1P. A. Kharecha and J. E. Hansen
NASA Goddard Institute for Space Studies and Columbia University Earth Institute
2880 Broadway, New York, NY 10025


Peaking of global oil production may have a large effect on future atmospheric CO2 amount and climate change, depending upon choices made for subsequent energy sources.

We suggest that, if estimates of oil and gas reserves by the Energy Information Administration are realistic, it is feasible to keep atmospheric CO2 from exceeding approximately 450 ppm, provided that future exploitation of the vast reservoirs of coal and unconventional fossil fuels incorporates carbon capture and sequestration.

Existing coal-fired power plants, without sequestration, must be phased out before mid-century to achieve this limit on atmospheric CO2. We also suggest that it is important to “stretch” oil reserves via energy efficiency, thus avoiding the need to extract liquid fuels from coal or unconventional fossil fuels. We argue that a rising price on carbon emissions is probably needed to keep CO2 beneath the 450 ppm ceiling.


M. King Hubbert, the late petroleum geologist and Shell oil company consultant, articulated the notion that oil production would peak when about half of the economically recoverable resource had been exploited. His successful prediction of peak oil production in the continental United States (Hubbert, 1956) has encouraged numerous analysts to subsequently apply his model or variations thereof to global oil production. The concept of peak extraction of a finite nonrenewable resource constrained by geology and geography has received support from similar patterns of growth, peak production, and decline of mineral resources (van der Veen, 2006), natural gas (Lam, 1988), and coal (Milici and Campbell, 1997) in specific regions.

There is intense disagreement about when global peak oil might occur, but it is widely accepted that peak oil will occur at some point this century (Wood et al., 2003; Kerr, 2005). Despite the obvious relevance of “peak oil” to future climate change, it has received little attention in projections of future climate change. For instance, in the CO2 emissions scenarios outlined in the Special Report on Emissions Scenarios (SRES) of the Intergovernmental Panel on Climate Change (IPCC, 2000), socioeconomic and technological changes are employed as determinants of future energy use, without explicitly addressing the consequences of peak production of fossil fuels.

In this paper we emphasize the magnitudes of estimated fossil fuel resources, and the relevance of these limitations to the question of how practical it may be to avoid “dangerous anthropogenic interference” with global climate as outlined in the U.N. Framework Convention on Climate Change (UNFCCC, 1992). We are motivated by the conclusion of Hansen et al. (2007a,b) that “dangerous” climatic consequences are likely at an atmospheric CO2 level of 450 ppm and possibly at even lower levels. Thus we investigate whether the atmospheric CO2 amount can be kept to 450 ppm or less via constraints on the use of coal and unconventional fossil fuel resources .

In estimating atmospheric CO2 levels for given emission scenarios we employ a simple pulse-response function fit to the Bern carbon cycle model (Joos et al., 1996). Such a linear approach is expected to underestimate atmospheric CO2 levels for large emission cases. However, our main interest is in defining requirements for keeping atmospheric CO2 at ~450 ppm or less, and for such cases the pulse response function may be a reasonable approximation and it has the merit of simplicity and transparency.

We do not attempt to resolve the debate about the magnitude of oil and gas resources; rather, we consider reasonable alternative assumptions. We recognize that the magnitude of recoverable oil and gas resources depends on economic incentives and penalties. Thus we discuss the possible need for a growing price on carbon emissions to the atmosphere, if CO2 is to be kept to a low level.


We use fossil fuel CO2 emissions from the historical (1750-2003) analysis of the United States Department of Energy’s Carbon Dioxide Information Analysis Center (CDIAC; Marland et al., 2006). The record is extended through 2005 with data from British Petroleum (BP, 2006), with the BP data for each fuel adjusted slightly by a factor near unity such that the BP and CDIAC data coincide exactly in 2003.

