Coastal waters flow through deteriorating wetlands on Aug. 22 in Plaquemines Parish, La. According to researchers at the National Oceanic and Atmospheric Administration (NOAA), Louisiana’s combination of rising waters and sinking land give it one of the highest rates of sea-level rise on the planet. (Drew Angerer/Getty Images)

IN 2015, international negotiators struck the Paris climate agreement. Nearly four years later, it remains the world’s most promising tool to get all the planet’s major carbon dioxide emitters working together to cut the heat-trapping gases they release into the atmosphere. But that does not mean it is working as well as expected — or as well as humanity needs it to.

Last month underscored the extent to which world leaders are failing future generations. That starts with President Trump, who recklessly denies that there is a problem. At a United Nations climate conference, delegates were surprised to see that Mr. Trump even showed up. They laughed when U.N. climate envoy Michael Bloomberg suggested that the president could use what he heard to develop a climate policy. Mr. Trump’s mindless intention to remove the United States from the Paris agreement next year, at the first possible opportunity, not only makes the United States the object of international contempt — it also deeply undercuts the accord’s effectiveness.

At a conference such as this, delegates should have been able to pressure major powers to increase their emissions-cutting ambitions. Instead, China’s representative said his country would merely “fulfill its obligations,” noting that “certain countries” — that is, the United States — would not even meet the minimum commitments they made in Paris. Mr. Trump’s abdication of leadership enables others to shrug off further obligations and still appear to be more far responsible. He is letting China off the hook just when the world’s largest emitter needs to feel more pressure. Because simply meeting initial Paris commitments would not be enough: The world would still be on track to warm by 2 degrees Celsius by 2100, which could be catastrophic for human society.

To underscore just one category of risk, the United Nations also released a major report on how the world’s waters are changing. Higher temperatures melt land-based ice and expand the volume of ocean water. Rising greenhouse emissions could boost global sea levels by substantially more than three feet by 2100, an increase from the last U.N. estimate. Even under more favorable scenarios, by only 2050, major cities such as Los Angeles and Miami might face a “100-year” flood every year. If humanity fails to prevent the loss of major Antarctic and Greenland ice masses, future generations would see far worse.

Higher seas are only one danger. Warmer waters wreak havoc on sea organisms forced to migrate away from major fisheries — and the fishermen who depend on the resource. More acidic water dissolves the world’s corals. Retreating glaciers disrupt freshwater availability. “Marine heat waves” kill off ocean life.

For the moment, some of the most pronounced consequences have been concentrated in areas such as the Arctic. But it will be increasingly impossible for humans all over the planet to ignore the accelerating costs of uncontrolled climate change. No one will be laughing then.