AND THEN there was one. The Associated Press reports that, at a U.N. climate conference in Bonn, Germany, this week, Syrian representatives said their country will join the Paris climate agreement. That means every country on the planet has embraced the pact.

Except one. The United States, a signatory, recently declared that it will repudiate the agreement. Once a world leader helping to corral other nations to solve common problems, the United States has become a drag on the global response to the largest environmental threat of this century. It is now alone in its rejection of Paris.

President Trump has played down the importance of climate change. Last year he told The Post that the nation simply faces much graver threats. He is wrong. Even if scientists' warnings turn out to have been too dire, climate change still poses massive risks to human society, and the world must respond. A recent study underscored this point.

The study, published in Nature Geoscience, suggests that the world may have not missed its chance to avoid dangerous climate change after all. A team of European researchers recalculated the planet's carbon budget — that is, the amount of carbon dioxide that humans can emit going forward before risking dangerous temperature increases. They found that humanity might be able to release more than previously thought and that restraining global temperature rise to 1.5 degrees Celsius, a goal many presumed was unreachable, remains possible.

Yet, even if this encouraging news is correct, the world is still warming quickly. Humans are still responsible. A transformation in the way nations produce and use electricity must occur over the next several decades. The paper's conclusions offer an opportunity for world leaders to do what they should have done before and mitigate the problem before it gets too severe. The researchers found that meeting the 1.5-degree goal would require emissions to stabilize in the next decade and rapidly decline afterward. That, in turn, would require an ambitious international campaign to cut emissions — precisely what the Paris agreement was designed to spur.

Mr. Trump's objections to Paris are mostly fiction. It does not demand that the United States sacrifice while others do little. In fact, the president may adjust the United States' Paris commitment whenever he wants. The deal merely asks nations to submit voluntary emissions plans, without specifying the level of ambition or the policy tools to be used. It also specifies no sanctions for countries that fail to meet their commitments. If the commitment that President Barack Obama submitted in Paris is not to Mr. Trump's liking, he could replace it. Meantime, staying in the treaty would make other nations more likely to follow through on their pledges and would build international expectations that all nations will act together, which is the only way climate change will ever be effectively addressed.

Altering the U.S. commitment would be unfortunate, but it would be less unreasonable than leaving entirely. Every nation from Syria to China to Brazil to Saudi Arabia is finding a way to sign up and stay in. Every nation but one.