TORONTO — When President Barack Obama vowed this week to reverse U.S. dependence on "foreign oil," did he also mean Canada's? It's a question Americans might want to consider.

They could start by noting an important fact: Canada is the single-largest exporter of oil to the United States. Indeed, the U.S. imports more oil from Canada than from Saudi Arabia. Nearly one-fifth of all U.S. oil imports come from north of the 49th parallel. At current trends, a third of U.S. oil imports will soon come from Canada.

Canadians are accustomed to America's willful ignorance of its northern neighbor. They don't care that only 21 percent of Americans know the capital of Canada (hint: it's not Toronto). But when polls indicate that just 4 percent of U.S. residents know we're the biggest supplier of their fossil fuel addiction, even politicians lose their diplomatic cool.

The day of Obama's inauguration, Liberal Party leader Michael Ignatieff, who leads the main opposition to Canada's Conservative Party government, had this to say: "The Americans have to wake up and realize we ship more petroleum to the United States than Saudi Arabia. So when we talk, when Canadians talk, Americans have to listen."

Canadians are particularly sensitive about this issue because a large part of the oil going south involves what many consider to be environmental destruction on a staggering scale.

It comes from the Athabasca tar sands, an oil deposit about the size of Florida in the northern wilderness of Alberta, a province in western Canada. With 175 billion barrels of proven reserves, it's the biggest deposit outside of Saudi Arabia.

Unlike the easily accessible liquid gold in the Gulf region, this deposit is made up of a tar-like mess of bitumen and sand. Extracting the bitumen requires a massive mining process described in "Tar Sands: Dirty oil and the future of a continent," a new, disturbing, and well-researched book by Alberta-based journalist Andrew Nikiforuk.

"To coax just one barrel of bitumen from the Athabasca sand pudding, companies must mow down hundreds of trees, roll up acres of soil, drain wetlands, dig up four tons of earth to secure two tons of bituminous sand, and then give those tons a hot wash," Nikiforuk writes. "Every other day open-pit mines move enough earth and sand to fill Yankee Stadium."

Multiply that amount by 1.3 million barrels of oil (the current daily production) or 3 million barrels a day (the level expected by 2015), and the enormity of an enterprise that has so far attracted $164 billion in investment from major oil companies begins to take shape.

The elaborate heating and washing processes needed to extract one natural resource — oil from tar sands — devours extraordinary amounts of two others. Natural gas is used in quantities that would heat 4 million North American homes every day and enough water from the Athabasca River is used in one year to supply a city of 2 million people.

The result? Producing a barrel of oil from the tar sands spews out three times as many greenhouse gases as does producing a barrel of conventional oil. It largely explains why Canada signed the 1992 Kyoto Protocol and then developed, paradoxically, one of the worst carbon emission records in the world.

Nikiforuk also notes that "the world's biggest energy project has spawned one of the world's most fantastic concentrations of toxic waste, producing enough sludge every day (400 million gallons) to fill 720 Olympic pools."

The toxic sludge — which includes arsenic, cyanide, and benzene — is stored in man-made dams that cover 23 square miles of forest and muskeg, Canadian for stinky bog. They leak or seep into groundwater and are suspected of causing an increase in rare cancers in a community downstream. Last spring, 500 migrating geese mistook the snow-covered tailings ponds for lakes and landed. Their gruesome deaths outraged Canadians and made international headlines.

Nikiforuk cites numerous examples of compliant government agencies turning a blind eye to ecological concerns or rubber-stamping oil company plans for new tar sands projects. The projects fill government coffers with billions of dollars in royalties. But the result is a "petrodollar" that has sharply risen in value, thereby reducing exports in manufactured goods and throwing tens of thousands of workers out of work.

The truly outrageous punch line is this: Virtually all the oil from the tar sands goes to the U.S., while the eastern half of Canada remains largely dependent on foreign oil.

In the last few months, the global recession and the drop in oil prices has put a halt to tar sands expansion. But activity there has slumped before, only to return with a vengeance.

More worrying signs for the oil barons are Obama's criticism of "dirty" oil and the fact that his advisers have pointed fingers at Canada. Also, U.S. Congressman Henry Waxman of California, the author of a 2007 bill that seemed to ban imports of tar sands oil — ignored by the Bush administration — is now chairman of the House Committee on Energy and Commerce.

It's sure to be a main topic of discussion when Obama makes Canada his first foreign visit soon. Canada's prime minister, Conservative Party leader Stephen Harper, will urge Obama to keep buying our dirty oil. Many Canadians, frustrated that Americans never listen, will in this case hope they stay true to form.