What Will It Take To Stabilize Carbon Dioxide Concentrations? 

Currently, world energy-related carbon dioxide emissions are increasing at a rate of about 2.1 percent per year. Carbon dioxide concentrations, on the other hand, are rising by only about 0.6 percent per year (see figure below). There are two major reasons for the difference: 

• First, the base from which growth in the atmospheric carbon dioxide concentration is calculated is much larger than the base from which increases in annual emissions are calculated. Before the industrial revolution, the weight of carbon dioxide in the atmosphere was about 2,163 billion metric tons,^a and in the early stages of industrialization the concentration increased slowly—at a rate of about 0.04 percent per year. 
• Second, the Earth’s oceans and soils absorb carbon dioxide. Over time, about 42 percent (at current emission rates, between 11 and 12 billion metric tons) of the net carbon dioxide emitted through the burning of fossil fuels and deforestation has been absorbed by the planet and has not accumulated in the atmosphere. The other 58 percent has been added to the atmospheric balance. One of the uncertainties in projecting future concentrations is whether the same absorption ratio will hold for future emissions. 

Figure Data

In pre-industrial times, the concentration of carbon dioxide in the atmosphere was about 280 parts per million (ppm). The atmospheric concentration of carbon dioxide at present is about 380 ppm, and according to the IEO2008 reference case projections, by 2030 it would be about 450 ppm.^b If the growth of world carbon dioxide emissions continues unabated, the concentration of carbon dioxide in the Earth’s atmosphere could reach 560 ppm by the middle of the 21st century. 

Many possible actions beyond those currently projected in the business-as-usual baseline would be needed to stabilize the atmospheric concentration of carbon dioxide at a level below 560 ppm (still double the pre-industrial level). There is no unique path for achieving any stabilization goal. In addition, a number of “wild cards” could alter the relationship between emissions rates and atmospheric concentrations—such as the Earth’s capacity to absorb carbon, which some scientists believe could be diminished by global warming. Each of the options outlined below could be expected to mitigate 1 billion metric tons or more annually by 2030, relative to the IEO2008 reference case projection. It is beyond the scope of this analysis to project either the upper bound or the economic cost of each option. 

• Reductions in energy demand growth. Reducing the growth of energy demand in residential and commercial buildings would require adoption of more energy-efficient lighting systems (such as compact fluorescent bulbs and, eventually, light-emitting diodes) and of more efficient heating, cooling, and refrigeration systems, as well as energy-efficient building shell retrofits and new construction. In the transportation sector, it would require more fuel-efficient vehicles and more use of public transit and telecommuting. In the industrial sector, more combined heat and power and more efficient processes would be needed to lower energy demand per unit of industrial output. 
• Increases in nuclear electricity generation. According to the World Nuclear Association, the achievement of 740 gigawatts of installed nuclear electricity capacity by 2030—36 percent more than projected in the IEO2008 reference case—is possible. If additional nuclear power displaced only coal, such an increase would achieve a reduction of about 1 billion metric tons annually by 2030. 
•  Increased use of nonhydropower renewables for electricity generation in the OECD economies. For nonhydropower renewables to provide 20 percent of the electricity consumed in the OECD economies in 2030, the use of renewables would have to increase by an average of 7.4 percent annually from 2010 to 2030, as compared with the 2.5-percent average increase in the IEO2008 reference case. The increase would yield 1 billion metric tons of abatement annually by 2030. 
• Increased use of hydropower and nonhydropower renewables for electricity generation in the non-OECD economies. Assuming that there are more opportunities for hydropower expansion in the non-OECD economies than in the OECD economies, if the combined use of hydropower and nonhydropower renewables in non-OECD countries grew by 3.5 percent per year from 2020 to 2030, as compared with 1.3 percent in the IEO2008 reference case, 1 billion metric tons of carbon dioxide emissions would be avoided annually by 2030. 
• Increased use of renewable fuels for transportation. If new technologies were employed to minimize carbon dioxide emissions from input fuels and indirect emissions of other greenhouse gases, so that an additional 20 quadrillion Btu of biofuels was consumed in the transportation sector, assuming a life-cycle savings of 80 percent in carbon dioxide emissions compared to conventional petroleum, 1 billion metric tons of carbon dioxide emissions could be avoided by 2030. 
• Carbon capture and storage. It is unlikely that significant amounts of carbon capture and storage will be implemented before 2020. When the technology does become available commercially, its application to about 250 gigawatts of coal-fired generation capacity with a 90-percent removal rate would result in the mitigation of 1 billion metric tons of carbon dioxide emissions annually. The IEO2008 reference case does not include carbon capture and storage. Although there are some small projects in pilot phases around the world, the assumption is that without binding constraints on carbon dioxide emissions throughout the projection period there would be no economic incentive to engage in carbon capture and storage. 
• Anthropogenic sequestration. The latest assessment by the Intergovernmental Panel on Climate Change estimates that about 3.7 billion tons carbon dioxide equivalent per year is sequestered by anthropo- genic activity, including projects such as reforestation and other land-use programs. A 27-percent increase in such activity by 2030 would represent an emissions reduction of 1 billion metric tons. 

