The answer depends on the exact question you're asking. Here is my view of the scientific consensus on a range of questions:
1) Did global warming cause Katrina? Or Rita? Or any single storm?
As far as I know, there exists not a single peer-reviewed article that connects global warming with the increased ferocity of any single storm. The commonly used dice analogy provides a good explanation of why the case is so hard to make. Assume the weather is determined by rolling a six-sided die, with a six corresponding to a massive hurricane. Now assume that by adding greenhouse gases to the atmosphere, we've loaded the die to make six come up twice as frequently.
Now, we roll the die once, and it comes up six. Did it come up six because it was loaded? After all, a normal die has a 16% chance of coming up six, so it's absolutely possible that the die would have come up six even without the loading.
So the answer to this question is "maybe, maybe not, we just don't know," and I think it's likely to stay that way.