As eager as TransCanada is to build the new pipeline, there is sufficient pipeline capacity for now to carry current production of crude from the Alberta oil sands to American refineries. With relatively minor adjustments, there will be enough space on existing transborder pipelines to handle expected flow until 2018 or later, analysts said.

It is only after 2020, when production of Canadian crude is expected to double from today’s 1.5 million barrels a day, that the pipeline crunch becomes severe. Canadian companies are already planning to expand current pipelines and build new ones to carry oil to the coast of British Columbia for export to Asia.

Notably, however, one such proposed project, Enbridge’s Northern Gateway pipeline from Alberta to Kitimat, British Columbia, has been stopped for at least a year by the Canadian government because of strong opposition on environmental grounds from local landowners and indigenous populations.

Nonetheless, Stephen Harper, the Canadian prime minister, said in a television interview this week that if the United States blocked the Keystone pipeline, Canada would look to China as a market for its oil.

“I am very serious about selling our oil off this continent, selling our energy products off to China,” Mr. Harper said.

The oil sands formation in western Canada, sometimes referred to as tar sands because of the density of the extracted oil, contains an estimated 1.75 trillion barrels of recoverable oil, the second-largest known deposit of oil in the world after Saudi Arabia’s. Extracting, transporting and refining it, however, is energy intensive, producing 15 percent to 80 percent more carbon emissions over its life cycle than average petroleum products. Thus James Hansen, an eminent climatologist at the National Aeronautics and Space Administration, has warned that if development of the oil sands deposits goes forward unchecked it means “game over,” in his words, for the global climate.

Removing and burning all that oil, Dr. Hansen has warned, would spew so much carbon dioxide into the atmosphere that it would be impossible to stabilize the climate and avoid disastrous global impacts.