Substantial reductions in greenhouse gas emissions from the electricity sector are achievable over the next two to three decades. They can best be realized through a portfolio approach involving the widespread deployment of multiple technologies: energy efficiency; renewable energy; coal, natural gas, and biomass with carbon capture and storage; and nuclear. However, to enable this portfolio approach, the viability of the following two technologies must be demonstrated during the next decade to make them ready for widespread commercial deployment starting around 2020: (1) the technical and commercial viability of CCS for sequestering CO₂ from electricity production and (2) the commercial viability of evolutionary nuclear plants in the United States. Achieving substantial greenhouse gas reductions in the transportation sector over the next two to three decades will also require a portfolio approach involving the widespread deployment of energy efficiency technologies, alternative liquid fuels with low life-cycle CO₂ emissions, and light-duty-vehicle electrification technologies.

As noted in Chapter 1, the United States emits some 6 billion tonnes (6 gigatonnes) of CO₂ into the atmosphere each year (see Figure 1.3); about 5.6 gigatonnes are attributable to the energy system. The potential for reducing greenhouse gas emissions from this system before 2020 is limited, but the potential for reducing emissions after 2020 is significant, especially in the electricity sector, if certain technologies can be successfully deployed at commercial scales.

Electricity is produced in stationary facilities, which in principle makes it easier to effectively monitor and control their greenhouse gas emissions. The options for reducing the electricity sector’s emissions are apparent from an inspection of Figure 2.15, which provides estimates of life-cycle CO₂-equivalent¹⁷ (denoted CO₂-eq) emissions per kilowatt-hour of electricity produced. Coal and natural gas plants are by far the largest emitters of greenhouse gases from electricity generation. In fact, their CO₂-eq emissions are far higher than those of any of the other technologies represented. As shown in Figure 1.8, coal and natural gas plants collectively supplied about 70 percent of electricity demand in 2007.

Achieving substantial reductions in CO₂ emissions from the U.S. electricity