It may not look like much anymore, but in its heyday La Chaumière was the “premier nightclub in all Saint Louis”, recalls Cheikh Badiane. When the ocean tide was low, the long beach extending far into the distance was wide enough for crowds to gather for football matches on the sand. But in recent years, the ageing fisherman says, “so many catastrophes have happened.” La Chaumière is closed. The Koranic school along the waterfront is no more. A few years ago, during a particularly terrible flood, a small house next to a mosque collapsed, killing the carpenter who lived there. These days, when the storm-surge comes, the waters reach all the way to the war memorial a couple of hundred metres inland. Inch by inch, home by home, Saint Louis is being washed into the sea.

A crowded island city built among waterways, Senegal’s former colonial capital—dubbed the “Venice of Africa”—is especially exposed to a changing climate and rising oceans. The thin peninsula on which fishermen like Mr Badiane live has the Atlantic on its western side and the mouth of the Senegal river on its east. A botched attempt, in 2003, to reduce flooding by digging a canal made things worse, putting a whole neighbourhood under water. A study commissioned by the Senegalese government a decade later found that 80% of the city will be at risk of flooding by 2080. “Saint Louis is a city of water,” says Mr Badiane. “If we’re not careful it will all disappear.”

St Louis is not just an example of a city that is extremely vulnerable to climate change, it may also be a vision of the future. Many of West Africa’s fast-growing cities are at risk of sinking slowly beneath the waves. Across the globe seas are expected to rise by a further half-metre or so on average in the next 50 years. Low-lying West Africa will be particularly badly hit. The major cities built by European colonial powers a century or more ago are overwhelmingly found on fragile sandy shores, often among lagoons and mangrove estuaries at the openings of rivers used for transport and trade (see map). In Nigeria Lagos, for instance, straddles a string of islands. Much of Mauritania’s capital, Nouakchott, is below sea level. It is protected only by a belt of dunes, which may itself be breached by the waves.

West Africa’s coastal cities might not yet be the most visible victims of rising seas. Several cities in Asia have witnessed more dramatic disasters. Half of Jakarta, Indonesia’s capital, was submerged under nearly four metres of water in 2007, which forced half a million people from their homes. But the speed at which West Africa is urbanising, and the particularly low level of income at which it is doing so, will greatly magnify the impacts of swelling tides. “These cities are the future megahubs of the continent,” explains Kamal Amakrane of the un’s Global Centre for Climate Mobility (gccm). The World Bank reckons some 42% of West Africa’s gdp is generated in coastal areas, which are also home to around 33% of the region’s population.

The problem is not only surging seas. It is also, simultaneously, sinking cities. In much of West Africa, subsidence—the lowering of the land surface itself—is often one of the biggest causes of urban coastal flooding, says Rafaël Almar, a geophysicist and oceanographer at France’s Research Institute for Development. Lagos, for instance, is sinking by as much as 87mm a year, due in part to uncontrolled development and badly maintained drainage systems. Most of the region’s coastal cities also pump water from aquifers on which they are built, literally shaking the earth beneath them, notes Marcus Mayr of the un’s Green Climate Fund.

This means the ground is weakening just as soaring temperatures and dwindling fresh water push more and more of those living in West Africa’s desiccating hinterlands towards the coast. Indeed, no continent is projected to see faster rates of population growth and urbanisation in its low-lying coastal areas than Africa. A report published by the Intergovernmental Panel on Climate Change in 2021 found that some 116m Africans could live in such areas by 2030. And nowhere will see faster growth than the West African littoral. Lagos, some reckon, is already growing by 1m people a year. By 2100, according to some estimates, the roughly 1,600km coastal stretch from there to Abidjan, the capital of Ivory Coast, may form a single, sprawling megalopolis containing as many as 500m people.

Managed well this could be an immense driver of economic growth. But the gccm warns that on current trends these coastal cities will in fact cease being population magnets by 2050. As the effects of flooding and erosion mount, whole neighbourhoods will become uninhabitable—turning cities themselves into sources of climate migration.

St Louis illustrates some of the difficulties in holding back the waves. France and the World Bank paid for an emergency dyke to be built after a particularly catastrophic flood in 2007. But costly protective schemes are not a long-term solution for most cities in poor countries. Several dykes in Senegal have collapsed, as did a seawall in Ghana. The World Bank instead touts “nature-based” alternatives, such as the mangroves and coral reefs that once protected the coasts. But some of these “themselves are threatened by climate change”, notes Nick Simpson of the Overseas Development Institute, a think-tank in London.

Even if the world stopped carbon emissions today, inexorably rising seas are already “baked in”, says Mr Amakrane. This means many people will have no choice but to move to higher ground. Along the beachfront in Saint Louis houses have been marked for demolition. More than 3,000 residents have been resettled on the other side of the city. Mr Badiane is resigned to moving, too. “Everyone must leave,” he sighs.