UNDER PRESIDENT Barack Obama’s leadership, the world finally began addressing one of the greatest challenges human beings have ever faced, a multi-generational struggle to keep the planet temperate and accommodating to human life. President Trump’s move to rip up Mr. Obama’s climate policies are beyond reckless. Children studying his presidency will ask, “How could anyone have done this?”

Climate science is complicated, but the basics are easy enough for those schoolchildren to understand. When humans burn fossil fuels, they emit heat-trapping gases into the atmosphere. Releasing vast amounts of these gases for decades changes the atmosphere’s chemistry, creating an ever-thicker blanket. The world has therefore warmed and will continue to warm; the more fossil fuels burned, the hotter the planet will get.

The human species still has time at least to moderate the trajectory. But on the course Mr. Trump set Tuesday, the prospect will be for sharp environmental disruption. Among many other things, scientists have predicted more and more intense heat waves, more volatile weather, more abrupt changes in the landscape, more destruction from invasive pests, more illness from microbes flourishing in warmer fresh water and more urban flooding. Americans alive today will saddle future generations with the costs of acting too late, when addressing the issue sooner would have been cheaper and far less destructive.

Even as climate science has steadily improved, the U.S. climate debate has descended into a partisan mess, with a once-great American political party embracing rank reality-denial. The nation has now reached an anti-intellectual nadir, elevating a man who called climate change a “hoax” to the presidency and a climate-change denier to head the Environmental Protection Agency. The country reaped the fruits of this decision Tuesday, when Mr. Trump signed an executive order to unwind Mr. Obama’s Clean Power Plan and several other important climate rules.

The practical effects will be serious though not immediate. The Trump administration will have to rewrite federal regulations, which takes time and will encounter stiff resistance from environmental groups with many lawyers. Between now and 2020, other federal policies will continue to put some downward pressure on emissions. After 2020, the absence of the plan will be felt. According to an Energy Information Agency assessment released in January , energy-related greenhouse-gas emissions would have declined significantly between 2020 and 2030 — not by enough, but it would have been a decent start. Without the plan, these emissions will stay roughly the same over that crucial decade. And, by the way, energy experts predict no coal renaissance in Appalachia, despite Mr. Trump’s campaign promises, because the economics simply do not make sense in a country awash in cheap natural gas.

The nation had a climate policy. Now it does not. If Mr. Trump has a plan that would significantly cut greenhouse emissions in a smarter way than Mr. Obama’s Clean Power Plan — indeed, a few senior Republican statesmen offered one just a few weeks back — he should propose it. Instead, the president has put the country on a know-nothing path to an endangered planet.