Megacities are both booming and increasingly exposed to the risk of disastrous floods

Jakarta and Dhaka dethroned Tokyo’s long-held status as the world’s biggest city, with 42 million, 37 million and 33 million people respectively. Mexico City and Sao Paulo were overtaken by Shanghai and Cairo among the global top 10. Bangkok, Delhi, Karachi, Lagos, Luanda and Manila were some of the fastest growing among metropolises of more than 10 million.

Many of these very regions have been hit by a devastating run of floods in recent weeks. The monsoon belt from Southeast Asia to West Africa is a swathe of the globe that’s urbanizing the fastest and also the one where catastrophic rainfall is set to increase most dramatically.

Nearly 1,000 people have been killed in a wave of storms that have stretched from Sri Lanka to Vietnam, with more than 442 dead in the north of Indonesia’s Sumatra island and at least 160 fatalities in southern Thailand.

Such disastrous events are hardly unprecedented. Most early civilizations grew up along inundation-prone river valleys, as evidenced by the near-universality of deluge myths. In the same rural areas of Southeast Asia that have been among the worst-hit by the rains of recent weeks, homes were traditionally built on stilts under steeply-pitched roofs to allow water to run off without doing harm. Local traditions often warn against building near wild rivers prone to bursting their banks.

The sophistication of this vernacular technology is under-appreciated, but—as with modern urban flood modelling—it’s inadequate to the challenges we’ll face as our planet warms.

With each degree that the local temperature rises, the atmosphere’s ability to hold moisture goes up by about 7%. That’s an immense amount when you consider that a cyclone can easily hold half a billion tonnes of water. Indigenous knowledge, like modern flood maps, is grounded in a historical understanding of how rainwater behaves—but the heating of our planet is making all those old predictions irrelevant.

These risks are greatest in the expanding megacities. The current rural population of about 1.5 billion will barely grow before heading into permanent decline in the 2040s, according to the UN, but two-thirds of the population growth between now and 2050 will be in cities. About half a billion new urbanites will be in just seven countries, most of them in the Asian and African monsoon belts: India, Nigeria, Pakistan, the Democratic Republic of Congo, Egypt, Bangladesh and Ethiopia.

Unlike rural dwellers who can often site their settlements in more stable locations, city migrants rarely have much choice on where to live. That’s why so many shanty towns are built on land once neglected as too risky, from the landslide-prone hillsides of Brazil’s favelas and Venezuela’s barrios to the swamps that gave rise to slums in Mumbai’s Dharavi, Bangkok’s Khlong Toei and Lagos’s Makoko.

Precious few of these places have the sort of wealth to handle the engineering challenges of weather-proofing their built environment. Out of 1.8 billion flood-threatened people worldwide, just 11% are in high-income countries.

Unlike famine and infectious disease, urban floods are rarely the result of absolute poverty. Instead, they’re most often the outcome of development that’s failing to keep pace with the problems it brings in its wake—cities whose allure is drawing people in so fast that infrastructure is incapable of moving at the same speed.

The most damaging flood over the past week in Thailand was in Hat Yai, a bustling tourist and shopping destination close to the Malaysian border that’s home to a special economic zone and one of the country’s busiest airports. In Sri Lanka, the fast-growing capital Colombo was worst-hit.

That puts a grave responsibility on municipal and national governments. All are counting on cities as their engines of growth over the coming decades, but they’ll need to work hard in the face of natural disasters that will perpetually threaten to tear apart their urban fabric. The great centres of India, straining under water shortages and choking urban pollution, show what can happen to a country when urbanization starts to fail.

Bringing fresh water and global connections with them, rivers and coastlines have long been the lifeblood of the world’s great cities. As rising seas and devastating floods now make those same places increasingly unliveable, we must confront the possibility that these life-giving attributes could be their doom as well. ©Bloomberg

The author is a Bloomberg Opinion columnist covering climate change and energy.