The short answer from most opponents (liberal and otherwise) is that the tar sands will increase greenhouse gas emissions, endanger water supplies, increase the potential for toxic disaster and undermine the efforts being made to transition the energy mix to more sustainable sources.

Personally, my initial inclination is to support the pipeline because being a liberal, I am not chained to the sway of the congregation (that's more a conservative thing) Also, as an environmentalist AND a realist, my opinion is that we should pick our fights. It seems to me that coal burning is a far greater detriment to climate patterns than oil is. Also, it's clear that there isn't anything we can do about the Canadian commitment to extracting bitumen from northern Alberta. If we ditch the pipeline, TransCanada will continue with the extraction anyway and will simply transport it using potentially even more dangerous methods than the proposed pipeline.

But now I'm looking deeper into this and finding evidence that there is more to the issue than meets the casual eye. Of course this doesn't surprise me. For instance I'm finding out that tar sand oil is not the same thing as regular crude, the hazards are far greater. To be honest the potential for 42,000 jobs doesn't mean that much to me. I'm not in the oil business and I'm not a politician looking for votes. So, my main concern regarding this issue is the environment and our efforts to switch to more sustainable sources of energy. The only reason at this point why I might still support the pipeline is if it represents a safer option for handling Canadian bitumen than the alternatives.