A lot of electrons have flowed over Web sites, blogs, and e-mails on this topic -- some merely raising the question while others making a heartfelt, passionate -- and, some would say, overblown -- case that government inaction on climate has been a major culprit in this week's catastrophe.
RealClimate -- a scientist-run blog site -- offers what may be a much more authoritative, if not definitive, answer: "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. Weather events will always result from a combination of deterministic factors (including greenhouse gas forcing or slow natural climate cycles) and stochastic factors (pure chance).
Due to this semi-random nature of weather, it is wrong to blame any one event such as Katrina specifically on global warming - and of course it is just as indefensible to blame Katrina on a long-term natural cycle in the climate.
It's easy to want to believe otherwise -- to lay the blame for New Orleans' ruin at the feet of all those companies, politicians, and paid-for scientists that have led the United States to be the world's laggards on addressing climate. But that may not be the case. And doing so feeds into the neocons' assertion that those advocating action on climae aren't looking at the science.
My humble opinion: let's take the rhetoric down a notch and focus on the human tragedy at hand.