The Donald Trump administration approved the Keystone XL pipeline knowing the tar sands crude oil it would deliver from Canada is even more polluting than the Obama administration thought when it turned the project down in 2015.

Recent government studies of a different tar sands pipeline found that the project's greenhouse gas emissions "may be five to 20 percent higher than previously indicated," the State Department noted on March 23 in its decision approving the Keystone XL permit.

Because both projects would carry tar sands crude, that suggests a similar understatement of emissions in the detailed but dated environmental review of the Keystone XL, which the Trump administration does not want to revisit. The final environmental assessment of the TransCanada project was published in January 2014.

The gloomier picture of emissions emerged during a review, published in January 2017, of a pipeline upgrade proposed by Enbridge that is still awaiting a presidential permit. The project, Line 67 (also called the Alberta Clipper, or the "Keystone XL of the Great Lakes"), has remained largely below the radar.

The estimate was produced using the most up-to-date studies and tools, including a model known as GREET, developed by the federal Argonne National Laboratory, which calls it the "gold standard" for this kind of calculation.

The model provides what's known as a "well-to-wheels" analysis of carbon emissions, taking into account every step, from the moment the fuel is first brought out of the ground through processing, refining and final combustion.

This kind of detailed analysis is the standard way to assess the climate change impact of a petroleum project. And the Keystone XL's critics now say the project's numbers ought to be run again.

Environmental groups have filed suit to overturn Trump's approval of the pipeline, arguing that it lacks an up-to-date environmental impact statement, as required under the National Environmental Protection Act (NEPA).

To be legal, they say, a review would have to consider all kinds of new information that wasn't available the last time around—including the latest GREET modeling. New data on oil prices, supply and demand, and the cumulative impacts of other pipelines also would have to be assessed.

State Department analysts had closely reviewed the complicated question of the tar sands' overall carbon footprint in a series of Keystone XL environmental impact statements. The review published in 2014 led President Barack Obama to reject the project in 2015.

Over the next few years, the State Department did a similar, but more thorough analysis for the Enbridge Line 67 project, which would expand existing pipelines carrying Canadian oil to midwestern refineries.

It was in that analysis that the darker picture of the tar sands was painted.

Compared to other sources of oil that American refineries might use, the tar sands would be about 17 percent more polluting on average, the State Department's final 2014 review of the Keystone XL estimated.

But the answer looked even worse in the Enbridge study.

Tar sands crude carried by Line 67 would have a carbon footprint of 632 kilograms per barrel, the GREET model estimated. That compares to an average U.S. refinery mix of 521 kilograms per barrel of carbon dioxide emissions. The difference is 111 kilograms per barrel—21 percent dirtier, not 17 percent.

To be sure, all of these estimates are approximations, filled with uncertainties and assumptions. But the more pessimistic Line 67 analysis, using the best information the government has available, would be just as relevant to the Keystone XL. That is why the passing reference to the heavier carbon footprint was tucked into Trump's Keystone XL decision.

Among the specialists who noticed the new math was Joshua Axelrod, a Canada analyst at the Natural Resources Defense Council, which is among the groups challenging the Keystone XL in court.

The environmental assessment of Enbridge's Line 67—performed during the Obama administration—found that the pipeline "could contribute incrementally to global climate change." It said that in aggregate, "emissions from these sources can have a large cumulative effect," and that the climate change impacts "would likely be long term."

The Trump team is disdainful of how the Obama administration applied NEPA. It has revoked an Obama executive order requiring all agencies to fully consider climate impacts in every environmental review conducted under the law, a landmark 1970 statute.

But in a March 27 letter to the State Department assessing the Line 67 environmental impact statement, the Environmental Protection Agency praised the document's discussion of the Enbridge line's greenhouse gas emissions. It particularly praised the work on the relative carbon intensity of tar sands.

"EPA believes these discussions are clear and informative, and take a hard look at potential impacts," the letter said.

The phrase "a hard look," a NEPA term of art, is the formal legal standard for any environmental impact statement to pass judicial muster.