Whether peak oil is good news for the climate ultimately depends on what replaces oil as our staple fuel source. It will be unsurprising to most that replacing dwindling oil reserves with coal would do little to solve the climate problem, but how much coal remains is also highly uncertain, according to Prof. David Rutledge of Caltech, who spoke to the press at the AGU this morning.

Andy Dessler of Texas A&M touched on this in a guest commentary posted here last month. In short, Dessler called for a global IPCC-like assessment of our fossil fuel reserves, pointing to a new analysis by Rutledge that shows the world’s available coal reserves are far lower than traditional estimates would suggest. If Rutledge’s estimates are correct, combustion of all remaining conventional oil, gas, and coal reserves would produce an atmospheric concentration of carbon dioxide of approximately 470 ppmv in 2100, near the stabilization target that many climatologists argue we must achieve to avoid the worst impacts of climate change.

While Rutledge’s estimates suggest that the worst-case scenarios of the IPCC may be unachievable, Ken Caldeira of Washington’s Carnegie Institute had a more sobering message. Caldeira and colleagues used a climate and carbon cycle model to look at how running out of oil could affect future climate scenarios. Their analysis showed that if we replaced oil with liquefied coal fuel promptly, we would reach 2°C above pre-industrial temperatures in 2042 instead of 2045. Replacing oil with renewables, however, would delay reaching 2°C above pre-industrial temperatures by 11 years. This is simply because per coal emits more carbon dioxide per unit of energy than oil; add to that the energy costs associated with conversion of coal to liquid fuels, a likely option if we run out of oil.

Even if Rutledge is correct in his estimates of available reserves, we’d still easily have enough to overshoot the 2°C target it seems, and we’ll need Herculean efforts to avoid it, a point that has been repeated numerous times over the past few days. Kharecha, who has been working on this issue with James Hansen, called for a zeroing of carbon emissions from coal by 2030 and stabilization of atmospheric concentrations at 350 ppmv.

But Rutledge’s estimates have important implications for the amount of change we can expect in the longer term. I asked him about the feasibility of reassessing global fossil fuel reserves and he highlighted the political improbability of it, but he suggests the IPCC include an additional scenario in their next assessment to account for lthe real possibility of lower available coal reserves.

Olive Heffernan