In truth, the agreement does not require any country to do anything; after the failure of the 1997 Kyoto Accord, the United Nations, which oversees climate change negotiations, decided that it simply did not have the authority to force a legally binding agreement. Instead, negotiators in Paris aimed for, and miraculously achieved, a voluntary agreement, under which more than 190 countries offered aspirational emissions targets, pledged their best efforts to meet them and agreed to give periodic updates on how they were doing.

Paris did not, in short, legally constrain Mr. Trump from doing the dumb things he wanted to do. Which he already has. In the last few months, and without consulting a single foreign leader, he has ordered rollbacks of every one of the policies on which President Barack Obama based his ambitious pledge to reduce America’s greenhouse gas emissions by 26 percent to 28 percent below 2005 levels by 2025 — most prominently, policies aimed at reducing greenhouse gases from coal-fired power plants, automobiles and oil and gas wells.

But if withdrawing from the agreement will not make Mr. Trump’s domestic policies any worse than they are, it is still a terrible decision that could have enormous consequences globally. In huge neon letters, it sends a clear message that this president knows nothing or cares little about the science underlying the stark warnings of environmental disruption. That he knows or cares little about the problems that disruption could bring, especially in poor countries. That he is unmindful that America, historically the world’s biggest emitter of carbon dioxide, has a special obligation to help the rest of the world address these issues. That he is oblivious to the further damage this will cause to his already tattered relationship with the European allies. That his malfeasance might now prompt other countries that signed the accord to withdraw from the agreement, or rethink their emissions pledges.

Perhaps most astonishing of all, a chief executive who touts himself as a shrewd businessman, and who ran on a promise of jobs for the middle class and making America great again, seems blind to the damage this will do to America’s own economic interests. The world’s gradual transition from fossil fuels has opened up a huge global market, estimated to be $6 trillion by 2030, for renewable fuels like wind and solar, for electric cars, for advanced batteries and other technologies.