FROM A certain perspective, Environmental Protection Agency Administrator Scott Pruitt's move to rip up the Clean Power Plan, President Barack Obama's signature climate change policy, hardly seems radical. The EPA chief repealed a rule he claims was illegal. Indeed, it was something of a stretch for the EPA to regulate planet-warming carbon dioxide under the Clean Air Act, a law originally written to handle more traditional pollutants. The Obama administration took an expansive approach, pushing the law as far as it could so that Mr. Obama's climate negotiators could credibly promise that the United States would cut its carbon emissions if other nations would, too.

Yet the Obama EPA's Clean Power Plan did not, in our view and that of many experts, break the law. The Supreme Court has already ruled that the EPA must regulate carbon dioxide under the Clean Air Act. There is a strong chance that the court would have upheld the Obama administration's now- ­defunct strategy to do so, or at least much of it. But now the country may never know, because its chief environmental officer wants not to do as much as he can to protect the atmosphere, but to do as little as the law demands, threatening the painstaking progress the Obama administration made in coordinating an international response to climate change.

Mr. Pruitt has not yet proposed a replacement plan; he began a public comment process that may lead to a new set of regulations on carbon emissions. It is technically possible that his EPA will decide to instate new rules that are serious, if less aggressive than Mr. Obama's. But everything about Mr. Pruitt's history — relentlessly suing the Obama administration, denying climate science, shutting environmentalist voices out of the EPA — suggests he will aim to do the minimum the courts will allow.

As if to confirm that reading, Mr. Pruitt announced his Clean Power Plan rollback in coal country, declaring that "the war against coal is over." Economist Paul Krugman has pointed out that 18 times more jobs have been lost at department stores than in the coal business since the turn of the century. Nevertheless, it is on behalf of a dirty, dying and relatively small industry that Mr. Pruitt will go down as one of the nation's worst environmental stewards. He may believe that the Obama administration's climate plan was radical. But his approach — anti-scientific, ideological, a betrayal of his office — is far more so.

There is another notable villain: Congress. For decades, lawmakers have dawdled while the greatest environmental challenge of our time has worsened, each wasted year making it harder to address. Once the science became clear — and it has been for years now — it became legislators' responsibility to craft a law specifically designed to address the carbon issue. Instead, they punted to the executive branch, leading to the chaotic regulatory seesaw now on display. The right response to Mr. Pruitt's radical action is a reassertion of congressional responsibility.