The remote northern corner of Alberta is home to the tar sands, a sprawling deposit of thick, heavy oil that is among the most greenhouse gas–intensive forms of petroleum to produce. In the past decade Canada has become the U.S.'s primary supplier of imported petroleum—ahead of Saudi Arabia—and more than half of it comes from this Florida-size reserve, the only place in the world where oil is mined, not drilled. Should President Barack Obama sign off on construction of the Keystone XL pipeline this year, the flow of tar sands oil, known as bitumen, into the U.S. would increase.

Sourcing more oil from Canada achieves the politically desirable goal of making the U.S. less dependent on OPEC. But bitumen exacts a heavy toll on the environment. As compared with conventional Saudi oil, it emits twice as much greenhouse gas per barrel because of the resources needed to process it. And although it is net-positive— providing between 7 and 10 Btu (British thermal units) of energy for every 1 Btu put into the tar sands—it is less so than conventional petroleum. Once it is mined, bitumen requires large amounts of gas-heated water to melt and separate it from the coarse grains of sand to which it is bound. At that point, the bitumen is still too tarry to flow, so it has to be chemically manipulated with heat and pressure to become yellowish crude oil, diesel, jet fuel or other typical hydrocarbon products. Or it can be diluted with light hydrocarbon liquids to become pitch-black “dilbit” (for “diluted bitumen”), capable of traveling via pipeline to the U.S.

Some environmental scientists see tapping the oil sands as a disastrous tipping point for global warming. In an analysis of how to restrain warming to an increase of two degrees Celsius or less above preindustrial levels, the International Energy Agency suggested that tar sands production should not exceed 3.3 million barrels a day. Yet approved tar sands production would surpass five million barrels a day—a fact that NASA climatologist James Hansen calls “game over for climate change.”

Of course, the true challenge is reducing the use of all fossil fuels, not just oil. U.S. coal-fired power plants produce 10 times more carbon dioxide than Albertan oil sands. Even so, power plant emissions have begun to decline, while the Canadian Association of Petroleum Producers notes that CO2 pollution from oil sands has risen 36 percent since 2007. As the U.S. weighs construction of the Keystone XL pipeline, the problem of tapping the oil sands is only getting stickier.