Asia has added one billion people to its urban population in the past 30 years. That's more than every other region in the world combined. Moving from a 10per cent to 50per cent urban population took Latin America 210 years, North America 105 years, and Europe 150 years. It took Asia only 95 years. In fact, in China, the transition happened in just 60 years.

But the biggest difference is not simply speed - it's how these cities have grown. "Megacities" - home to more than 10 million citizens - are proliferating in Asia at breakneck speed. The three most densely populated large cities in the world are in South Asia. By 2025, 21 of the world's 37 megacities will be Asian.

This pace of urbanisation has not only led to traffic snarls and massive pressure on resources like water and sanitation, it has also created slums - 61per cent of the world's slum dwellers are in Asia - and contributes to rising crime.

Air pollution has also reached levels that seem barely tolerable. A staggering two-thirds of Asia's cities can't come close to pollution standards set by the European Union. Half a million Asians die each year because of pollution, a situation that is only likely to worsen, considering carbon-dioxide emissions grew 97per cent in Asia in the first eight years of the new millennium and are expected to further triple by 2050 if nothing is done.

What this means is simple: crowded cities whose growth in numbers is not matched by a growth in infrastructure are vulnerable - susceptible to crime, pollution, and, among other risks, flooding. More than 550million urban Asians were already considered at risk of coastal and inland flooding in 2010. This is projected to rise to 760million by 2025.

But there is hope. The problems are huge, fast accelerating, and expensive to fix. But green urbanisation, if managed properly, can also provide solutions.

Bigger cities whose growth in services outpaces that of industry will ultimately face less pollution, since the service sector is cleaner.

Building decent infrastructure can allow manufacturers to relocate: where North Americans and Europeans have suburbs, Asia can keep its vibrant downtowns and instead provide incentives and transport links to enable manufacturers to move to satellite cities, keeping dirty industry at bay.

Asia is not without good examples. In Delhi and Shanghai, metros connect to satellite cities. In Japan, South Korea and elsewhere, eco-cities are becoming the trend. Singapore has shown it can be both compact and "smart".

Building green technology is no longer for the future - it's starting now. There are pilot bio-digester plants in the Philippines; pilot waste-to-energy plants in Singapore; and pilot biomass energy production in Thailand. Baoding city in Hebei has created 20,000 "green" jobs in the past three years alone. South Korea plans to have more than a million green jobs by the end of next year and Japan's green sector also expects to create over two million jobs by 2020.

Asia's urbanisation challenges are unique - and its solutions will be too. For instance, mass transport systems must link satellite cities to ports and megacities without excessive reliance on private vehicles, otherwise Asia's cities will only be trading one form of pollution for another.

Perhaps the bigger challenge is protecting the poor and managing urban migration.

This will take a delicate mix of innovation, public fortitude and more than a little common sense. Issuing land titles, removing slumlords and providing rudimentary basic services can help.

Asia should not ignore the warning signs of recent floods. This is not just bad luck, nor is it merely bad weather. It is a reminder that policymakers and the private sector must act now to ensure the green urbanisation opportunities of today are not lost to the Asian megacities of tomorrow.