Our estimates for the remaining fossil fuel reservoirs and their potential growth (Figure 1) are from the United States Energy Information Administration (EIA, 2006). For the coal reservoir we also show the more optimistic estimate of the Intergovernmental Panel on Climate Change (IPCC, 2001a), this larger estimate being more typical of estimates by other analysts. All of the estimates are shown in units of gigatonnes of carbon (1 Gt C = 1 Pg C = 1015 g C) for the sake of uniformity, as well as in units of CO2 concentration equivalent (1 ppmv CO2 ≈ 2.1 Gt C).

Proven reserves are the amounts of these fuels that are estimated to be economically recoverable under current economic and environmental conditions with existing technology. Reserve growth is defined by EIA (2006) as expected additions to proven reserves based on realistic expected improvements in extraction technologies.

The reservoir estimates may be larger if they were made under the assumption of very high fuel prices, or if it is assumed that greater technology advances will allow recovery of a much higher percentage of fossil fuels in existing fields. On the other hand, if a substantial carbon tax is applied to CO2 emissions in the future, the reservoir may decrease, as it becomes unprofitable to extract resources from remote locations or to squeeze hard-to-extract resources from existing fields. Because of these uncertainties, we also consider estimates of the fossil fuel reservoirs alternative to those shown in Figure 1.

Unconventional fossil fuels are those that exist in a physical state other than conventional oil, gas and coal. The contribution of unconventional fossil fuels to CO2 emissions is negligible to date (IPCC, 2001a). We do not include unconventional fossil fuels in the scenarios that we examine, because we are interested primarily in scenarios that keep atmospheric CO2 close to the limits, of the order of 450 ppm or less, that Hansen et al. (2007a,b) estimate for the transition to “dangerous” climate change. However, it should be borne in mind that unconventional fossil fuels could contribute huge amounts of atmospheric CO2, if the world should follow an unconstrained “business-as-usual” scenario of fossil fuel use.

We examine four CO2 emissions scenarios for the period 1750-2150. The first case, labeled the Business-As-Usual (BAU) scenario, assumes continuation of the ~2% annual growth of fossil fuel CO2 emissions that has occurred in recent decades (EIA, 2006; Marland et al., 2006). This 2% annual growth continues for each of the three conventional fuels until half of each reservoir has been exploited, after which emissions are assumed to decline 2% annually.

The second scenario, labeled Coal Phase-out, is meant to approximate a situation in which developed countries freeze their usage rate of coal by 2012 and within a decade developing countries similarly halt increase in coal use. Between 2025 and 2050 it is assumed that both developed and developing countries will linearly phase out emissions of CO2 from coal usage. Thus in Coal Phase-out we have global CO2 emissions from coal increasing 2% per year until 2012, 1%/year growth of emissions between 2013 and 2022, flat emissions from 2023-2025, and finally a linear decrease to zero CO2 emissions from coal in 2050. These rates refer to emissions to the atmosphere and do not constrain consumption of coal, provided the CO2 is captured and sequestered. Oil and gas emissions are assumed the same as in the BAU scenario.

The third and fourth scenarios include the same phase-out of coal, but investigate the effect of uncertainties in global oil usage and supply. The Fast Oil Use scenario adopts an alternative approach for calculating the peak in global oil emissions/usage, following the method of Wood et al. (2003). It assumes that 2% annual growth in oil use continues past the midpoint of oil supplies, until the ratio of remaining reserves to emissions decreases to 10 from the current value of ~50. This scenario causes ‘peak oil’ to be delayed 21 years to 2037. Finally, the fourth scenario, Less Oil Reserves, uses the same trends as in Coal Phase-out but omits the oil reserve growth term. This fourth scenario may be most relevant to a situation in which a high price on carbon emissions discourages exploration of oil in remote locations.


The level of atmospheric CO2 that threatens dangerous consequences for humanity and the planet is not yet known with precision. However, evidence has been presented (Hansen et al., 2007a,b) that global warming of more than 1°C above the level in 2000 is probably dangerous, as judged by likely long-term effects of such warming on sea level, species extinctions, and regional climate disruptions. In turn, given nominal estimates for climate sensitivity and the likely course of other smaller climate forcings, such a limit on global warming suggests an upper limit on atmospheric CO2 in the neighborhood of 450 ppm or less.

