The tortoise pace of repairing fractured sewer and water lines in the Lower Ninth originally fed suspicions that the city would take the Urban Land Institute's advice and redevelop the neighborhood with higher income homes and condos near the river and green space way back-a-town, replacing the derelict houses. But thanks largely to the efforts of Harris and ACORN, the city in March included the Lower Ninth in its rebuilding plan, which provides seed money for redevelopment. In fact, one of the first two new houses built in the neighborhood belongs to Harris's grandmother, Josephine Butler, the five-foot-tall (1.5 meters), 85-year-old matriarch of her clan. "Nobody," says Harris, "not Ray Nagin, not George Bush, is going to tell her what to do."

THE REALITY REMAINS DAUNTING for those trying to rebuild, or trying to decide whether to come back at all. The risk of catastrophic flooding is rising year by year, with no end in sight—in no small part because the city is sinking.

Even before it was covered by millions of tons of floodwater, New Orleans had sunk well below sea level, because of the draining and compacting of the backswamp and the pumping of groundwater. According to the latest satellite measurements, the city continues to sink at around two-tenths of an inch (0.5 centimeters) each year. The rate is faster in Lakeview and fastest of all in neighborhoods to the east and west. In St. Bernard Parish, subsidence tops out at nearly an inch (2.5 centimeters) a year. Some sections of the MRGO levees have sunk up to four feet (1.2 meters) since they were built, according to Roy Dokka, an LSU geologist who co-authored the satellite study, and Katrina breached many of the low spots.

"This is a place where people shouldn't be living, yet we're here," says Dokka. "But subsidence isn't going to kill people. It's the ever increasing vulnerability to storm surges and our inability to prepare for them."

Sinking is only part of the city's elevation challenge. Over the thousands of years when the delta beneath the city was being formed, sea level was almost stable. But as climate change warms the oceans and melts glaciers, sea level is rising by three millimeters a year. In February a United Nations panel on climate change predicted that seas would be more than a foot (0.3 meters) higher by 2100. And one of the nation's top climate scientists thinks that forecast is far too modest. James Hansen, director of the NASA Goddard Institute for Space Studies in New York City, notes new data from satellites showing accelerated melting of the vast ice sheets in Greenland and West Antarctica. "If we go down the business-as-usual path," he says, "we will get sea level rise measured in meters this century."

The impact on New Orleans? A meter of sea level rise would be enough to turn New Orleans into the new Big Easy Reef—or a new Amsterdam, behind massive dikes. That's assuming that big hurricanes don't come more often; chances are they will. Hurricane frequency in the Atlantic waxes and wanes over a decades-long cycle that is now on the upswing. For this year, hurricane forecasters are predicting seven to ten hurricanes in the Atlantic Basin, with up to five reaching Category 3 or above—more than double the average from 1950 to 2000. The Gulf Coast faces 50-50 odds of being hit by a Katrina-size storm this summer. Already, tropical storms in the Atlantic are 50 percent more common than at the previous peak, in the 1950s, say Peter Webster and Judith Curry of the Georgia Institute of Technology. The frequency of truly monster storms—Categories 4 and 5—has doubled since 1970.

These trends have persuaded some researchers that the natural cycle is not the only factor driving up hurricane activity. Global warming is boosting sea-surface temperatures in hurricane alley—the tropical Atlantic and Caribbean—and warm seas are rocket fuel for stronger hurricanes. Before Katrina made landfall, it had exploded from a Category 3 storm to a Category 5 in 12 hours, partly because it stirred up a deep pocket of warm water in the Gulf. Only when it reached the Louisiana coast did the storm weaken again to a Category 3, sparing New Orleans an even greater catastrophe. If global warming produces stronger storms on top of the decadal cycle, 2005, with Katrina, Rita, and two other mega-hurricanes in the Atlantic, could be a stormy precursor of the coming century.