A demonstrator during a protest in Paris. (Etienne Laurent/European Pressphoto Agency)

ON MONDAY, the journal Nature Climate Change published a study finding that global warming’s effects on major world cities could be far more devastating than previously understood. Some cities, it found, could be a staggering 14.4 degrees warmer on average by the end of the century, causing a 10.9 percent decline in gross domestic product as people work less, air and water quality declined, and more energy was needed to cool buildings.

On Thursday, President Trump took a major step toward making this dystopia a reality.

In announcing that he will pull the United States out of the Paris climate agreement, Mr. Trump dealt a blow to the effort to slow climate change — but not only that. By joining Syria and Nicaragua as the only nonparticipants in the most consequential diplomatic effort of this century, he also dealt a blow to the U.S. leadership that has helped promote peace and prosperity for the past seven decades under Republican and Democratic presidents alike. Under their guidance, the United States acted with selflessness and enlightened self-interest. The traits reflected in Mr. Trump’s decision are self-defeating selfishness, insecurity and myopia.

A variety of factors contributed to the nation’s post-World War II economic boom, but prominent among them was energetic internationalism. The General Agreement on Tariffs and Trade (GATT), which obliged countries to meet regularly and discuss improving the atmosphere for global trade, was one of the spectacularly successful U.S.-backed institutions that helped gradually remove barriers to economic exchange and innovation.

(Daron Taylor/The Washington Post)

The Paris agreement had the promise to be the 21st century’s GATT, providing a framework in which countries would regularly convene and in which each nation would be expected to offer what more it could do to advance an essential global goal that no country could achieve alone — not freer trade, in this case, but heading off climate change’s worst effects. The agreement bore an American stamp. It was fairer and more flexible than previous attempts to strike a global climate deal, with particular sensitivity to U.S. concerns that emissions limits not be imposed on any country.

The agreement was the world’s best hope to ensure that big developing nations such as China and India did their share, addressing GOP concerns that these countries would refuse to sacrifice along with the United States. It did not lock in exactly how the United States and other nations would help. Rather, it created an international expectation of voluntary commitments from every nation, enforced by diplomatic pressure. All of Mr. Trump’s arguments for withdrawing, in other words, are unfounded. He could have adjusted, even minimized, the U.S. commitment without trashing the framework.

The president said Thursday that the United States might rejoin the Paris agreement after a period of renegotiation. But given the extent to which other nations already accommodated American demands, the prospect of a radically different treaty is fanciful. So what tangible benefit does this irrational decision bring to Americans? None. None at all.