Cleared up once and for all
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.
2) Can we identify a trend in hurricane intensity over the past few decades?
There have been several peer-reviewed publications arguing that the data show a clear increase in hurricane intensity over the past few decades. Kerry Emanuel started the controversy with a paper in Nature (PDF) that came out just before Katrina flattened New Orleans. Then Webster et al. published their paper in Science (PDF) that essentially confirmed the Emanuel results.
However, there have also been legitimate concerns about the analyses, in particular the quality of the data.
There certainly are scientists out there who consider the evidence totally convincing and have entirely accepted the idea that global warming is leading to more intense hurricanes. And there are those who are wholly unconvinced. My sense is that most scientists think there’s a good chance these analyses are right, but would not consider the case a slam-dunk. Science is by nature conservative, and important claims are accepted only when the evidence supporting them is overwhelming. That is not the case yet
So I estimate the answer to this question is somewhere around “probably.”
3) Will climate change make future hurricanes more intense?
Most scientists agree that increasing sea-surface temperatures will lead to increased intensity. The real question is how much. In a letter to Nature, Emanuel claims a 2°C increase in sea-surface temperature corresponds to a 10% increase in wind speed, which is quite significant.
There is a general consensus that it’s very likely that global warming will lead to more intense hurricanes in the future. The exact amount of the intensity increase, however, is much more uncertain.