Such a low limit on atmospheric CO2 implies that the vast coal and unconventional fossil fuel reservoirs (Figure 1) cannot be exploited unless the resulting CO2 is captured and sequestered. This conclusion does not depend upon details of the scenarios for fossil fuel use or upon the likely errors due to our approximation of the carbon cycle. Instead it depends on the fact that a substantial fraction—approximately one-quarter—of anthropogenic CO2 emissions will remain in the air more than 500 years (Archer, 2005), which for practical purposes is an eternity.

Given these basic considerations, we have focused on scenarios in which it is assumed that coal use will be phased out by mid-century except for uses in which the CO2 is captured. We find that, with such an assumption, it is possible to keep maximum atmospheric CO2 in the vicinity of 450 ppm, provided that the EIA estimates of oil and gas reserves and reserve growth are realistic.

Surely the goals for atmospheric CO2 amount will need to be adjusted as knowledge about climate change and its impacts improves. Recent evidence of sea ice loss in the Arctic and accelerating mass loss from the West Antarctic and Greenland ice sheets suggest that the dangerous level of warming may be even less than 1°C above the 2000 global temperature. Thus, even minute details about the magnitude of fossil fuel reserves and the rate at which the reserves are exploited may prove to be important. We find that the maximum atmospheric CO2 amount varies by ~15 ppm depending upon the rate at which a fixed oil resource is consumed. This difference decreases with time, however, so the size of the oil (and gas) fossil fuel reservoirs exploited is a more important consideration.

The size of economically recoverable oil and gas resources is flexible, depending upon the degree to which fossil fuels are priced to cover their environmental costs. Thus we have argued (Hansen et al., 2007a; Hansen, 2007) that avoidance of dangerous climate change depends upon placing a significant rising price (tax) on CO2 emissions. One effect of a rising carbon price would be to slow the rate at which fossil fuel resources are exploited, thus reducing the maximum atmospheric CO2 amount, as illustrated above. More important, a carbon price would also result in some of the oil and gas being left in the ground, primarily deposits that exist at great depths or in extreme environmental locations. Given that the world inevitably must move beyond petroleum for its energy needs, it is appropriate to encourage that transition soon, and thus minimize dangerous climate change.

Hirsch et al. (2005) have noted that it requires decades to remake energy infrastructures, and thus the peaking of inexpensive supplies of oil and gas has the potential for severe economic dislocations if steps are not taken to encourage appropriate technological development and implementation. This consideration only adds to the desirability of prompt actions to slow down the use of readily available oil and gas, thus stretching out these supplies, while encouraging innovations in energy efficiency and alternative energies.

Finally, we note that as knowledge of climate change and its impacts improves it is possible that even lower limits on atmospheric CO2 and the net anthropogenic climate forcing than discussed here may prove to be highly desirable. Potential steps for achieving reduced anthropogenic climate forcing include the following:

(1) A freeze on additional coal-fired power plants (without CO2 sequestration) beginning in 2010, with a linear phase-out of all such existing plants between 2010 and 2030. This action reduces the maximum atmospheric CO2 from 440 ppm in our standard Coal Phase-out scenario to 422 ppm.

(2) Intensive efforts to reduce non-CO2 anthropogenic climate forcings, especially methane, tropospheric ozone, and black carbon. Hansen and Sato (2004) estimate that the maximum potential savings from such reductions are equivalent to 25-50 ppm of CO2.

(3) Anthropogenic draw-down of atmospheric CO2. Farming and forestry practices that enhance carbon retention and storage in the soil and biosphere should be supported. In addition, burning biofuels in power plants with carbon capture and sequestration can draw down atmospheric CO2 (Hansen, 2007), in effect putting anthropogenic CO2 back underground where it came from. CO2 sequestered beneath ocean sediments is inherently stable (House et al., 2006), and other safe geologic sites are also available.