PRESIDENT TRUMP was not the only populist conservative leader who drew criticism at the Group of Seven meetings last weekend in France. In between discussions of Mr. Trump’s trade wars and historically bad idea to readmit Russia to the G-7 was concern that Brazil’s new president, Jair Bolsonaro, is presiding over an environmental catastrophe, as fires burn in the irreplaceable ecological wonder that is the Amazon River basin. The Amazon rainforest is a crucial resource for all of humanity; the trick will be to persuade the current Brazilian government to act without crossing Mr. Bolsonaro’s extreme sensitivity to any perceived insult to Brazil’s sovereignty.

NASA reported last week that satellites have confirmed an uptick in fires in the Brazilian Amazon, resulting in the most vigorous fire season since 2010. “August 2019 stands out because it has brought a noticeable increase in large, intense, and persistent fires burning along major roads in the central Brazilian Amazon,” the NASA statement explained. “While drought has played a large role in exacerbating fires in the past, the timing and location of fire detections early in the 2019 dry season are more consistent with land clearing than with regional drought.” This indicates that deforestation, not preparing already cleared land for meat or soybean production, is a major culprit.

This is no surprise. Official Brazilian government figures suggest that deforestation is rising under Mr. Bolsonaro, who prioritizes development of the Amazon over environmental protection. Mr. Bolsonaro dismissed those numbers as “a lie,” igniting a fight with experts within and without Brazil. A recent New York Times analysis found that enforcement actions from the country’s primary environmental authority have dropped 20 percent during the first six months of this year, compared with the first half of 2018.

The good news, if one can call it that, is that deforestation does not yet appear to have hit the breakneck levels of the 1990s and early 2000s. But that is little reason for comfort. Amazon deforestation must be stopped. Experts worry that the rainforest is getting ever closer to a tipping point at which the water recycling that underlies the ecosystem will break down, transforming the region from rainforest to savanna or some other landscape. This “forest dieback” scenario would see massive amounts of heat-trapping carbon dioxide, stored within the Amazon’s vast amount of flora, released into the atmosphere. The rainforest will face increasing stress as global warming progresses, making the region more vulnerable to forest fires. It needs no further challenge from humans.

Yet Mr. Bolsonaro has held up a $22.2 million emergency aid package that the G-7 nations pledged to help fight the current fires, as he fights with French President Emmanuel Macron. The Brazilian president often accuses those worried about the fate of the Amazon of colonialism. Unfortunately, Mr. Macron played into Mr. Bolsonaro’s hysterics as he stressed the interest other nations have in the Amazon’s future.

Mr. Bolsonaro should stop fighting and take the aid money, and he should allay concerns donor nations might have that he will divert it to agricultural interests, which led Germany and Norway to freeze rainforest aid this year. Meanwhile, Mr. Macron and other world leaders should be willing to pony up more — and to take care that their tone expresses concern rather than command.