Here’s all you ever really need to know about CO2 emissions and climate:

• The peak warming is linearly proportional to the cumulative carbon emitted
• It doesn’t matter much how rapidly the carbon is emitted
• The warming you get when you stop emitting carbon is what you are stuck with for the next thousand years
• The climate recovers only slightly over the next ten thousand years
• At the mid-range of IPCC climate sensitivity, a trillion tonnes cumulative carbon gives you about 2C global mean warming above the pre-industrial temperature.

[CG:  More than 2 deg C warming and we’re into “Oh my god we’re all gonna die.” territory.  Keeping warming under 2 deg C seems seems prudent to me.  We will not have the opportunity for a do-over with this experiment.  Back to Pierrehumbert…] 

This graph gives you an idea of what the Anthropocene climate looks like as a function of how much carbon we emit before giving up the fossil fuel habit, without even taking into account the possibility of carbon cycle feedbacks leading to a release of stored terrestrial carbon…

Assuming a 50-50 chance that climate sensitivity is at or below this value, we thus have a 50-50 chance of holding warming below 2C if cumulative emissions are held to a trillion tonnes. Including deforestation, we have already emitted about half that, so our whole future allowance is another 500 gigatonnes…

Proved reserves of conventional oil add up to 139 gigatonnes carbon… To the carbon in conventional petroleum reserves you can add about 100 gigatonnes C from proved natural gas reserves… If one assumes that these two reserves are so valuable and easily accessible that it’s inevitable they will get burned, that leaves only 261 gigatonnes from all other fossil fuel sources. How does that limit stack up against what’s in the Athabasca oil sands deposit? the associated carbon adds up to about 230 gigatonnes — essentially enough to close the “game over” gap. But oil-in-place is not the same as economically recoverable oil. That’s a moving target, as oil prices, production prices and technology evolve. At present, it is generally figured that only 10% of the oil-in-place is economically recoverable…

Are the oil sands really the “biggest carbon bomb on the planet”? As a point of reference, let’s compare its net carbon content with the Gillette Coalfield in the Powder river basin, one of the largest coal deposits in the world. There are 150 billion metric tons left in this deposit, according to the USGS. How much of that is economically recoverable depends on price and technology… the carbon in the Powder River coal amounts to 67.5 gigatonnes, far below the carbon content of the Athabasca Oil Sands. So yes, the Keystone XL pipeline does tap into a very big carbon bomb indeed.

But comparison of the Athabaska Oil Sands to an individual coal deposit isn’t really fair, since there are only two major oil sands deposits (the other being in Venezuela) while coal deposits are widespread. Nehring (2009) estimates that world economically recoverable coal amounts to 846 gigatonnes, based on 2005 prices and technology. Using a mean carbon ratio of .75 (again from Table 6 here), that’s 634 gigatonnes of carbon, which all by itself is more than enough to bring us well past “game-over.” The accessible carbon pool in coal is sure to rise as prices increase and extraction technology advances, but the real imponderable is how much coal remains to be discovered. But any way you slice it, coal is still the 800-gigatonne gorilla at the carbon party.

… building Keystone XL lets the camel’s nose in the tent… the pipeline itself is really just a skirmish in the battle to protect climate, and if the pipeline gets built … that does not mean in and of itself that it’s “game over” for holding warming to 2C. Further, if we do hit a trillion tonnes, it may be “game-over” for holding warming to 2C (apart from praying for low climate sensitivity), but it’s not “game-over” for avoiding the second trillion tonnes, which would bring the likely warming up to 4C.  The fight over Keystone XL may be only a skirmish, but for those who seek to limit global warming, it is an important one. It may be too late to halt existing oil sands projects, but the exploitation of this carbon pool has just barely begun. If the Keystone XL pipeline is built, it surely smooths the way for further expansions of the market for oil sands crude. Turning down XL, in contrast, draws a line in the oil sands, and affirms the principle that this carbon shall not pass into the atmosphere.