PRESIDENT TRUMP has arguably been most effective in ripping up important environmental protections. Will the same be true of Jair Bolsonaro, inaugurated this week as president of Brazil? The world had better hope not.

Brazil is home to the largest uninterrupted forest in the world, a unique ecological treasure on which every other nation depends. Functioning as the Earth’s lungs, the Amazon takes in carbon dioxide — more than 2 billion tons of the greenhouse gas per year — and breathes out fresh oxygen. Massive amounts of planet-warming carbon dioxide is stored in the region’s trees and other flora. Deforestation, meanwhile, releases ever more of that trapped carbon dioxide into the air, even as it cuts the amount of mature forest eating up greenhouse gases. Worldwide, deforestation is responsible for 15 percent of annual planet-warming emissions. As the Amazon rain forest shrinks, fighting climate change is far harder. With wholesale deforestation, the job would be near-impossible.

Brazil in recent years appeared to recognize its special responsibility to preserve this irreplaceable planetary resource. Though some 3,000 square miles of rain forest disappeared last year, the country’s deforestation rate has nevertheless dropped sharply over the past decade or so. The country adopted a forest code forbidding Amazon landowners from clearing more than 20 percent of their land. Preserves set aside for indigenous peoples served as crucial havens from development. Recognizing the climate crisis, Brazil committed to cutting greenhouse emissions by 43 percent by 2030 in its submission to the Paris climate agreement.

Enter Mr. Bolsonaro, sometimes called Brazil’s Donald Trump. The new president has proposed running a highway through the forest, expressed hostility toward international environmental organizations, said that indigenous communities must “adapt or simply vanish,” called their preserves chickenpox on the landscape and promised to merge the Environment Ministry, which houses the authorities that oversee Amazon protections, with the pro-farming Agriculture Ministry. Immediately after assuming the presidency, he moved authority over indigenous preserves to the Agriculture Ministry and ordered new monitoring of international nongovernmental organizations. From here, already overstretched environmental watchdogs could see their resources and support only shrink.

Mr. Bolsonaro does not have a free hand. He cannot amend the forest code, eliminate indigenous protections or pull out of the Paris agreement without the cooperation of the legislature, in which his party does not have a majority. Meanwhile, the rest of the world can have some impact, too. Western consumers, companies and governments should eschew products coming from deforested tracts, and they should press Brazilian business partners to do the same.

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