Standing on the shores of Nyangai, a small island off the coast of Sierra Leone, Melvin Kargbo points to his old football field, now below an expanse of seawater. Never large, Nyangai has shrunk from around 700 metres in length a decade ago to about 90 metres now. Most of its mango and coconut trees have been felled by the waves. The remaining residents live in a tight cluster of houses that flood frequently. Even so, they want to stay put. “We cannot leave this land,” says Mr Kargbo, a 35-year-old teacher. “I do not think there is a better life for us anywhere else.”

He will probably be forced to find one. Global sea levels are rising as glaciers melt in the warming climate. Storms are becoming more erratic, accelerating coastal erosion. Some 15 years from now, Nyangai is likely to be under water.

It is not just Nyangai. Africa has contributed just 3% to global carbon-dioxide emissions since the industrial revolution, but it will be hit harder than other places by the effects of the warming climate: longer droughts, deadlier heatwaves and less predictable storms and floods. With no power to slow warming, adapting is the only option. But money is short.

For Mr Kargbo and his family, adapting will mean moving. The UN estimates that climate change may compel up to 113m Africans, 5% of the continent’s population, to leave their homes by 2050. Many will end up in cities such as Freetown, Sierra Leone’s capital. Yvonne Aki-Sawyerr, the mayor, says politicians need to “buckle up” to ensure cities remain habitable.

Ms Aki-Sawyerr has made a start. Her team has installed cooling covers on Freetown’s busiest and sweatiest markets. They have planted mangrove trees on the edges of flood-prone slums to serve as buffers against the rising sea. Earlier this month, Ms Aki-Sawyerr held a “heat summit” for mayors from six cities across west Africa to discuss how to keep their residents cool, for instance by planting more trees and installing cooling roofs on public buildings.

How they will pay for it is not clear. Adapting to climate change in Africa will cost between $30bn and $50bn a year over the next decade, estimates the UN, around 2-3% of the continent’s GDP. African governments do not have the money. Most of the funds for Freetown’s adaptation efforts come from bilateral aid and aid-funded multilateral organisations. As America and other rich countries continue to cut aid, especially for projects responding to climate change, nobody else is likely to step in.

That will ultimately make adaptation more expensive. The earlier it happens, the more money it saves by reducing damage to crops and infrastructure. According to one estimate by the World Bank, every dollar invested in infrastructure saves $4 in reconstruction costs. Effective adaptation, the UN reckons, could also shrink by about a third the number of people forced from their homes by climate change.

For some, it is already too late. The sandbags that Mr Kargbo wants the government to ship to Nyangai will not keep the sea at bay for long. But money to move and rebuild on the mainland could help him keep his head above water, even as the island is swallowed by the sea.