Note: This post is part of an ongoing series about North American pipelines. For an introduction and links to the wide-ranging coverage--from safety to legal issues to the business and economics to vulnerabilities--see this regularly-updated intro post.

On Monday, the House passed a bill that would force the Obama administration to make a final decision on TransCanada's controversial Keystone XL pipeline by November 1. The Keystone XL project (which regular DeSmogBlog readers should be familiar with) would funnel tar sands oil from Alberta's massive reserves down to Gulf Coast refineries in Texas.

This isn't the place to discuss in too much depth the various and plentiful problems with Alberta tar sands itself -- from extraction to transportation to refining to combustion, it's the dirtiest oil on the planet. From a climate perspective, the Alberta tar sands contain enough carbon to lock the planet into climate chaos. In the words of NASA climatologist Jim Hansen, "if the tar sands are thrown into the mix it is essentially game over."

Because Keystone XL is so controversial, and because its construction could be such a tipping point in the climate fight, a broad and diverse coalition of scientists and activists are digging in their heels for a big fight, and planning a multi-week action at the White House. (Here's more on how to get involved.)

But since this is a post about pipelines, I'm going to focus on how tar sands pipelines are different than those that carry conventional crude, how they're much more prone to leaks and spills, and how those spills are particularly bad for the environment.