Canada's oil sands are one of the most carbon-intensive sources of crude in the world, and for American climate activists, the Keystone XL pipeline represents a "line in the sand" on climate policy.

But greenhouse gas produced by the oil sands is a fraction of the amount spewed by U.S. coal-fired power plants. In 2010, the oil sands produced 48 million tons of carbon-dioxide emissions. Coal-fired power plants in the state of Wisconsin alone produced 43 million tons.

No wonder Canadian politicians challenge environmentalists who rally in Washington to stop the Keystone pipeline but are not chaining themselves to the White House fence to demand the shutdown of power stations.

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Quoting the chief economist from the Paris-based International Energy Agency, Natural Resources Minister Joe Oliver says oil-sands emissions amount to "not peanuts, but less than peanuts" in the grand scheme of global climate change. And he notes that emissions from U.S. coal-fired power plants are 40 times greater than those from the oil sands.

Political calculations may have persuaded the environmental community that the Keystone XL battle can be won in the short term, compared to the long, hard slog that it would take to win regulations forcing coal-fired utilities to cut emissions or shut down.

Coal served as fuel for 42 per cent of U.S. electricity generation in 2011. In some key battleground states such as Ohio, the figure is more like 80 per cent. Coal is mined in key Democratic states such as Pennsylvania, West Virginia and Ohio. Many Midwestern states rely heavily on it for their power, and there could be a nasty political fight to impose costly regulations on those utilities.

However, many plants are ripe for retirement, and low-cost natural gas is rapidly gaining ground as the fuel of choice. That switch is an opportunity for the United States to reduce its greenhouse-gas emissions even without regulations.

The oil sands are both a carbon-intensive source of crude and a rapidly growing one, with emissions expected to soar if the industry can achieve its growth forecasts of increasing production to 4 million or 5 million barrels in 2030 from 1.6 million.

For the environmentalists, the political calculus also boils down to a simple equation: one man, one decision. President Barack Obama will have to make a straight call – yes or no. That's why the Sierra Club decided for the first time in its 120-year history to engage in civil disobedience in opposition to Keystone XL.

"With coal plants, you've got to be fighting all over the place," said David Pumphrey, an energy and security analyst at Washington-based Center for Strategic and International Studies. "But you can rally around the Keystone project and turn it into a slogan and make it into an icon in the climate fight. And it becomes less about facts and more about ideology."