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.