For thousands of years, people have built their great metropolises right up against the contours of coasts, in estuaries and deltas, confident of the ebb and flow of the tides.
Domination of the seas allowed many coastal cities to become trading powerhouses and cultural melting pots.
But now that strength is becoming a liability as climate change swells the oceans, redrawing the map and putting hundreds of millions at risk.
That’s the warning of an unpublished draft report by the UN’s climate science advisory panel, seen exclusively by AFP, which outlines the vast and urgent climate threats facing the planet.
From the air, you can see the problem.
In city upon city, office towers, warehouses and homes crowd the water’s edge as if they are jostling to take a dip, major roads hug the shore, landing strips jut dauntlessly into the frothing waves.
These cities are on the “frontline”, the Intergovernmental Panel on Climate Change (IPCC) report says.
“Difficult choices will need to be made as sea level continues to rise, floods and storm surges become more frequent and intense, warming increases ocean acidity and intensifies heatwaves,” it adds.
Rising seas will likely spark mass migration and eventually force whole cities to be abandoned to the flood.
But profound consequences for megacities, small islands, arctic communities and deltas will come long before that—well within the lifetimes of most alive today.
In some places, people already know all too well the destructive power of water.
“I once was a bride of an affluent family,” Yasmin Begum tells AFP in the bare corrugated metal hut she now shares with her relatives in the Bangladesh capital Dhaka.
“My in-laws and parents, they had everything—cattle, nice house, farmland. But the river took it all away.”
The family had lived a relatively prosperous life in the southern island district of Bhola, an exposed tendril of land at the outer limits of one of the world’s most populated deltas.
Then one night 12 years ago, the raging Meghna river devoured everything they had.
Begum, now 30 and a mother of three, scratches a living as a Dhaka housekeeper, while her husband drives a cycle rickshaw through the city’s steamy streets.
They escaped one flood only to be at risk of another: Their new home, in one of the narrow-alley slums clustered precariously close to a flood barrier, is just metres from the riverbank.
Roughly a tenth of the world’s population and assets are based less than 10 metres (33 feet) above sea level.
“Structurally, a lot of cities are set up in almost the exact wrong place for a world with rising seas,” says Ben Strauss, CEO and chief scientist at Climate Central.
Ports and airports are built on the very flattest, lowest land possible.
“Sea level used to be stable,” Strauss explains.
“Not anymore.”
Land currently home to 300 million people will be vulnerable to annual flooding by 2050, according to his research.
Those least able to afford to protect themselves will likely be hit hardest.
All urban areas are increasingly stifled by air pollution and extreme heat, but those at the coasts are also deluged by flooding intensified by the rising water level and lashed by storms whipped into greater fury by warmer seas.
Saltwater washes across agricultural fields and rice paddies, while ecosystems—crucial for economies, cultures and climate protection—are gravely degraded by human activities.
Cities like Venice and Jakarta are also sinking, while low-lying island nations, particularly in the Pacific, are at risk of disappearing under the waves.
The report says slashing emissions now can reduce the risks.
“But sea level rise is accelerating and will continue for millennia,” it says.
The greenhouse gases already in the air have essentially baked-in the amount that waters will rise until around 2050.
By 2100, the IPCC has predicted water levels could be 60 centimetres (24 inches) higher, even if global warming is limited to well below two degrees Celsius—the cornerstone target of the 2015 Paris Agreement.
Longer term, it says, the prospects for many coastal cities are “dismal” without deep cuts to emissions.
Meanwhile, humanity continues to pump pollution into the atmosphere, locking in global heating that is melting ice sheets and glaciers and expanding oceans.
In just a few decades we may have emitted enough to eventually drown some of the world’s great cities, Strauss says.
By 2050, an at-risk region like South Florida might have developed a multi-decade plan to “empty out”, he says.
“We live our lives knowing we’re going to die, but I don’t think we often think of our cities in those terms,” he tells AFP.
“Most of our coastal cities are mortal. A lot of them will end by flood in the long run.”
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