It was a close shave, but New Orleans just managed to escape Hurricane Gustav’s onslaught on Monday. But the stark truth is that the city’s days are numbered. Its fate was sealed in 1717, when French explorer Jean-Baptiste Le Moyne de Bienville chose a sharp bend in the Mississippi River to found “Nouvelle-Orleans”, in the teeth of opposition from his chief engineer, who warned him of flooding ahead.

And it didn’t take long before the first flood struck. Today New Orleans is a hopeless case. It can play around with levees, floodgates and sluices, but the seas are rising higher and the natural flood defences of surrounding marshlands are disappearing, But at least New Orleans has a network of flood defences. Miami is far more vulnerable and in 1926 was totally swamped by a hurricane storm surge, although it was then only a small town. And many other large cities around the world face other appalling natural disasters.

Istanbul (population c 12 million)

The question is not will a big earthquake strike Istanbul but when. Fifteen large earthquakes have hit the city since the 4th century, the last in 1894, and the next one is due.

The Turkish city lies on a fault line that is slowly being opened up like a zip. In 1999 the Izmit earthquake struck a mere 50 miles to the east of the city, killing between 20,000 and 45,000, including many in Istanbul itself. A large quake in the metropolis is expected in the next 30 years or so.

The impact of the disaster will be exacerbated by shoddy building construction; the jerry-built buildings in Izmit collapsed like a pack of cards. The city’s narrow streets and fires from ruptured gas lines will make the disaster even worse. In fact, Istanbul is so dangerous that some earthquake experts of my acquaintance refused to go to a conference there several years ago.

Phoenix (more than one million)

The Arizona city could become the first in the United States to run out of water. It is built in a desert, with a yearly rainfall of only around 20cm (8in), and much of its water supply is taken from the Colorado River, which is running out of water as seven states also tap its water.

The territory around the fast-growing city is also in the grip of a decade-long drought. But much of the problem Phoenix faces comes from its huge water demand for swimming pools, golf courses, lawns and much else. Phoenix also faces the threat of huge wildfires, which are worsening as the drought has taken grip.

Bombay (more than 18 million)

Rain is killing India’s most populous city. On July 26, 2005, record-breaking monsoon rains devastated Bombay and killed more than 400 people. The city’s decrepit drainage system, built in the 19th century, could not cope with the deluge, and Bombay has little open space to soak up heavy rains.

As the surrounding mangroves have been stripped away to reclaim more land, the city also faces floods and cyclone surges from the sea, which is rising higher. Eventually, Bombay could disappear under the waves.

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