My friend Jared Bernstein rightly points out that blocking the Keystone pipeline will not keep the tar sands oil in the ground. There are other ways to bring the oil to market and the industry will undoubtedly pursue these channels if opponents of the pipeline are successful.
But there is an important point here. These other methods of getting the oil to consumers are more expensive. We know this because the industry would not be pushing the pipeline if it was not the lowest cost way to get the oil to the market.
In this way opposition to the pipeline is effectively raising the cost of tar sands oil. That is exactly what we should want to see. In a sane world we would have a carbon tax, which would discourage the use of oil in general and in particular oil that was associated with large amounts of carbon emissions.
For political reasons, a carbon tax seems a non-starter at the moment. With the failure of Washington to act responsibly, the Keystone protesters are effectively imposing their own carbon tax on tar sands oil by raising its price. It's far from perfect, but it's certainly a reasonable course of action under the circumstances.