In his national address about rebuilding New Orleans, President Bush explained the government's inadequate response by stressing the severity of Hurricane Katrina. "It was not a normal hurricane -- and the normal disaster relief system was not equal to it." The "unexpected" intensity of Hurricane Katrina was a central talking point for the Bush administration as the inadequacy of its response became clear in the days after Katrina struck. But now, with Category 4 Hurricane Rita bearing down on the Texas coast, it's clear that the era of "normal" hurricanes is over.

Scientists from the National Center for Atmospheric Research and the Georgia Institute of Technology presented research recently that showed a steady increase in Category 4 and 5 hurricanes between 1970 and 2005. While environmentalists and their conservative opponents argue about whether global warming is the result of human activities or natural cycles, scientists have documented that temperatures have risen steadily and that the average temperature of the world's oceans increased by 1 degree Fahrenheit.

While one might argue that nobody expected terrorists to fly commercial jetliners into skyscrapers in a major American city until the Sept. 11 attacks, scientists have been predicting more intense and devastating hurricanes for decades -- predictions that the conservative ideologues who run the Bush administration have pointedly ignored. They have spent the last 15 years: a) denying that global warming was happening; b) arguing that even if it was happening, we couldn't prove that it was being caused by human activity; and c) arguing that if it was real and was caused by human activity, adaptation (e.g., improving levees) is more cost-effective than prevention (e.g., reducing global-warming emissions).

Having long pooh-poohed "adaptation" and other efforts to prepare for global warming as pointless, environmentalists are not without culpability. In his 1992 best-seller, "Earth in the Balance," Al Gore wrote, "Believing that we can adapt to just about anything is ultimately a kind of laziness, an arrogant faith in our ability to react in time to save our own skin." In response to the EPA's 2002 report on global warming, one Greenpeace executive told the New York Times, "Adaptation is like the 'wear sunglasses and a hat' theory of fighting ozone depletion." And in an open letter to environmental fundraisers last year, the Sierra Club's Carl Pope attacked preparedness as "accommodation," writing, "We simply won't build a seawall around Florida, much less around the Gangetic Delta in Bangladesh -- and a seawall won't stop a hurricane, or save coral reefs."

But today, in the wake of reports that a strategic investment to restore the Louisiana coast and improve the levees would have saved hundreds of lives and tens of billions of dollars, environmentalist opposition to global warming preparedness is both bad policy and bad politics. Between environmentalists who opposed adaptation and anti-government conservatives who didn't really want to invest in preparation, mitigating global-warming disasters has never had a champion in Washington.

Rather than continuing to argue about whether global warming is natural or not, it's time for environmentalists to acknowledge that it's a little late to prevent something that is already happening and get on with the business of preparing for the impacts. What we need to do now is relatively straightforward, though by no means easy. Every coastal area of the United States should immediately assess its vulnerability to hurricanes, intense storms and sea-level rise. Scientists say that if Antarctica's ice sheets collapse into the ocean, as the Larson B ice shelf did in March 2002, sea levels around the globe could rise by more than 15 feet, creating floods that will make the New Orleans death toll look tiny in comparison. What city or state in America has a plan to deal with that?

No less alarming is the prospect that melting ice sheets could shut down the northern Atlantic Gulf Stream, as described in a report commissioned by the Pentagon and reported in Forbes magazine last year, dramatically cooling Northern Europe and North America in a period of years and devastating food production. What plans do the Department of Agriculture, the Army Corps of Engineers and state water agencies have for dealing with that?

Regardless of whether global warming is caused primarily by human activity, reducing human contributions of carbon dioxide to the atmosphere can only help. A rapid transition to clean-energy sources makes good sense, not just for mitigating global warming, but also for economic and security reasons. Solar, wind and geothermal power tend to be more geographically distributed and are thus more resilient than coal and natural-gas power plants in the face of disaster-triggered blackouts. If done right, investments in clean energy and efficiency will generate enough good jobs, corporate profits and treasury revenues to more than pay for themselves.

Hurricane Katrina has just given us a terrifying glimpse into the kind of disasters we will be facing as our planet gets hotter. Global warming adaptation and preparation will be no less a challenge than prevention. The magnitude of investments that need to be made will be enormous and need to start now. Rather than continuing to argue about the source of global warming, it's time to move smartly along toward preparing for -- and reducing -- its impact.

Ted Nordhaus and Michael Shellenberger are partners of American Environics (www.AmericanEnvironics.com) and directors of the Breakthrough Institute (www.theBreakthrough.org). Their book, "The Death of Environmentalism and the Birth of a New American Politics," will be published by Houghton Mifflin in 2006.

© 2005 San Francisco Chronicle