Guest Commentary by Andrew Dessler

There is an emerging view among some experts that recoverable fossil-fuel reserves are far smaller than previously thought. If so, the Intergovernmental Panel on Climate Change’s (IPCC) highest emissions scenarios could be unrealistically high, thus limiting the worst-case climate change during the 21st century. This view of a constrained fossil-fuel supply points to a potential convergence of thinking about policies and actions needed to address the seemingly divergent problems of energy supply and climate change.

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One of the standard techniques for estimating future oil production is the method pioneered by M. King Hubbert. Briefly, Hubbert’s theory involves fitting historical production data to a logistics curve; extrapolation of the curve allows estimation of future production. Hubbert successfully predicted domestic U.S. oil production would peak in the early 1970s, and his theory predicts that world oil production should be peaking about now, which production figures seems to be confirming. Application of Hubbert’s theory to world-oil production suggests that total future recoverable conventional oil is ~1.2 trillion barrels or 7 Zeta Joules (ZJ). Figure 1 shows oil consumption from emissions scenarios produced by the IPCC, along with this estimate based on Hubbert’s method. All IPCC scenarios assume integrated oil production greater than the Hubbert estimate of recoverable oil.

For coal, the canonical wisdom is that there are hundreds of years of availability. However, several recent analyses have cast doubt on this. A recent NRC report on U.S. coal availability concluded, “… there is probably sufficient coal to meet the nation’s needs for more than 100 years at current rates of consumption. However, it is not possible to confirm the often-quoted assertion that there is a sufficient supply of coal for the next 250 years.” Prof. David Rutledge of Caltech has applied a Hubbert-like analysis to coal production data and concluded that the availability of coal is overstimated. The Energy Watch Group, a German think tank, has performed a detailed country-by-country analysis of coal reserves. Integrating their estimate of future coal production, we get 11 ZJ for world coal production from 2008 to 2100 — about one-ninth of the coal reserves cited by the IPCC (Table 4.2 of the fourth assessment report). As is the case for oil, the IPCC’s scenarios’ projected coal consumption generally exceeds this estimate of available resources.

If these analyses of oil and coal production are correct, then the IPCC’s high emissions scenarios are not achievable. Using these new and lower estimates, Rutledge estimates that combustion of all remaining conventional oil, gas, and coal reserves would produce an atmospheric concentration of CO2 of 470 ppmv in 2100, near the stabilization target that many climatologists argue we must achieve in order to head off the worst-case climate impacts. Kharecha and Hansen recently concluded that atmospheric CO2 could be kept from exceeding about 450 ppm if we consume all of our reserves of conventional oil and gas but restrict coal usage to amounts similar to the recent availability estimates of the Energy Watch Group and Rutledge.

There are, of course, enormous reserves of unconventional sources of fossil fuels, such as tar sands or oil shale — reserves of oil shale alone could exceed 1 trillion barrels. However, the issue with unconventional sources is not the size of the reserves, but the rate at which they can be produced. Shale, for example, needs to be heated to release the oil, and one estimate suggests that it takes 12 GW of power to produce one million barrels of oil per day from shale. Additionally, oil production from shale requires significant quantities of water, which is problematic because oil shale resides mostly in the water-stressed western U.S. These and other resource requirements limit production rates from shale to, at most, a few million barrels per day — not an insignificant amount, but also not enough to meaningfully affect emerging constraints in our supply of fossil fuels.

Our goal as a society should be to adopt policies that head off, to the maximum extent possible, both climate-change and energy-supply risks. First and foremost, we need better estimates of our reserves of oil and coal. The NRC report on coal, for example, was essentially a plea for better information on coal availability. The situation is not much better for oil. Saudi Arabia, for example, claims reserves of a quarter of a trillion barrels. There is some evidence that these estimates are inflated, and Saudi Arabia will not provide relevant data to back up their high reserve estimate.

Given the high stakes involved, this lack of information is simply unacceptable. The world therefore needs an IPCC-like assessment of our fossil fuel reserves. While some countries may balk at providing the relevant data needed to verify their reserve estimates, the world must compile the necessary information to assess this risk. Second, we should recognize that solutions to the problems of energy supply and climate change are, to a great degree, aligned. Solutions such as energy efficiency, the cheapest and most cost effective “new” source of energy, and renewable energy sources, such as solar, wind, and nuclear, can help solve both energy supply and climate problems.

A few solutions, however, address only one problem and not the other, and these policy pathways should be avoided. For example, coal combined with carbon sequestration, where CO2 from coal combustion is sequestered rather than released to the atmosphere, is a poor choice if coal is less abundant than expected. Geoengineering, active manipulation to bring about a cooler global climate, is also a poor choice because it does nothing to address energy availability.

Many people are rightfully concerned about emissions of carbon dioxide doubling, tripling, or even quadrupling this century. There is however another risk we should worry about: namely, that recoverable fossil fuel reserves are far smaller than previously thought. If these new estimates of fossil-fuel availability turn out to be correct, then reducing our carbon emissions could be easy — because we will be out of fossil fuels. Constraints on energy supply could, like climate change, challenge our very way of life and would not be good news. We must therefore take the opportunity we have now to work toward a common goal of energy sufficiency and climate-change mitigation through development of suitable alternative energy sources that address both problems.

Acknowledgments: I thank Drs. Dave Rutledge, Werner Zittel, Alex Dessler, and Hans Juvkam-Wold for their help on this project.

Figure caption: Cumulative oil production between 2008 and 2100 from the IPCC emissions scenarios, in units of ZJ. The bars show the spread over all scenarios in each scenario family; medians emissions are the tick marks and the marker scenario is the black dot. The dotted line is the Hubbert estimate of oil production. IPCC values are from Fig. 4-8 of ref. 11, adjusted by subtracting 2.8 ZJ from the scenarios to account for production from 1990-2008.

Andrew Dessler is a professor in the Dept. of Atmospheric Sciences at Texas A&M University and a contributor to Grist.