If Katrina had struck the Gulf Coast just three decades earlier, the results might have been quite different.
It would have still been fierce, to be sure, but perhaps not quite as deadly as it was in the summer of 2005.
Researchers have found that if Katrina had hit in the early 1970s it would have had less moisture to fuel its powerful storm system, and less rain. That would have meant less water to pummel an outdated system of levees, and possibly, fewer failures.
And the finger of blame points quite clearly at global warming.
Evidence of Enhancement
Global warming didn't create Katrina. But research indicates it enhanced it.
"Our estimate is that rainfall from Katrina was about 7 percent enhanced by global warming," says Kevin Trenberth, head of the climate analysis section of the National Center for Atmospheric Research in Boulder, Colo.
Trenberth's research is augmented by the work of a number of other scientists who are finding evidence that the greenhouse effect, produced primarily by the burning of fossil fuels, is boosting the power of great storms to an alarming degree. Kerry Emanuel, a leading hurricane expert at the Massachusetts Institute of Technology, believes major storms have increased in intensity and duration by a whopping 50 percent just since the 1970s.
Other research that reaches similar conclusions will be published shortly.
So scientists who have been saying that we won't see much effect from global warming for at least 50 years may be wrong. We may have seen a preview of what's to come when Katrina slammed into the mouth of the Mississippi.
Not everyone agrees, of course. Major storms are among the most complex systems on the planet, and some scientists argue that the effect of global warming is too meek to have much impact. So there's a fair amount of extrapolation in all of this, but some of the top experts in the world are finding new reasons to believe that we're in for a very rough ride.
A Little Means a Lot
What they are suggesting is that very small, very subtle, changes can produce very dramatic effects. Two main players are at work here: ocean surface temperatures, and airborne water vapor.
"The tropical sea surface temperatures (where major storms are born) have gone up around a half of a degree Celsius since 1970, and we believe that is due to increases in greenhouse gases in the atmosphere," Trenberth says. "Global warming, if you like."
That's less than one degree Fahrenheit. That's not much, but it's enough to increase the amount of water that evaporates into the atmosphere.
"There's an increase of somewhere between 3.5 [percent] and 4 percent in water vapor" available in the tropics since 1970, he adds.
"When you are talking about storms like Katrina, they reach out over a huge area and grab the moisture. It spirals into the storm from an area that is quite a bit bigger than just the area you see in terms of the storm itself."
Thus he estimates that the amount of moisture available to Katrina was increased by about 7 percent, which both fueled the storm and raised precipitation.
Rain, Rain Everywhere
"In this storm there were rainfalls exceeding 12 inches just north of New Orleans," he says. "There was a big swath of heavy rain extending all the way up to about the Canadian border. And that's the key thing.
"So our estimate is that the rainfall was about 7 percent enhanced by global warming."
Tons of rain flowed down the Mississippi or into Lake Pontchartrain, creating what Trenberth calls a "swell of rainwater," enhanced by about 7 percent.
"I suspect that enhancement must have contributed to the breaking of the levees," he says. "One way to think about this is in the manner of the straw that breaks the camel's back. You can only take so much, and these levees were designed in times that didn't take into account the enhancement or global warming factors, let alone Category 5 storms."
"Seven percent might just be enough to push it over the edge," he adds.
Trenberth acknowledges that seven is not a big number.
"It's not a dominant factor," he says. "The storm is primarily a natural storm."
There likely will be much disagreement over all of this in the months ahead. But nearly all experts will agree on one point. Global warming didn't create Katrina. Many are now starting to think, however, that it probably contributed to its fury.
And it will probably happen again, which worries people like economist Gary Yohe of Wesleyan University, who has made climate change a subspecialty.
Yohe notes that "common wisdom" holds that "while climate change did not cause Katrina, she was likely much stronger because temperatures in the waters on the Gulf of Mexico exceed 80 to 85 degrees this year. A continued trend of warmer waters in the tropics will turn what would have been Category 1 storms into Category 2s, and so on up to turning Category 4 storms into Category 5s."
It would be comforting to think he's wrong. But he probably isn't.
Lee Dye's column appears weekly on ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.