When Gibril Ali talks of “black gold”, he does not mean oil. He buys it in sacks, not barrels. He sells it to his customers in Nairobi, Kenya’s capital, from a metal shack. But in Kenya there is enough money in charcoal to make “big, big people” get rich by transporting it, he says.

Charcoal has fuelled Africa’s urbanisation. It is dense in energy and hard to regulate, much like cities themselves. Despite concerns about pollution, lost trees and illicit money flows, it refuses to go away. Nearly 200m Africans cook mostly with charcoal, including 27% of urbanites (see chart). The share has grown since 1990, offsetting the falling use of firewood. If there has been an African energy transition, it is from one kind of wood fuel to another.

The appeal of charcoal is its convenience. By weight it contains nearly twice as much energy as the wood from which it is made. That makes it easier to transport, helping megacities to outgrow their sketchy infrastructure. In Kinshasa, a Congolese metropolis of 18m people, almost everyone cooks with it. Never before, writes Jean-Baptiste Fressoz, a French historian, have cities reached such a size while relying so much on wood.

In the African countryside, tens of millions of people use earth kilns to make charcoal, outnumbering all the world’s coal miners. The process involves heating wood under low-oxygen conditions to remove moisture and volatile gases. Logs and branches are stacked into a pile, covered with grass and soil and burned for several days. This process yields between one and two tonnes of charcoal for every ten of dry wood. It emits much more carbon than transporting or burning it.

The wood often comes free, because producers harvest from their own land or from forests. Many are farmers who sell charcoal to traders when they need a bit of cash. Others are businessmen who bring in work teams from afar, dish out chainsaws, then pay off a landowner or chief. Locals often complain that outsiders take their trees. Vigilantes have attacked charcoal trucks in Kenya and Uganda.

The value of Africa’s charcoal trade is thought to be in the tens of billions of dollars, maybe more than Africans make from cocoa or coffee. The money trail often leads to soldiers or officials. The UN Security Council banned charcoal exports from Somalia because jihadists were making millions selling it to the Gulf, where shisha smokers prize the aroma of acacia wood.

Charcoal production leaves destruction in its wake. In Mabalane district in Mozambique, two-thirds of woodland was affected by it in the decade to 2018, estimates a study by Fernando Sedano, then at the University of Maryland, and others. But in some places agriculture is the bigger culprit. Farmers might clear land mainly to grow crops, making kilns as they go.

In Kitui county, about 200km east of Nairobi, Joseph Kisovi Martha describes a dilemma. In ten years all the trees will be gone, he says, sitting near a stump. But he felled the tree himself. How else, he asks, is he to pay school fees for his children?

To address the downsides of the charcoal trade, governments oscillate between trying to make it legal, through licensing and forest management, and outlawing it. Abrupt bans create an illicit market, says Tuyeni Mwampamba, a Tanzanian researcher. Enforcement is a messy negotiation between traders, officials, police, soldiers and local residents. Lorry drivers move at night and pay bribes at checkpoints. In Uganda, where production is banned, charcoal is smuggled across the border before being reimported with a seal from South Sudan.

Other policies encourage “clean cooking”. Liquid petroleum gas is cleaner than charcoal and in Kenya is exempted from VAT. Firms are putting meters on gas canisters so consumers can pay as they go, rather than facing a big cost upfront. More than half of urban Kenyans now use LPG as their main fuel. Burn Manufacturing, which makes efficient stoves, has stopped selling its charcoal model there and is promoting electric ones instead, subsidised by carbon credits. Its boss, Peter Scott, argues that charcoal is “dying out” in Kenya, though not in countries like Madagascar or Zambia, where alternatives are scarce.

Ending the use of charcoal would have economic consequences. The trade is informal and decentralised; gas flows through big firms, without creating as many jobs. Charcoal provides roughly 275 days of work for each terajoule consumed, compared with 95 for electricity and 15 for LPG. Gas imports eat up foreign currency.

For now, consumers mix and match energy sources. Charcoal production in Africa is still growing by 2% a year. Mr Ali’s neighbours use it for slow cooking, like boiling beans. Nearby shops sell gas and ethanol fuel. He is not worried about competition. “As long as there is the ghetto, they will always use [charcoal],” he says.