We're told the 9/11 attacks changed everything for America--that they ushered us into a new and more dangerous world, where we could no longer afford old illusions. If we take its full lessons, the disaster of Hurricane Katrina challenges us even more profoundly.

The 9/11 attacks were horrific, wrenching for the global community that witnessed them, devastating for those who lost loved ones and friends. But for most in New York City, life quickly resumed, although with an overlay of loss and fear. Although the deaths in New Orleans were fewer, most of the city is now largely a wasteland. Its residents are exiled from their homes, many losing everything they had. It's an open question whether most will ever be able to return to resume their work, their lives, and their contributions to a culture that's given so much to the world. To rebuild New Orleans so it doesn't just become a raunchier Disneyland with better music is a task unlike any our nation has ever undertaken.

9/11, we were told, required Americans to place unprecedented trust in their president and his advisors, and to scrap longstanding rules of international law and domestic liberties. It justified a preemptive war against Iraq and Bush's reelection, despite all his failures. Of course it might never have occurred if the US hadn't supported bin Laden to begin with, or if our policies hadn't so embittered the Islamic world that a small number of men were willing to murder thousands of innocent people. But we'd all agree the attacks had a profound global impact.

So what are the lessons of New Orleans? We may call hurricanes acts of God, but Katrina was a level 1 storm, the lowest, until blistering temperatures in the Gulf of Mexico supercharged it to level 5. The storm's virulence was likely related to global climate warming, much like the recent forest fires that ravaged Southern California, floods that covered much of Bangladesh, and European heat waves that killed 35,000 people two summers ago. Ironically, Mississippi Governor Haley Barbour played a key role, as an energy lobbyist, in convincing the Bush administration to break its campaign promise to support limits on the carbon dioxide emissions that fuel global warming.

This disaster was fueled by more than global climate change. Engineers and software writers talk of "common mode failures," where one mistake magnifies another and the cumulative impact is greater than all the separate parts. The New Orleans levees might never have been breached had the Bush administration not reversed Clinton administration policies prohibiting development of coastal wetlands that once buffered the impact of storms. The levees might have been buttressed and repaired had the administration responded to a 2001 FEMA study warning that a hurricane striking New Orleans was one of the three most likely disasters in the US. But instead of honoring the Army Corps of Engineers' request to strengthen and renovate levees and pumping stations, the Bush administration cut the flood control budget of the New Orleans Corps of Engineers by $71 million, 44 percent of its budget. They needed the money for the Iraq war and for $130 billion a year in tax cuts given to a tiny group of wealthy Americans.

Finally, this catastrophe built on the slow-burn disaster that's been hitting America's poorest communities for decades. The wealthy and comfortable could evacuate New Orleans and did, though their lives were severely disrupted. But in one of the nation's poorest cities vast numbers of citizens had nowhere to go, no transportation or money with which to leave, and no friends or relatives with extra space to house them. They are the people left desperately trying to get out while the helicopters and resources of a third of the Louisiana National Guard are deployed in Iraq. And they will be the ones most damaged and most forgotten when the floodwaters eventually recede.

We were told we had to change in the wake of 9/11 or face future terrorist attacks. I suspect there will still be more attacks on American soil, following London and Madrid, and that our Iraqi invasion makes this far more likely. But it's also probable that unless we change, New Orleans will not be the last of America's great cities to collapse in desperation and ruin. Immediate relief efforts are critical, but we also need to address root crises: global warming, runaway development, deterioration of critical infrastructure, and a malign neglect that leaves more and more Americans poor and desperate.

A year ago, the world's second largest reinsurance company, Swiss Re, warned that the economic costs of climate-related disasters threatened to reach $150 billion a year within ten years. We're already seeing storms of exceptional virulence accompanying the heating of our oceans by a single degree. Given that New Orleans may cost as much as $100 billion, what will be the level of destruction as global temperatures continue to increase?

The development patterns that destroyed Louisiana wetlands are being repeated throughout America, with the support of an administration intent on removing all limits on private economic activity. The aging levees are part of a deteriorating national infrastructure that will take billions of dollars to address. The poverty that leaves people helpless to respond to disasters of whatever kind continues to grow, accelerated by government policies that transfer resources away from the poorest.

9/11 may have indeed changed our world forever, though the brief window of real discussion it fostered quickly closed, and we were left with false myths about how the only way to view the situation was as a war between ultimate good and ultimate evil. We now have a chance to heed the lessons of New Orleans and Katrina, with consequences potentially far worse than 9/11 if we don't. We can start conversations in every corner of our country about the disaster's lessons and causes, and how to move forward in a way that honors the exiled and addresses the disaster's complex roots. We can call for accountability, from our media and political leaders. We can treat this tragedy as a call to commitment in a way that too few of us did after 9/11. It's up to us how we respond to the power of its warning.

--Paul Rogat Loeb, author of The Impossible Will Take a Little While: A Citizen's Guide to Hope in a Time of Fear and of Soul of a Citizen: Living With Conviction in a Cynical Time. See www.paulloeb.org.