A new report by the Asian Development Bank is warning that Asia's rapidly growing cities must “go green” in order to prevent disasters such as mass flooding and out-of-control pollution.

The report says Asia, which is urbanizing at a faster rate than anywhere else, is already home to almost half of all the world's city dwellers. In just over a decade, it will have 21 of 37 megacities worldwide.

But ADB says the unprecedented urban population growth has been costly, and has resulted in a sharp rise in pollution, slums, and widening economic and social inequalities.

And it warns the region faces a “bleak and environmentally degraded future” if its cities do not become more sustainable. It says over 400 million people in Asian cities may be at risk of coastal flooding and about 350 million at risk of inland flooding by 2025 if current patterns persist.

The report also says carbon emissions could reach 10.2 metric tons per capita by 2050 at the current rates – a level it says could have “disastrous consequences” for both Asia and the rest of the world.

But the report says more people living close together could make it easier and less expensive for governments to provide essential services such as piped water and sanitation.

The bank also says other benefits of urbanization, including rising education levels, the growth of middle classes and declining birth rates could positively impact the environment.