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.
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.