For many of the options listed above, the magnitude of the required changes relative to the reference case projections points to the difficulty of achieving stabilization at an atmospheric concentration that is at or below twice preindustrial levels. The effectiveness of reductions in electricity demand as a way to decrease carbon dioxide emissions depends on the fuel mix, the efficiency of generation, and the resultant carbon intensity of electricity supply (carbon dioxide emitted per kilowatthour of generation). For example, because coal-fired generation is more carbon-intensive than natural-gas-fired generation, achieving a given level of reduction in carbon dioxide emissions would require a smaller cut in coal use than the cut in natural gas use that would be required for the same reduction in emissions. Similarly, as the overall carbon intensity of electric power production declines, larger reductions in electricity demand will be needed to achieve a given level of emission abatement (see figure). 

Over time, increases in the efficiencies of generation technologies, such as new natural gas combined-cycle generation, will mean that demand reductions avoid smaller amounts of carbon dioxide emissions. With the average efficiency of electricity generation improving over time, the 2030 reference case intensity of 0.48 metric tons carbon dioxide per megawatthour of electricity supplied is lower than the 2005 historical carbon intensity of 0.56 metric tons per megawatthour supplied. As a result, if more non-carbon-emitting electricity supply is added, such as nuclear and renewables, the demand reduction requirement for the same amount of carbon dioxide emissions savings increases over time. 

There are wide ranges of estimates both for the marginal cost levels required to achieve various reduction levels and for the corresponding impacts on GDP. Policies to achieve emission abatements can have a large effect on the cost estimates, as can the rate of development of low- or non-carbon technologies. Specific questions that would have to be answered in order to estimate costs include: 

• Are all greenhouse gases included in the analysis? Are emissions credits freely traded? 
• Is nuclear power allowed to grow at a rapid pace? 
• Are biomass and other renewable technologies allowed to penetrate rapidly?  
• What discount rates are used for future costs and benefits? 
• Do new technologies, such as carbon capture and storage, enter the technology base early enough to be employed in the abatement strategy? 

If, by 2030, world GDP were 1 percent lower as a result of mitigation efforts, it would mean an annual cost of about $1.5 trillion (in constant 2000 dollars). The costs must of course be weighed against future benefits in the form of avoiding human-caused climate disruptions.

^aScientists typically measure carbon dioxide concentrations by the weight of the carbon only, because some carbon exchanges (fluxes) do not involve carbon dioxide. For this analysis, however, the weight of carbon dioxide is used for consistency with the rest of the chapter. 

^bThe concentration levels calculated here are based only on energy-related carbon dioxide. Taking into account other sources of carbon dioxide and concentrations of other heat-trapping gases, total greenhouse gas concentrations will be somewhat higher.