REPUBLICANS HAVE pledged that they would not just repeal Obamacare, but also replace it with a serious alternative. They should hold themselves to the same standard on climate change. If President Trump moves to eviscerate the nation’s efforts to combat global warming, Republicans must have a believable alternative plan ready that would reduce emissions as much or more than current regulations would, enabling the United States to meet its international commitments.

Mr. Trump can start the process of undermining federal efforts to reduce the emissions of heat trapping gases any time he wants — and he appears poised to begin soon. Much of the nation’s climate policy was not explicitly written into law by Congress; rather, the Environmental Protection Agency assembled it during the Obama years, based on general authorities Congress gave the agency years ago in the Clean Air Act. Using this administrative approach, the EPA has over the past decade finalized a series of rules on both vehicles and power plants, most notably the Clean Power Plan, which demands that electric utilities cut their emissions from existing power stations. What all this means is that the country’s climate policy is quite vulnerable to ongoing legal challenges and to attacks from the new administration — much more so than policies Congress has specifically authorized, such as Obamacare.

Now that the Senate has confirmed the president’s pick to lead the EPA — former Oklahoma attorney general Scott Pruitt — The Post’s Juliet Eilperin and Steven Mufson report that Mr. Trump is preparing executive orders demanding that the agency rewrite the Clean Power Plan and a water quality rule, appearing to confirm predictions that the new administration would be hostile to environmental concerns. At best, a rewrite would signal that the administration will put enforcement of the Clean Power Plan on hold as a long, formal process of regulation drafting and litigation takes its course. The country could also see its premier global warming policy replaced by a substantially weakened rule that lends the United States little international credibility — and harms what had been accelerating global efforts to fight emissions collectively.

For all the talk of job-killing federal overreach, the EPA estimated that the Clean Power Plan would cost the economy only $5.1 billion to $8.4 billion in 2030 — far less than a drop in what is now a $17 trillion economy — and that ignores the value of various health and climate benefits on the other side of the ledger. Even for those on the fence about climate science, it should be clear enough that the United States must participate in the international effort to hedge against very bad potential climate outcomes.

If Mr. Trump or other Republicans have a better way of doing this, they should detail it before doing anything to the Clean Power Plan. In fact, a coterie of senior former GOP officials have offered a conservative path forward, proposing to replace EPA climate rules with a substantial carbon tax whose revenue would be rebated straight back to Americans. This is what conservative governing looks like. What do Mr. Trump and the rest of his party want to do?