FORT MCMURRAY, Alberta—Where does the U.S. get the bulk of the imported petroleum to support its oil addiction? Contrary to popular belief, it's not Saudi Arabia—it's Canada, which supplies the U.S. with more than two million barrels of oil per day, or more than a quarter of all its imported oil. And more than half of that output comes from bitumen melted out of the buried sands of northeastern Alberta—a sprawling deposit of extra-heavy oil underlying more than 142,000 square kilometers, or an area roughly the size of Florida.

This Canadian province—only slightly smaller than Texas—exported more than $52 billion worth of crude oil in 2011 because it's "one of the few places in the world where you can actually mine oil," notes Cameron Brown, director of advocacy at the Alberta Ministry of International and Intergovernmental Relations.

View a slide show of the oil sands process.

If oil is the "devil's excrement" as a Venezuelan oil minister once complained, then bitumen is the worst of it. Bitumen is essentially tar, hence the appellation tar sands, and it requires roughly 12 barrels of water to separate one barrel of it from the sand, although only three of those barrels are consumed, thanks to recycling. That water also has to be hot to separate the clingy hydrocarbon—at least 50 degrees Celsius, which requires burning natural gas to heat it.

Even then, the mined bitumen is too tarry to flow, so it is chemically manipulated further with heat and pressure in a process known as "upgrading" to become yellowish crude oil, diesel, jet fuel or other typical hydrocarbon products. Or it is diluted with light hydrocarbon liquids to become pitch-black "dilbit" (for "diluted bitumen"), capable of traveling via pipeline to the U.S. This kind of extra-heavy crude oil from sandy deposits is not commercially developed anywhere else in the world, although other countries such as Venezuela, Russia and even the U.S. have similar resources.

And that's just for the tar sands close enough to the surface—no more than 80 meters deep—to be mined. For deeper deposits, volumes of superhot pressurized steam are pumped underground to melt out the bitumen so it can be sucked up to the surface by production wells running in parallel. Such so-called in situ production requires less water but far more energy to get the bitumen flowing, resulting in greenhouse gas emissions some 2.5 times higher than those from mining. Thus far, the majority of bitumen production has been via mining but, this year, in situ surpassed it as the primary production method—a trend likely to continue for the foreseeable future.

As it stands, oil sands projects either under construction or already approved will raise production above five million barrels per day, including ExxonMobil's Kearl Lake project, expected to start mining oil in 2013. "We have environmental impacts now, and these impacts are about to get a lot bigger," notes oil sands policy analyst Marc Huot of the Pembina Institute, an environmental group working for responsible development.

The biggest environmental impact may be all the energy required to extract the oil from the tar sands—the burning of the resulting fuels in vehicles notwithstanding. Some scientists, such as NASA climatologist James Hansen, are convinced that using such resources—he calls them "the dirty needle" of an oil addict—will mean "game-over for climate change." Oil sands are among the most greenhouse gas–intensive forms of petroleum to produce. "Things that don't make sense from a planetary perspective are being actively encouraged," Hansen notes. "As long as fossil fuels are the cheapest fuel we will burn them and not solve the problem."

The fundamental problem, however, is the growth of fossil-fuel addiction itself. Of the trillion or so barrels of oil produced since the dawn of the Oil Age in the 19th century, a full quarter have been burned in just the first decade of the 21st century. Yet U.S. coal-fired power plants produce more than 30 times more CO2 than Albertan oil sands facilities—45 million metric tons of greenhouse gases versus nearly two billion metric tons. "If you think that using other petroleum sources is much better, then you're delusional," says chemical engineer Murray Gray, scientific director of the Center for Oil Sands Innovation at the University of Alberta. "Increasing coal use worldwide gives me a lot more pause than the pattern of petroleum consumption."

Even the oil sands ultimate consumption in a gasoline, diesel or jet engine only results in 500 kilograms of CO2-equivalent per barrel of refined petroleum products, meaning total oil sands emissions from well to wheel are considerably lower than those of this nation's more than 500 power plants burning coal to generate electricity.

That said, whereas CO2 emissions from coal-fired power plants in the U.S. have declined, greenhouse gas emissions from oil sands have doubled since the turn of the century and look set to double again by the end of this decade—the primary source of emissions growth for the entire country of Canada. As a result, the province of Alberta's emissions grew faster than they did for states such as Texas or Florida. In fact, the only thing that may slow such emissions growth is the development of lighter, cheaper oil in places such as North Dakota's Bakken Shale. But even with such reduction, the problem of tapping tar sands for petroleum just keeps getting stickier.

View a slideshow of the oil sands process.