Although its cradle is the sparsely wooded savannah, humankind has long looked to forests for food, fuel, timber and sublime inspiration. Still a livelihood for 1.5bn people, forests maintain local and regional ecosystems and, for the other 6.2bn, provide a-fragile and creaking-buffer against climate change. Now droughts, wildfires and other human-induced changes are compounding the damage from chainsaws. In the tropics, which contain half of the world's forest biomass, tree-cover loss has accelerated by two-thirds since 2015; if it were a country, the shrinkage would make the tropical rainforest the world's third-biggest carbon-dioxide emitter, after China and America.
Nowhere are the stakes higher than in the Amazon basin-and not just because it contains 40% of Earth's rainforests and harbours 10-15% of the world's terrestrial species. South America's natural wonder may be perilously close to the tipping-point beyond which its gradual transformation into something closer to steppe cannot be stopped or reversed, even if people lay down their axes. Brazil's president, Jair Bolsonaro, is hastening the process-in the name, he claims, of development. The ecological collapse his policies may precipitate would be felt most acutely within his country's borders, which encircle 80% of the basin-but would go far beyond them, too. It must be averted.
Humans have been chipping away at the Amazon rainforest since they settled there well over ten millennia ago. Since the 1970s they have done so on an industrial scale. In the past 50 years Brazil has relinquished 17% of the forest's original extent, more than the area of France, to road- and dam-building, logging, mining, soyabean farming and cattle ranching. After a seven-year government effort to slow the destruction, it picked up in 2013 because of weakened enforcement and an amnesty for past deforestation. Recession and political crisis further pared back the government's ability to enforce the rules. Now Mr Bolsonaro has gleefully taken a buzz saw to them. Although congress and the courts have blocked some of his efforts to strip parts of the Amazon of their protected status, he has made it clear that rule-breakers have nothing to fear, despite the fact that he was elected to restore law and order. Because 70-80% of logging in the Amazon is illegal, the destruction has soared to record levels. Since he took office in January, trees have been disappearing at a rate of over two Manhattans a week.
The Amazon is unusual in that it recycles much of its own water. As the forest shrivels, less recycling takes place. At a certain threshold, that causes more of the forest to wither so that, over a matter of decades, the process feeds on itself. Climate change is bringing the threshold closer every year as the forest heats up. Mr Bolsonaro is pushing it towards the edge. Pessimists fear that the cycle of runaway degradation may kick in when another 3-8% of the forest vanishes-which, under Mr Bolsonaro, could happen soon. There are hints the pessimists may be correct. In the past 15 years the Amazon has suffered three severe droughts. Fires are on the rise.
Brazil's president dismisses such findings, as he does science more broadly. He accuses outsiders of hypocrisy-did rich countries not fell their own forests?-and, sometimes, of using environmental dogma as a pretext to keep Brazil poor. "The Amazon is ours," the president thundered recently. What happens in the Brazilian Amazon, he thinks, is Brazil's business.
Except it isn't. A "dieback" would directly hurt the seven other countries with which Brazil shares the river basin. It would reduce the moisture channelled along the Andes as far south as Buenos Aires. If Brazil were damming a real river, not choking off an aerial one, downstream nations could consider it an act of war. As the vast Amazonian store of carbon burned and rotted, the world could heat up by as much as 0.1°C by 2100-not a lot, you may think, but the preferred target of the Paris climate agreement allows further warming of only 0.5°C or so.
Mr Bolsonaro's other arguments are also flawed. Yes, the rich world has razed its forests. Brazil should not copy its mistakes, but learn from them instead as, say, France has, by reforesting while it still can. Paranoia about Western scheming is just that. The knowledge economy values the genetic information sequestered in the forest more highly than land or dead trees. Even if it did not, deforestation is not a necessary price of development. Brazil's output of soyabeans and beef rose between 2004 and 2012, when forest-clearing slowed by 80%. In fact, aside from the Amazon itself, Brazilian agriculture may be deforestation's biggest victim. The drought of 2015 caused maize farmers in the central Brazilian state of Mato Grosso to lose a third of their harvest.
For all these reasons, the world ought to make clear to Mr Bolsonaro that it will not tolerate his vandalism. Food companies, pressed by consumers, should spurn soyabeans and beef produced on illegally logged Amazonian land, as they did in the mid-2000s. Brazil's trading partners should make deals contingent on its good behaviour. The agreement reached in June by the EU and Mercosur, a South American trading bloc of which Brazil is the biggest member, already includes provisions to protect the rainforest. It is overwhelmingly in the parties' interest to enforce them. So too for China, which is anxious about global warming and needs Brazilian agriculture to feed its livestock. Rich signatories of the Paris agreement, who pledged to pay developing ones to plant carbon-consuming trees, ought to do so. Deforestation accounts for 8% of global greenhouse-gas emissions but attracts only 3% of the aid earmarked for combating climate change.