ENVIRONMENTALISTS HAVE drawn a line in the sand on the Keystone XL pipeline. It’s the wrong line in the wrong sand, far away from any realistic assessment of the merits — as yet another government analysis has confirmed. It’s past time for President Obama to set aside politics and resolve this bizarre distraction of an issue.

The State Department’s latest study — the product of more than five years of investigation — largely confirms the conclusions of previous assessments and those of many independent energy experts: Allowing the firm Trans­Canada to build Keystone XL, which would run across the Canadian border to Steele City, Neb., is unlikely to have significant effects on climate-change-causing greenhouse gas emissions. That’s because its construction, or its rejection, would not significantly affect the extraction of tar sands bitumen, an oil-like substance, in Alberta. Even if the president rejects Keystone XL and no other pipelines out of Alberta are built, the crude could still travel by rail and barge — with marginally higher greenhouse emissions and a higher likelihood of accident. One hundred eighty thousand barrels of Canadian crude already moves on train cars every day.

“The net effects of the pipeline’s impact on our climate will be absolutely critical to determining whether this project is allowed to go forward,” Mr. Obama said last year. The State Department gave the president an answer on that score. Now he should assent to the pipeline’s construction and move the country onto more important climate-related issues.

Environmentalists try to justify of their opposition to Keystone XL with a series of unlikely assumptions. If world oil prices end up significantly lower than projected for a long time, and if the Canadian government proves incapable of establishing any pipeline and sea routes out of the country, and if the price of rail transport remains as high as the State Department’s generous projections, then some tar sands extraction projects wouldn’t be economically viable. Advocates also contend that the passionate movement against the pipeline can be useful to achieve more consequential ends and therefore should be supported, as though cultivating irrational thinking is an acceptable basis for public policy. Neither view — one unconvincing, one cynical — reflects well on the country’s environmentalists.

If the pipeline wouldn’t heat the planet, it also wouldn’t significantly slash gasoline prices, the State Department found. The real downside to rejecting the project concerns jobs (construction would create at least several thousand), relations with Canada and the message that arbitrary ­decision-making would send to investors and other nations.

The country needs its environmentalists to do serious work. The United States will significantly cut its greenhouse gas emissions if and when people reduce their appetite for carbon-heavy fossil fuels, opting instead to use less or find greener alternatives. That is why we strongly favor placing a significant and rising tax on carbon dioxide emissions, a simple, universal carbon policy for the whole economy and pressing other nations to do the same. While we encourage Congress to act on carbon pricing, Mr. Obama’s Environmental Protection Agency can set and enforce emissions limits on power plants, the biggest emitters in the country, without lawmakers’ permission. That effort, too, requires defense against hostile politicians. Congress also needs to be convinced to invest more in scientific research.

Fighting for good climate policy may be more difficult than waging a symbolic war against a lone pipeline. But the battle for policies that might actually work is the one to which environmentalists must devote their time, enthusiasm and money.