Coral Davenport, writing for the New York Times, argues that “when it comes to the pipeline’s true impact on global warming, energy and climate change experts — including former Obama administration officials — say Keystone’s political symbolism vastly outweighs its policy substance.”

Experts say that Mr. Obama’s eventual decision on the pipeline will have a marginal impact on global warming emissions, while those dull-sounding E.P.A. rules and treaty talks will determine his enviromental legacy.

Experts say that to make a serious dent in American carbon emissions, Mr. Obama’s administration would have to enact policies that would force the two most polluting sectors of the nation’s economy — cars and coal plants — to slash their emissions.