If Americans needed a reminder of the serious environmental risks posed by the proposed Keystone XL pipeline, they got it last week with the oil spill in Mayflower, a suburb of Little Rock, Arkansas. More than 12,000 barrels of crude spilled into the town like a black river, coating lawns and forcing evacuations.

It's still not certain what caused a leak from the ExxonMobil pipeline that carries Canadian crude oil from Illinois to the Texas Gulf Coast. But what is clear is that such pipeline spills are not the rarity that the oil industry likes to claim. The Arkansas incident was a veritable drop in the bucket compared to the 2010 pipeline rupture that dumped 843,000 gallons of oil into Michigan's Kalamazoo River — or what the much-larger Keystone pipeline could cause.

Transporting oil form Alberta's tar sands is thought to be far more dangerous than the typical pipeline operation since the crude produced there is heavier and more corrosive — more like peanut butter than 10W-40. That also makes it much more difficult to clean up. In Michigan, for instance, they are still dredging the river from nearly two years ago because the oil sinks to the bottom.

Even with improved pipeline safety standards signed into law last winter by President Barack Obama, the risk is undeniable. So it raises an obvious question: Is Keystone worth putting communities along its route in danger?

The answer is no — not simply because a pipeline rupture would be devastating, particularly in upper plains states that rely on the Ogallala Aquifer for their water supply — but because the benefits accrued to the United States would be so modest. Indeed, some experts see the pipeline as a net loser for the U.S. economy because it's likely to drive gasoline prices higher.

That point deserves some explanation. The lack of a Keystone XL pipeline today has helped create a glut of oil in North America that's benefited much of the middle of the country where gasoline prices are hovering about 30 cents per gallon cheaper than in Maryland.

If the 1,660-mile pipeline is built, the oil will be more readily available for export, and prices will inevitably climb to match the global benchmark. That's no small difference. Canadian producers stand to pocket between $2 billion and $4 billion more — money that could come straight out of U.S. motorists' pockets.

Pipeline proponents argue that Canada is a much more reliable supplier than some exporting countries upon which the U.S. now depends. But the problem with that argument is that the U.S. already buys the vast majority of what Canada produces. Indeed, Keystone gives the Canadians more options to sell it elsewhere.

What about the jobs that would be created? While it's certainly true that thousands of temporary construction jobs might be generated while the pipeline is being built, the overall impact on the U.S. economy is minuscule and short-lived. U.S. dependence on foreign oil is already in decline because domestic production is up and so is vehicle fuel efficiency.

But what ought to give President Obama pause in considering whether to authorize the international project is the potential impact that enabling greater exploitation of tar sands crude could mean to address climate change. Canada may be sitting on oil resources second only to Saudi Arabia, and that means releasing massive amounts of carbon dioxide and other greenhouse gases into the atmosphere.

And it's worse than merely pumping oil out of the ground. The process of extracting it from tar sands is harmful, too. That's why calling Keystone a step toward "energy security" seems so laughable. It's hardly secure if it brings the U.S. closer to rising seas and coastal flooding, droughts, severe weather and other effects of global warming.

Images of Arkansas waterfowl covered in thick crude may not pull at the heartstrings of pro-Keystone members of Congress, but the practical implications of enabling Canada to sell its oil to other countries and worsening climate change ought to get everyone's attention. These outcomes would be hugely expensive and harmful to national security. Canada might find an alternative to the Keystone even if it means sending it out by truck or train, but there's little reason why the U.S. needs to directly enable this folly.

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