My problem with oil pipelines in general is - based on my experiences in this field - they leak. Companies spend lots of money and time, and yet human error, equipment failure, and the sheer distances involved mean that there are always going to be leaks and they have a good probability of being big problems.
Therefore, I don't object to pipelines, but I do feel all levels of government need to quit being so hands-off in the regulatory world and actually take a serious interest in regulating pipeline standards. Right now the regulatory world around pipelines (of which my job is a part) is very piecemeal.
The objections as to source are, frankly, idiotic. For one, the oil sands are demonized by one side and lauded by the other. From my experience in the area, the truth lies in the middle. Basically, if people want to eliminate production from the oil sands, they'd better have an alternative to fossil fuels.
As for GHG emissions, the City of Toronto emits far more GHGs than the entirety of the oilsands development in Alberta, which (contrary to popular belief) incorporates a much larger area than just the region around Ft. McMurray and the Athabasca river. There is oil sands development stretching from Peace River to Cold Lake.
Basically, I wouldn't have an objection to Keystone XL so long as the operating company has hefty conditions on it to prevent spills beyond the current legislation in force. I would certainly prefer a pipeline to rail transport. Train derailments are typically much worse than pipeline breaks - while they are fewer, they tend to be much more catastrophic. In general, so long as the states and countries involved enforce their existing legislation diligently and perhaps add additional requirements, the pipeline is a no-brainer from an economics, environmental, and security perspective.
EDIT: I love CNN. "The pipeline from Alberta, Canada to Steele City, Nebraska." FIVE Nebraska's can fit inside the province of Alberta. Think you might want to be a little more precise on where that pipe is actually coming from?
