Figure 1. Nitrogen Oxides Emissions Reductions due to Changes in Scale, Generation Share, and Emissions Rate

Figure 2. Sulfur Dioxide Emissions Reductions due to Changes in Scale, Generation Share, and Emissions Rate

In contrast to nitrogen oxides and sulfur dioxide, Figure 3 shows that changes in scale and generation share account for roughly equal parts of the overall reduction in carbon dioxide emissions over the past decade. The drop in natural gas prices, the recession, and energy efficiency and renewables penetration in the market have all contributed to the reduction in carbon dioxide emissions. Changes in the emissions rate factor for carbon dioxide are practically nonexistent because a unit’s carbon dioxide emissions rate, which depends on its heat rate (efficiency), varies much less across units than the emissions rates of sulfur dioxide or nitrogen oxides. The effects of the 2008–2009 recession are also more visible for carbon dioxide than for the other two pollutants.

Figure 3. Carbon Dioxide Emissions Reductions due to Changes in Scale, Generation Share, and Emissions Rate

Overall, changes in emissions rates explain most of the overall decline in sulfur dioxide and nitrogen oxides emissions. Because regulations are the primary driver of changes in emissions rates, this decomposition suggests that regulation, rather than changes in market forces such as the price of natural gas and cost of renewables, explains most of the emissions reductions over this period. However, this doesn’t mean that the regulations have been responsible for a comparable portion of the costs of emissions reductions. The costs and benefits of regulation depend on the interactions among overlapping state and federal regulations—and on the influences of factors such as innovations in renewables technologies and increases in energy efficiency. We’ll return to these points in future blog posts.   This article appeared on the Resources for the Future website at http://www.rff.org/blog/2017/sources-decreasing-us-electricity-sector-emissions]]>