Devastating natural disasters like Hurricane Katrina raise endless questions: What caused such a powerful storm? Was there any way citizens' homes and belongings could have been spared? And there's the most uncomfortable question: Did we as a society contribute to this superstorm?
Every time there's a weather-related disaster, it brings into focus the debate about whether global warming contributes to severe weather. There's lots of research showing that climate change can produce vast fluctuations  in weather patterns. But there's also indications that our worst storms  all happened in the early 20th century—before major industrialization started contributing to climate change. So what's the answer?
It's not clear-cut, as Grist's Dave Roberts points out  :
There is no solid scientific case tying current hurricane frequency or severity to global warming. Global warming is widely expected to increase the severity of hurricanes and the frequency of severe weather events in coming years, but anyone who points at Katrina and says, "look, global warming!" is, to put it charitably, way out ahead of the scientific consensus.
There will likely never be any bulletproof scientific link between global warming and any particular severe weather event. Global climate is extraordinarily, almost incomprehensibly, complex. Billions of variables are involved. All climate scientists can say for now (and this seems to me unlikely to change) is that global warming raises the probability that severe weather events will occur. Consider an analogy: Crime rates rise during heat waves. But no individual criminal is likely to say, "I robbed that little old lady cause it's so damn hot." The reasons people commit crimes are many and varied. But this doesn't cast doubt on the link between heat and crime.
When greens discuss hurricanes and global warming, preventing hurricane-related deaths isn't their primary goal (though presumably all support it). Their primary goal is to raise awareness of, alarm about, and support for action on global warming, the ultimate effects of which will dwarf any individual severe weather event. If global warming represents the scale of disaster greens believe, then arguably the urgency of generating a large-scale response is great enough to warrant some fudging on strict rules of accuracy and precision. Many, many lives are at stake.