Congress has delivered to President Obama a bill commanding him to approve construction of the Keystone XL oil pipeline from Canada, accompanied by a warning from House Speaker John Boehner to ignore the “left-fringe extremists and anarchists” who oppose the project.

It was not immediately clear whom Mr. Boehner had in mind, unless he meant the 90 scientists, economists and Nobel laureates who appealed this week to Mr. Obama to reject the pipeline on the grounds that the United States should not be complicit in unlocking some of the dirtiest fuel on the planet. In any case, Mr. Obama should ignore the speaker and, as he has promised, veto the bill. Because the pipeline would cross an international border, the decision about whether to proceed is his to make, not Congress’s, and the State Department review that will help guide that decision is not yet complete.

The veto is the easy call. The tougher one — for the president and his secretary of state, John Kerry — is whether eventually to say yes or no to the pipeline, which would carry about 800,000 barrels of oil a day from Alberta’s tar sands to refineries on the Gulf Coast. In the great scheme of things, this would not be a big addition to a global oil output that now exceeds 90 million barrels a day. And the oil would come from a reliable friend, Canada. Building the pipeline would also provide about 3,900 temporary construction jobs over two years, but no more than 50 permanent jobs thereafter.

At the same time, both Mr. Obama and Mr. Kerry have declared, without reservation, that climate change is a grave and increasingly tangible threat to world stability. The Canadian tar sands oil can only add to that threat.