In other words, the scientific consensus appears to be that while global warming won't necessarily make hurricanes more frequent, it is likely to make them more powerful.
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.
Yet this is not the right way to frame the question.... 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....
Ultimately the answer to what caused Katrina is of little practical value. Katrina is in the past. Far more important is learning something for the future, as this could help reduce the risk of further tragedies. Better protection against hurricanes will be an obvious discussion point over the coming months, to which as climatologists we are not particularly qualified to contribute. But climate science can help us understand how human actions influence climate. The current evidence strongly suggests that:
- hurricanes tend to become more destructive as ocean temperatures rise, and
- an unchecked rise in greenhouse gas concentrations will very likely increase ocean temperatures further, ultimately overwhelming any natural oscillations.
Scenarios for future global warming show tropical SST rising by a few degrees, not just tenths of a degree. That is the important message from science. What we need to discuss is not what caused Katrina, but the likelyhood that global warming will make hurricanes even worse in future.
This is not good news for human beings. Having twice as many small hurricanes making landfall would likely be less destructive than having the same number of hurricanes, but with each on average releasing much greater energy. The case of New Orleans is paradigmatic in this respect. New Orleans could probably withstand a couple category two hurricanes every season; but one category four hurricane and it's curtains. If sea level rises just a foot or two, the same may well apply to cities like Miami -- not to mention the cities of the Gangetic delta.