Forests lock up a lot of carbon. Cutting them down accounts for around 20 per cent of the world’s emissions of greenhouse gases. On paper, halting deforestation should be the simplest way to cut emissions. Achieving a similar reduction by building wind turbines or nuclear-power stations, or by mandating more fuel-efficient cars and buildings, would take years and cost billions. In practice, however, halting deforestation is hard: much of the world’s rainforest has already succumbed to loggers and farmers. That is because it is difficult to align the interests of people who live in forests (now 20m in the Brazilian Amazon) with those of the rest of humanity.

The best way of doing so involves a mixture of two ideas: establishing clear property rights over land and paying its owners not to cut down trees. If these policies are to work anywhere, it will be in Brazil, which possesses 60 per cent of the world’s greatest tropical forest. Brazil has powerful motives for preserving the Amazon. Deforestation does terrible damage to the reputation of a country that is a pioneer in renewable energy. It also puts at risk the Amazon rain factory that enables Brazil to be one of the world’s biggest agricultural exporters.

Brazil now has a sophisticated system for monitoring deforestation from satellites and aeroplanes. It has set aside some 40 per cent of the Amazon as national parks or Indian reserves. It has laws that restrict deforestation in the rest. The problem is enforcing those laws over a vast area where many of the inhabitants dislike the rules. The first step is a proper land registry to confirm who owns what. Some 15-25 per cent of the Amazon is private property, which is supposed to be kept 80 per cent forested (though often is not). Most of the rest is nominally federal land, but in practice is up for grabs: title deeds are forged, people are killed and deforestation accelerates because of competing claims. Some farmers even clear trees as a way to solidify land claims: fines from Brazil’s environmental agency can create a paper trail that acts as proof of ownership.

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