How much can one oil pipeline affect global climate change? That's one of the fundamental questions probed by a new, final environmental impact assessment released January 31 by the U.S. State Department. At issue is the Keystone XL pipeline, which would carry 730,000 barrels a day of oil from western Canada, mostly from Alberta's tar sands, but also 100,000 barrels per day of oil fracked from the Bakken Shale in North Dakota. Much of that oil is already being produced and is being transported by other means, such as railroads.

The impact of a single pipeline in North America could hypothetically be trivial given that climate change is a global problem. Earth’s atmosphere does not distinguish between an individual molecule of carbon dioxide wafting up from the U.S. Midwest versus one spewed in the Middle East. And the State Department stands by its earlier assessment that Canada's tar sands will be mined and melted whether or not Keystone XL ever gets built.

In fact, the State Department finds in the new assessment that the proposed Keystone XL pipeline is the most environmentally friendly option compared to other transportation alternatives, such as railroads and tanker ships. Despite the significant (and unique, due to the oil's characteristics) risk of spills, a pipeline like Keystone XL is a safer, cheaper and more environmentally benign way of transporting oil, the assessment concludes.

But the State Department also received more than 1.5 million letters commenting on its initial draft of this environmental impact statement released in 2012, most concerned that the Department did not "adequately address the greenhouse gas and climate change effects of the extraction, processing and use of the crude oil" that Keystone XL would carry.

So the State Department has dug more deeply into the issue of greenhouse gases (pdf) as well and announced today that the Keystone XL pipeline would increase greenhouse gas emissions. Oil from Alberta's tar sands is one of the most polluting kinds of oil, the report notes, thanks to the energy cost of producing it in the first place as well as the pet coke and other byproducts that end up getting burned as well. The State Department also noted that just running the pipeline for a year once built would result in the same greenhouse gas pollution as roughly 300,000 cars over the same time span, and that the oil carried by the pipeline could add as much as 27 million metric tons of greenhouse gases to the atmosphere each year, most of that from its ultimate use as fuel.

That is just a finding, and the whole report is just evidence for decision makers to weigh. "It's not a decision," emphasized Jen Psaki, a spokeswoman for the State Department at a press briefing on January 31, noting that the 30-day comment period begins February 5. Now Secretary of State John Kerry will weigh this updated environmental impact assessment, along with climate and environmental priorities, Psaki added, and there is no timeline for a final decision.

Ultimately, that final decision on whether to approve the Keystone XL pipeline rests with President Barack Obama, and it will form a significant part of his climate change legacy. As Obama said during his speech on climate change last June: "Allowing the Keystone pipeline to be built requires a finding that doing so would be in our nation’s interest. And our national interest will be served only if this project does not significantly exacerbate the problem of carbon pollution."

The key word there may be "significantly." Even 27 million metric tons of greenhouse gases per year may seem small in a world that spews more than 34 billion metric tons of CO2 per year. And, if the State Department is right that the tar sands will be exploited anyway, then the CO2 emissions will happen anyway too, with or without Keystone XL. But as the President said in his 2014 State of the Union speech: "when our children's children look us in the eye and ask if we did all we could to leave them a safer, more stable world, with new sources of energy, I want us to be able to say yes, we did." It remains to be seen what, exactly, Obama meant by that.

Additional reporting by Dina Fine Maron.