RealClimate looks at Hansen and McKibben’s statements that the Keystone XL is essentially “game over” for the climate. All that really matters in the big picture is the absolute amount of carbon we release. How fast or slow we do it is of little consequence, because the effects last on the order of 10,000 years. If we’re aiming for 2°C of warming, or 450ppm CO2, and we assume that all of the world’s conventional oil and natural gas reserves are going to get burnt because they’re just too convenient, then we’re left with another 260 gigatonnes (GT) of carbon that can be released cumulatively from other sources. The Athabasca oil sands in total contain 230 GT (close enough to call it game ending) but not all of that will be producible economically. Even if we decide to go ahead, only a fraction of that will end up in the atmosphere. The Gillette coalfield in Wyoming’s Powder River Basin on the other hand contains about 70 GT of carbon in total, maybe half of that eventually being exploitable. Globally there are only 2 large tar sands deposits (the other being in Venezuela), but there’s a pretty large amount of coal… something like 800 GT of carbon equivalent appears to be economically accessible, and that’s far more than enough to fry us. So the Keystone XL pipeline and the tar sands in general are certainly significant battles, unlocking vast amounts of carbon, but in isolation, they’re not enough to end us. But then of course, they don’t exist in isolation. Going ahead on these non-traditional fossil fuel projects means we at some level intend to just Burn It All.