IT TOOK global negotiators a quarter-century to strike the Paris climate agreement, an international accord aimed at slowing global warming. The agreement represents the best hope for a world in which no one country acting alone can do enough to fight this global threat. Donald Trump could destroy the agreement with a stroke of a pen, and with it any hope that the world will keep the planet’s temperature within the boundaries scientists say are safe.

The Paris deal represents a major U.S. commitment, but it is not a treaty with the force of law. President Obama submitted the U.S. emissions goal; Mr. Trump could withdraw it just as easily. He also could deeply undercut or eliminate the basis for any future U.S. commitment. Mr. Trump has promised to rescind the Clean Power Plan, which obliges electrical utilities to reduce their greenhouse-gas emissions by cutting down on coal burning and increasing their use of natural gas and renewables. The Environmental Protection Agency drew up the plan using authorities that Congress gave it decades ago in the Clean Air Act. Mr. Trump could press the agency to revoke it or water it down.

Mr. Trump also would have considerable sway over the state of the nation’s land and water. He has promised to use that power to quash regulations on drilling and to open federal lands to coal, oil and gas production. He could do more; for example, he could attempt to use executive discretion over drilling royalties, pipeline construction, permitting, drilling leases, oil and gas exports, and other matters to reward firms and people he likes or to punish those he does not.

Then there’s the Antiquities Act of 1906 , which presidents have used to preserve historical monuments and natural treasures. Mr. Trump could reverse the process, shrinking or canceling protections for natural landscapes and marine preserves that Republican and Democratic presidents alike have set aside. The legal basis for doing so is untested — but not implausible.

To a degree, the courts could exert a check on Mr. Trump. In environmental matters, private parties can often sue to prevent major rules changes or to compel the government to enforce the law. But even hypervigilant judges could not prevent some major policy shifts, such as leaving the Paris climate accord.

Moreover, neither the judicial check nor a subsequent president who tried to pick up the pieces could compensate for the loss of time and focus as climate change accelerated . U.S. foot-dragging on warming would give other nations a pretext to avoid hard actions, too. The planet would warm more; the oceans would become more acidic; corals would bleach; sea levels would rise; deadly temperature extremes would get more frequent; species would die off; landscapes would change; shipyards would swamp; climate refugees would seek shelter. Some of these things are already bound to happen, to some extent. Mr. Trump is promising to exacerbate the problem, and he could do so by executive fiat.