In the bland, formal language of interagency correspondence, the Environmental Protection Agency has written a trenchant review of the State Department’s most recent effort to assess the consequences of building the Keystone XL pipeline. The E.P.A’.s letter, issued Monday, at the end of the public comment period on the department’s latest draft environmental impact statement, is hardly a favorable report card.

The letter commends the State Department for inching slowly toward an understanding of the significant difference in the amount of greenhouse gases caused by the energy-intensive production of Alberta’s heavy oil compared with the amount caused by the production of conventional oil. But it wishes that the department had given greater emphasis to the cumulative impact of these gases in the atmosphere over the next 50 years. It also says the department gave short shrift to the corrosive effect of the oil and its dangers to the vital aquifer underlying part of the pipeline’s latest route.

But the department’s biggest problem, the agency said, is a flawed assumption that distorts all of its analyses: that “oil sands crude will find a way to market with or without” the Keystone pipeline. This is a kind of magical thinking. If this pipeline won’t do, the State Department argues, other pipelines will be built or rail traffic will be ramped up. One way or another, the department says, oil sands production will go ahead full speed. For a variety of reasons, not least the cost of rail transportation, the E.P.A. has serious doubts.

One of the things we have found distasteful about the State Department’s views from the start is their air of inevitability. But we should not regard the pipeline or the eventual scale of tar sands production as inevitable. Nor should President Obama, whose decision is expected later this year, or Secretary of State John Kerry, who will advise him. In his Earth Day statement, Mr. Kerry, discussing global warming, said that “the science is screaming at all of us and demands action.” Blocking the pipeline is one obvious response.