greenhouse gases in the atmosphere as much
as we have.
They go on to provide an interesting analogy to try to
explain how global warming might increase the statistical likelihood of more
intense storms:
…we can indeed draw some important
conclusions about the links between hurricane activity and global warming in a statistical sense. The situation is
analogous to rolling loaded dice: one could, if one was so inclined, construct
a set of dice where sixes occur twice as often as normal. But if you were to
roll a six using these dice, you could not blame it specifically on the fact
that the dice had been loaded. Half of the sixes would have occurred anyway,
even with normal dice. Loading the dice simply doubled the odds. In the same
manner, while we cannot draw firm conclusions about one single hurricane, we can draw some conclusions about
hurricanes more generally. In particular, the available scientific evidence
indicates that it is likely that global warming will make - and possibly
already is making - those hurricanes that form more destructive than they
otherwise would have been.
The tendency is for statistically more intense (but not
more frequent) hurricanes. Climate
Science concludes, in the cautious language of good scientists, that “it would
be premature
to assert that the recent anomalous behavior can be attributed entirely to a
natural cycle”. However they conclude:
The current
evidence strongly suggests that:
(a)
hurricanes tend to become more destructive as ocean temperatures rise, and
(b) an unchecked rise in greenhouse gas concentrations will very likely
increase ocean temperatures further, ultimately overwhelming any natural
oscillations.
Or in plain language, global warming is likely to result
in more not less Katrina-like hurricanes in future years.
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