This year, for the first time, the number of people living in towns and cities will overtake the world's rural population.

This isn't just some curious footnote in human history. Rapid urbanisation, combined with factors like climate change, is increasing the chances of a disaster that would dwarf anything we've seen before.

Eight out of 10 of the world's most populous cities are prone to earthquakes, the United Nations' new emergencies chief, John Holmes, told an international conference on disaster prevention in Geneva.

Even more worryingly, six of these cities are on or near the coast, leaving millions of people vulnerable to post-quake tsunamis, tidal surges and flooding.

"The combination of decaying infrastructure, land erosion, crowded conditions and a lack of rescue services could lead to a catastrophe on an unprecedented scale," Holmes said.

"Global warming makes this task still more urgent, and our responsibility ever more grave."

Mumbai, Mexico City, Bogota, Shanghai, Manila and Jakarta are just some of the cities of particular concern to disaster experts.

Holmes announced he would be commissioning a definitive study on disaster reduction much like the recent climate change report by Sir Nicholas Stern.

Top economist and Nobel Laureate Professor Amartya Sen is already on board.

Risk reduction includes everything from installing early warning systems to enforcing strict building codes.

Holmes said he hoped the report would press home the economic advantages of investing in disaster prevention.

"One of the most potent arguments for risk reduction is also the simplest: 'An ounce of prevention is worth a pound of cure,'" Holmes told the Geneva launch of the Global Platform for Disaster Risk Reduction.

A recent study showed that every $1 invested in disaster risk reduction today saved $4 in post disaster mop-up costs.

"Natural hazards need not lead automatically to human catastrophe," Holmes said. "By taking simple cost effective steps today we can reduce the risks and save lives tomorrow. Disaster risk reduction is an idea whose time has come."

The fact that the Global Platform has attracted more than 1,000 delegates would seem to suggest Holmes may be right.

Particularly encouraging is that around 120 governments have sent ministers or other representatives.

But underlining the urgency of the challenge ahead, officials from several countries had to pull out at the last minute - they were too busy dealing with disasters at home.

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