CENTRAL TO the rise of Asian economies in the last few decades has been the transformation of its cities as they lure rural dwellers in their millions.

But a new report by the Asian Development Bank (ADB) says that Asian countries must act swiftly to make their cities greener and more resource-friendly or they face a bleak future.

Since the 1980s, Asia has urbanised at a faster rate than anywhere else in the world. Almost half the world’s city dwellers live in Asia, and in just over a decade’s time, it will have 21 of 37 megacities worldwide. Over the next 30 years another 1.1 billion people are expected to join Asia’s already enormous urban population.

“Asia has seen unprecedented urban population growth but this has been accompanied by immense stress on the environment,” Changyong Rhee, ADB’s chief economist, said in the report, which runs in ADB’s annual statistical publication, Key Indicators for Asia and the Pacific 2012.

“The challenge now is to put in place policies which will reverse that trend and facilitate the development of green technology and green urbanisation.”

The downside of this expansion has been the rise of pollution, slums and a widening gap between rich and poor, all of which have contributed to environmental degradation.

Swelling urban populations mean that over 400 million people in Asians cities may be at risk of coastal flooding and roughly 350 million at risk of inland flooding by 2025.

And the impact of Asian environmental problems would have wider implications than just the region.

Urban CO2 emissions could reach 10.2 metric tons per capita by 2050 if left unchecked, a level which would have disastrous consequences for both Asia and the rest of the world.

On the upside, the report said that having so many people in relatively small areas makes it easier and more efficient to provide essential services. Combined with improvements in education, with the flight of factories from cities, the expansion of the middle classes and declining birth rates will all have positive impacts on the use of resources.

Governments need to invest in green infrastructure including public transport, the report said, and it hopes that the region would take advantage of new technologies which could lead to greener cities that will lessen the impact of climate change.