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Science Briefs

How Will the End of Cheap Oil Affect Future Global Climate?

Fossil fuel burning by humans has been the primary cause of the observed global warming during the industrial era. The carbon dioxide (CO2) emitted to the atmosphere from fossil fuel use accounts for about 80% of the rise in CO2 from its preindustrial level of 280 parts per million (ppm) to its current level of about 385 ppm, with the remainder due mainly to anthropogenic deforestation. Furthermore, in projections of 21st century global warming, CO2 emissions from fossil fuels remain the dominant cause of the problem.

Bar chart of fossil fuel reservoir estimates

Figure 1, at right. Fossil fuel-related estimates used. Historical emissions are shown in purple, remaining reserves in blue, and potential near-term additions to reserves in yellow. Unconventional fuels (all-yellow bar) have uncertain magnitude, but are commonly assumed to be vast. Carbon units are shown on the left, and their CO2 equivalents on the right. (IPCC = Intergovernmental Panel on Climate Change; WEC = World Energy Council.) Click for large GIF or PDF

Thus, it is important to develop plausible future paths of global CO2 emissions from the continued use of fossil fuels. Using a wide range of published supply estimates of the three fossil fuels — coal, oil, and natural gas (Figure 1) — we have developed several CO2 emissions scenarios that incorporate the notion of fuel production "peaks" (Kharecha and Hansen 2008). This notion was popularized by American petroleum geologist M. King Hubbert, who suggested that peak oil production occurs when about half of all usable oil reserves (past plus future supplies) are produced, after which time production continuously declines.

After "peak oil" occurs on a global scale, the age of cheap, readily available oil will effectively be over. Historical data reveal that principles of peak production — or consumption, for our purposes — can be applied to the other two fossil fuels as well.

In our study (Kharecha and Hansen 2008), we were motivated by the idea that "dangerous" anthropogenic global warming will occur if atmospheric CO2 exceeds about 450 ppm (see, e.g., Hansen et al. 2007). We devised five CO2 emissions scenarios for the years 1850-2100, each reflecting different assumptions for the fossil fuels (Figure 2). Then, using a simplified carbon cycle model, we converted these emissions trajectories into atmospheric CO2 concentration trajectories (Figure 3). It is important to note that all of our scenarios are purely illustrative in nature — they are not predictions.

Line plots of historic and projected CO2 emissions for five scenarios

Figure 2, above. Historical and projected anthropogenic CO2 emissions for the five main scenarios in our paper (BAU = business-as-usual, i.e. unconstrained emissions; LU = land use). Scenarios (b)-(e) represent cases in which, among other things, CO2 emissions from coal-fired power plants are phased out by mid-century globally. See Methods section of Kharecha and Hansen for further details. Click for large GIF or PDF

Line plots of historic and projected CO2 concentrations for five scenarios

Figure 3, above. 20th and 21st-century atmospheric CO2 concentrations for the five main case scenarios, showing the individual contributions of each fuel to total CO2. The difference between the "BAU" scenario and the mitigation scenarios highlights the crucial and urgent need to greatly reduce emissions from coal. The mitigation scenarios also show that the rate of fossil fuel use is not as important as the ultimate amount used. Click for large GIF or PDF

In our first scenario (Business-As-Usual, or "BAU"), CO2 emissions from each fuel are unconstrained, resulting in Hubbert-type trajectories. The remaining four cases are mitigation scenarios, which differ mainly in their assumptions about oil supplies and consumption patterns. The baseline assumption in these four scenarios is a complete phase-out of global coal emissions between the years 2025 to 2050. This phase-out can be accomplished either by simply ending the use of coal or by preventing the CO2 produced at coal-fired facilities from reaching the atmosphere.

We find, as expected, that unconstrained BAU emissions yield an atmospheric CO2 level that is more than double the preindustrial level, which would surely lead to "dangerous" climate change. Even in a "low-end" BAU scenario in which the lowest estimates from Figure 1 are assumed, we find that for much of this century, atmospheric CO2 exceeds the 450 ppm threshold adopted for this study (see Fig. 7 of Kharecha and Hansen).

However, CO2 levels in each of our four mitigation scenarios are kept significantly below 450 ppm for most of this century. By contrast, the scenarios used in most climate change projections lead to CO2 levels of over 500-900 ppm by 2100 — well into the "dangerous" zone for climate.

None of our scenarios include emissions from "unconventional" fossil fuels such as tar sands, methane hydrates, and oil shale. Although the amounts of such fuels are highly uncertain, they are widely believed to be very large. It is clear that they should only be used if their emissions are captured and sequestered.

Even if we assume high-end estimates and unconstrained emissions from conventional oil and gas, we find that these fuels alone are not abundant enough to take CO2 above 450 ppm. However, given world society's major dependence on these fuels, we suggest that they be used as efficiently and sparingly as possible. This would help minimize the socioeconomic impacts of "peak oil" as well as the temptation to extract oil from pristine areas such as the Amazon and the Alaskan wilderness — and it would also help reduce oil and gas CO2 emissions by reducing the need to resort to coal-to-liquids or unconventional fuels as substitute liquid fuels.

Because coal is much more plentiful than oil and gas, reducing coal emissions is absolutely essential to avoid "dangerous" climate change. The most important mitigation strategy we recommend, phase out of CO2 emissions from coal within the next few decades, is feasible using current or near-term technologies.

We conclude that the peaking of global oil production/consumption could have a major effect on future CO2 levels and therefore global climate, depending on choices countries make for substitute energy sources. We suggest that a fair yet effective price on carbon emissions is crucial for guiding energy policies in a climatically sound direction.

In addition to the above suggestions, our study also outlines several other general strategies the world can adopt as the likelihood that even 450 ppm is a dangerously high CO2 level becomes more evident. These include an earlier phase-out of coal emissions and/or use, large-scale reforestation, and use of carbon-negative biofuels grown on degraded or abandoned farmland.

Related Links:

News Release: NASA Study Illustrates How Global Peak Oil Could Impact Climate

News Release: Research Finds That Earth's Climate is Approaching 'Dangerous' Point


Hansen, J., Mki. Sato, R. Ruedy, P. Kharecha, A. Lacis, R.L. Miller, L. Nazarenko, K. Lo, G.A. Schmidt, G. Russell, I. Aleinov, S. Bauer, E. Baum, B. Cairns, V. Canuto, M. Chandler, Y. Cheng, A. Cohen, A. Del Genio, G. Faluvegi, E. Fleming, A. Friend, T. Hall, C. Jackman, J. Jonas, M. Kelley, N.Y. Kiang, D. Koch, G. Labow, J. Lerner, S. Menon, T. Novakov, V. Oinas, Ja. Perlwitz, Ju. Perlwitz, D. Rind, A. Romanou, R. Schmunk, D. Shindell, P. Stone, S. Sun, D. Streets, N. Tausnev, D. Thresher, N. Unger, M. Yao, and S. Zhang, 2007: Dangerous human-made interference with climate: A GISS modelE study. Atmos. Chem. Phys., 7, 2287-2312.

Kharecha, P.A., and J.E. Hansen, 2008: Implications of "peak oil" for atmospheric CO2 and climate. Global Biogeochem. Cycles, 22, GB3012, doi:10.1029/2007GB003142.

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