Carbon is irrelevant. If you tune in to some part of the “debate”, and the participants are going on about Eevill CO2 and Saving the Planet, turn it off and go elsewhere. It simply means all of those people are liars and/or too ignorant to pound sand.
If the pipeline is not built, another will be, from the tar sands deposits to the Canadian West Coast, completely outside the jurisdiction of the U.S. Government (much to the frustration of the IRS and the other TLAs). It will be cheaper than Keystone-XL; not only will it be shorter, it will be cheaper per linear measure because it will be built under Canadian regulations, which focus on (gasp! choke!) pipeline safety, rather than maximizing saliva generation when kissing environmentalist ass. The oil will be pumped to Vancouver, where it will be loaded on tankers and taken (mostly) to China, where it will be burned to provide power to industry.
The oil will be produced, whether Keystone-XL is built or not.
The oil will be burned, whether Keystone-XL is built or not.
The carbon will enter the atmosphere, whether Keystone-XL is built or not.
So in this particular case, both “anthropogenic global warming” and environmental degradation at the production zone are irrelevant. The carbon dioxide is gonna get into the atmosphere, and the tar sands deposits are gonna get mined, whether Keystone-XL is built or blocked. There are some (minor) issues regarding environmental impact of the pipeline, and those might be worth debate, but Saving the Earth is not an issue in this context, and anybody who starts bloviating, passionately or otherwise, about Global Warming and CO2 emissions is too stupidly self-centered to realize what’s going on and/or deliberately trying to confuse the issue. Shut ‘em out.