For years, civil engineers and environmentalists had warned that the Big Easy, as New Orleans is called, was a plum target.

Located below sea-level and swaddled by a complex system of dams, the city would be flooded if it were hit by a top-of-the-range hurricane, they said.

But other cities, too, may well be living on borrowed time.

They are located in areas that expose them to Nature's wrath -- to the instant violence of hurricanes or earthquakes or to the slower agony of drought.

The long list includes the Florida cities of Miami and Tampa, which lie in the pathway of hurricanes; Los Angeles and San Francisco, which straddle an earthquake fault line; and Las Vegas, Phoenix and Tucson, the fast-growing cities of the southwestern desert, which are gulping down their water reserves.

"Urban planners are irrelevant in this country," says Troyt York, president of the American Institute of Urban and Regional Affairs. "We have had so many disasters and none has essentially had an impact when it comes to planning."

York singled out Los Angeles, which a century ago was a small town nestled among orange groves in the California desert and is now a sprawling conurbation of 10 million.

"They have drought problems, earthquake problems, forest fires, they're built right on the San Andreas fault. It's only a question of time before we see a disaster... no-one should think about building cities in places like that."

According to a study published in May by the US Geological Surveyand Southern California Earthquake Center (SCEC), a major earthquake in the LA area could result in 3,000 to 18,000 fatalities, 142,000 to 735,000 displaced households, and more than 250 billion dollars in damage.

That compares with a current death toll of more than 700 from Hurricane Katrina, and costs tentatively estimated at around 125 billion dollars.

Other enormous migrations have occurred in the southeastern and southwestern United States, causing cities to spring up around the hurricane-prone coastlines of Florida, Louisiana and Texas and in the "Sun Belt" of Arizona, Nevada and New Mexico.

These desert cities are models of unsustainable development.

To survive in the scorching climate, their inhabitants live in bubbles of air conditioning in their homes, cars and offices, which thus makes them extremely vulnerable to disruptions in electricity and gasoline supplies.

Local aquifers -- created by rainfall that took thousands of years to seep through porous rock -- have been so depleted that these cities now import water over hundreds of kilometers (miles) from the Colorado River to nourish thirsty farm irrigation systems, lawns and golf courses.

As these cities grow and demand for water swells, so will tensions between farmers and urban dwellers and between states as to how the precious substance should be shared out.

In both the southeast and the southwest, the threat facing these new cities is made more acute by climate change.

The scientific evidence, while still sketchy, suggests that hurricanes will become more violent and possibly more frequent too as a result of global warming.

Yet many cities along the Gulf Coast and the eastern Florida coast are perched right on the beach, built on the wetlands, dunes and barrier islands that would have otherwise provided some protection from such storms, said Brenda Ekwurzel of the Union of Concerned Scientists, a lobby group that claims a membership of 10,000.

In the US west, higher temperatures are already altering precipitation patterns, bringing forward the date when the snows melt in the California and Colorado mountains that feed the rivers which provide LA and the Sun Belt with their lifeline.

"We're vulnerable to begin with, and climate change exacerbates the vulnerability," said Gregg Garfin, a scientist with the Institute for the Study of Planet Earth at the University of Arizona.

On the positive side, awareness of the problem is rising.

Some cities, such as Miami, LA and San Francisco have long worked hard to beef up construction standards and practise relief and evacuation procedures, and Florida is working to restore some of its wetland buffer.

Southern California, too, has added to its capacity for holding water to cope with seasonal changes in rainfall patterns.

In the water-stressed Sun Belt, a little progress is being made on conservation, but overall, "We've done surprisingly little to mitigate the effects of natural climate variability, never mind the effects of global warming," said Julio Betancourt of the US Geological Survey (USGS) in Tucson.