PerspectiveClimate Change

Farewell to Fossil Fuels?

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Science  10 Sep 2010:
Vol. 329, Issue 5997, pp. 1292-1294
DOI: 10.1126/science.1195449

One concrete goal adopted by some policy-makers is to reduce the risks associated with climate change by preventing the mean global temperature from rising by more than 2°C above preindustrial levels (1). Climate models indicate that achieving this goal will require limiting atmospheric carbon dioxide (CO2) concentrations to less than 450 parts per million (ppm), a level that implies substantial reductions in emissions from burning fossil fuels (2, 3). So far, however, efforts to curb emissions through regulation and international agreement haven't worked (4); emissions are rising faster than ever, and programs to scale up “carbon neutral” energy sources are moving slowly at best (5). On page 1330 of this issue, Davis et al. (6) offer new insights into just how difficult it will be to say farewell to fossil fuels.

The authors ask this question: What would future CO2 levels and global mean temperatures be if humans built no additional CO2-emitting devices (e.g., power plants, motor vehicles) and allowed existing CO2-emitting devices to live out their normal lifetimes over the next 50 years? Their answer is that this strategy would limit mean warming to 1.3°C (1.1° to 1.4°C) above that of the preindustrial era and limit atmospheric concentrations of CO2 to less than 430 ppm. They concede, however, that such a radical “age-out” scenario is unlikely, and that a major mobilization is needed to figure out how to power the world carbon-neutrally to stay below the 2°C threshold.

The bitter pill implicit in Davis et al. and other, earlier studies of emissions-reducing scenarios is that we are in no position to make this energy transition now, and that it will likely take decades of hard work (7). For example, Pacala and Socolow (8) analyzed a scenario that envisioned stabilizing atmospheric concentrations of CO2 at 500 ppm within 50 years. They found that reaching that goal required the deployment of seven existing or nearly existing groups of technologies, such as more fuel-efficient vehicles, to remove seven “wedges” of predicted future emissions (the wedge image coming from the shape created by graphing each increment of avoided future emissions). Those seven wedges, each of which represents 25 gigatons of avoided carbon emissions by 2054, are cited by some as sufficient to “solve” climate change for 50 years (9).

Unfortunately, the original wedges approach greatly underestimates needed reductions. In part, that is because Pacala and Socolow built their scenario on a business as usual (BAU) emissions baseline based on assumptions that do not appear to be coming true. For instance, the scenario assumes that a shift in the mix of fossil fuels will reduce the amount of carbon released per unit of energy. This carbon-to-energy ratio did decline during prior shifts from coal to oil, and then from oil to natural gas. Now, however, the ratio is increasing as natural gas and oil approach peak production, coal production rises, and new coal-fired power plants are built in China, India, and the United States (10).

Reducing emissions.

(A) Estimates based on one emissions scenario (red line) (8) found that technologies capable of removing 7 “wedges” of future predicted emissions could stabilize future CO2 emissions from fossil fuels. Other scenarios (blue and gold lines) (6, 11) suggest that removing up to 25 wedges will be needed. (B) Cost of producing electricity from silicon and thin-film photovoltaics (PV) are declining, but extensive development is needed to scale up cost-effective solar power to produce needed terawatts of primary power.

The enormous challenge of making the transition to carbon-neutral power sources becomes even clearer when emissions-reduction scenarios are based on arguably more realistic baselines, such as the Intergovernmental Panel on Climate Change's “frozen technology” scenario (11, 12). Capturing all alternate energy technologies, including those assumed within this BAU scenario, means that a total of ∼18 of Pacala and Socolow's wedges would be needed to curb emissions (13) (see the figure). And to keep future warming below 2°C, even under the Davis et al. age-out scenario, an additional 7 wedges of emissions reductions would be needed—for a total of 25 wedges (see the figure).

Maintaining world economic growth and keeping atmospheric CO2 concentrations below 450 ppm, even with continuing improvements in energy intensity (the amount of CO2 emitted per unit of energy, and a proxy for increasing energy efficiency and less consumptive lifestyles), will require ∼30 terawatts (TW) of power from carbon-neutral sources at mid-century (2). Some forecasts envisioned market forces spurring the creation of 10 carbon-neutral terawatts (2), but that now appears optimistic. The difficulties posed by generating even 1 TW of carbon-neutral power led the late Nobel Laureate Richard Smalley and colleagues to call it the “terawatt challenge” (1416); we have yet to mobilize enough talent and resources to meet it. It has proven difficut, for example, to tap the huge, if diffuse, solar flux on Earth using photovoltaic (PV) cells to generate electricity in a cost-effective manner, particularly for routine “baseload” generation. Costs of PV technologies are dropping (17) (see the figure), but greater economies of scale, perhaps accompanied by a switch to thin films—a less efficient technology, but less expensive in terms of cost per kilowatt—will help. Achieving massive market penetration of solar and wind electricity will require utility-scale systems that can store intermittent supplies of power until they are needed. Denmark, for instance, uses intermittent power to pump water into Norway's hydropower impoundments; the water is later released through turbines. That approach, however, isn't widely feasible in the United States, and other approaches that use compressed air, flywheels, and “flow batteries” to store power are expensive and need substantial research and testing.

Broad investment will be crucial to enabling such basic research findings to cross the “valley of death” and develop into applied commercial technologies. Carbon taxes (1) and ramped-up government research budgets (2) could help spur investments, but developing carbon-neutral technologies also requires, at the very least, reversing perverse incentives, such as existing global subsidies to fossil fuels that are estimated to be 12 times higher than those to renewable energy (18). We have to stop marching the wrong way before we can turn around.

Davis et al. show that breaking the world's fossil-fuel addiction will be hard. To create carbon-neutral power sources, we may well need programs with the scale and urgency of the Manhattan atom bomb project. One goal should be to develop technologies that can first meet the terawatt challenge, and eventually provide 30 carbon-neutral terawatts of power by mid-century; perhaps 10 TW each of primary power from “clean coal” and from nuclear and renewable technologies. Without these alternatives to fossil fuels, the age-out scenario painted by Davis et al. evokes Havana after the 1959 Cuban Revolution: no new cars, only constantly repaired but still running old Chevys in the streets, as a beautiful old city crumbles around them.

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