President Obama is facing a critical opportunity to take the country beyond its century-and-a-half reliance on oil. Earth Day today and this past Saturday's third anniversary of the BP Gulf of Mexico disaster, offer stark reminders of the stakes and mistakes inherent in crude.

To paraphrase Charles Dickens, the president made the best of decisions last summer when he dramatically improved mileage-and-emissions standards for U.S. cars and light trucks. Now, he could make the worst of decisions - approving the Keystone XL oil pipeline.

The pipeline would carry some of the world's most-polluting and expensive oil from Canada to Texas. It would threaten America's rivers, water supplies and the atmosphere. And we won't even get into the benefit of these new supplies. Much of the fuel the pipeline delivers would be exported.

There is no question that we are burning too much oil - too much for our economic security and our environmental security.

The president recognizes the risk that oil poses. He took the biggest single step of any nation to cut oil use and global warming pollution when he set the United States last summer on course to nearly double the gas mileage of our new car fleet, and halve the autos' greenhouse gas emissions, by 2025.

The tar sands oil that the Keystone pipeline would transport is among the dirtiest there is. Burning it would release at least 19 percent more carbon dioxide, the main global warming pollutant, than conventional crude oil.

And it threatens other pollution as well: When a pipeline operated by Exxon Mobil Corp. sprung a leak near Mayflower, Ark., on March 29, it wasn't just tar sand oil that came out. Benzene, toluene and hydrogen sulfide - toxic chemicals that help the sludge-like goo move through the pipeline - leaked, too. The dizziness and nausea that afflicted local residents were similar to the complaints voiced by people living along the Kalamazoo River in Michigan, where Canada's Enbridge Inc. pipeline spilled tar sands oil in 2010.

The consequences of our thirst for oil abound.

Three years ago the BP explosion killed 11 oil rig workers, polluted the Gulf of Mexico with 210 million gallons of crude oil, decimated sea life, and fouled four states' beaches. Have we not learned its lessons?

Yet Keystone XL's proponents present it as too good to pass up - Canadian oil that would substantially increase U.S. supplies.

That's not true. With much of it destined for overseas buyers, it would barely add a drop to U.S. tanks. And wherever it ends up, it is likely to raise, not lower, the price of gasoline. That is because extracting, transporting and refining it requires more energy - and dollars - than more conventionally produced gasoline.

The pipeline's supporters argue that it will create jobs. But the vast majority will be temporary, disappearing once construction is done. Only 127 permanent workers will remain to run the pipeline, according to Lara Skinner, a Cornell University researcher.

Rather than importing the Canadian tar sands, only to refine and then ship its oil overseas, we should use our best technology to cut our oil use in half. We can start by expanding our fleet of hybrid and electric vehicles, improving home heating and industrial efficiency, and accelerating the development of cleaner alternatives to oil.

That same American know-how, sent abroad, can help other nations cut their use of fossil fuels, too. We can export the world's best technology rather than import its worst oil.

Dan Becker is director of the Safe Climate Campaign. James Gerstenzang, the campaign's editorial director, formerly covered the environment and the White House for the Los Angeles Times.