Police are helping flood victims to evacuate the city. Credit: © AP Photo/Eric Gay

When hurricane Katrina hit New Orleans on 29 August, the city thought it had escaped the worst. The category-5 hurricane had weakened slightly to category 4 before making landfall, and residents were confident that they had avoided disaster.

But the following day, after the storm itself had passed, a 100-metre section of the levees protecting the area from the flood waters was breached, along with at least two other smaller stretches, inundating some 80% of the city.

By 31 August, estimates of the ultimate death toll were in the thousands, and all this with more than two months to go until the end of hurricane season. Here news@nature.com looks at the dire situation, and asks whether more is in store.

Did experts know this might happen?

Yes. New Orleans is protected by a series of flood walls called levees that help to hold back nearby Lake Pontchartrain, which in turn is connected to the Gulf of Mexico.

Parts of the city sit several metres below sea level. And the system's 565 kilometres of walls were built to withstand only category-3 hurricanes. So a direct strike from a severe storm has long been anticipated as one of the worst natural disasters that could befall the mainland United States (see 'Hurricane Ivan highlights future risk for New Orleans' ).

Could something have been done to prevent this?

The United States had had a really long run of good luck with hurricanes. Lots of building decisions were made thinking we would continue to have the benign conditions of the 1970s and 80s. Hugh Willoughby , International Hurricane Research Center, Miami

Yes. The levees could have been higher. The New York Times has reported that the estimated cost of protecting against a category-5 hurricane, the highest on the scale, is $2.5 billion. The natural marshlands that protect New Orleans from surrounding waters could also have been protected from degradation. A 30-year restoration plan, called Coast 2050, was published in 1998, but it put the bill at a staggering$14 billion. Damages from the current flooding are expected to run to tens of billions of dollars.

Part of the problem is that planners did not take into account the recent upswing in hurricane incidence, says Hugh Willoughby, a meteorologist at the International Hurricane Research Center in Miami, Florida.

"The United States had had a really long run of good luck with hurricanes. Lots of building decisions were made thinking we would continue to have the benign conditions of the 1970s and 1980s," Willoughby told news@nature.com.

"Unfortunately, 'Don't worry, be happy' is not a very good philosophy for dealing with this kind of thing."

How exactly did the levees fail?

It is still unclear why the 100-metre section of levee along the 17th Street canal was the one to break. It had recently been upgraded, and was constructed of concrete several feet thick, unlike the earthen structures elsewhere in the city.

Experts point out that Lake Pontchartrain was sloshing around in the wake of the storm, which might have caused water to tip over the edge of nearby levees. This water may have eaten away at the foundations of the wall, ultimately causing it to topple.

How long will it take to repair the damage?

All of the 20-odd pumping stations surrounding the 17th Street canal, the main route by which water is normally pumped out of the city, have been knocked out by the flood.

Keeping New Orleans free of water was a daily challenge even before Katrina struck. With almost the entire area between Lake Pontchartrain and the Mississippi River under water, clearing the flood will take at least a month.

For now, helicopters and barges are dropping sandbags and concrete highway construction barriers into the largest levee break in an attempt to plug the hole. As this story went to press, the waters were slowly starting to recede.

How many people will be affected?

Nearby towns already fear death tolls in the hundreds, and the overall number is expected to be in the thousands. It is potentially the worst natural disaster on US soil since the 1906 San Francisco earthquake, which claimed up to 6,000 lives.

Besides this, some 11,000 National Guard troops have been assigned to the region to distribute food supplies, rescue those stranded, and quell the looting that has sprung up in New Orleans.

Is climate change to blame?

It is impossible to say for certain.

There is evidence that hurricanes are becoming more intense, but this may be due to natural variation. New Orleans was last hit by a hurricane in 1969, marking the end of a particularly violent couple of decades. This was followed by a relatively quiet patch in Atlantic hurricanes, lasting until 1995. Since then, storms have been heating up again.

Hurricanes tend to be stronger when sea surface temperatures in the Atlantic are higher. But data on these temperatures only stretch back a couple of decades, since satellites began to be used to monitor the oceans. And computer models for climate change cannot predict small-scale, individual events such as hurricanes.

Nevertheless, sea surface temperatures are predicted to rise by a few degrees by 2100, meaning that devastating hurricanes may become more frequent. Whether these will make landfall or veer out to sea, however, is not known.

What can we expect from the rest of this hurricane season?

The Atlantic hurricane season traditionally lasts until November, so there could be more in store. So far this year, the region has produced 11 tropical storms, four of which have become hurricanes. The final tally could be around 20 storms with 10 hurricanes, says weather forecaster Julian Heming of the UK Met Office in Exeter, although not all of them will hit the mainland.

Can we expect more of the same next year?

"There's no reason to suggest it won't carry on as it has done," Heming says. "The past decade has seen a sudden switch to high activity." Monitoring such storms and evacuating people where necessary remains the best form of defence.

Of course, not all hurricanes will home in on major cities to such devastating effect. "Katrina probably picked the worst place to come ashore, with the possible exception of Miami," Heming says. But this week's events may well be a wake-up call.