Carnegie Mellon University
October 09, 2012

Press Release: Carnegie Mellon Researchers Find Greenhouse Gas Reductions Overestimated on Energy Transitions

Contact: Chriss Swaney / 412-268-5776 /

PITTSBURGH—Uncertainty is a way of life.

That’s why Carnegie Mellon University Green Design Institute researchers Aranya Venkatesh, W. Michael Griffin, H. Scott Matthews, and Paulina Jaramillo have found that ignoring uncertainty in any coal transition to natural gas can make a substantial difference in the analysis of the net environmental effect of generating electricity.

Life cycle assessment (LCA) is the study of impacts that occur from “cradle to grave,” or in the case of coal, from “mine to plant.” LCA can be used to inform policy in the areas of coal use and electricity generation.

In two research papers, one appearing in the October issue of Environmental Research Letters, and the other in Environmental Science and Technology, the researchers explored two of the primary forces that continue to affect the short-term environmental effects of making electricity: the increase in natural gas-fired generation as a result of low natural gas prices and the retirement of coal plants.

With respect to increased use of natural gas, many studies have looked simply at the difference in emissions between coal and natural gas power plants, suggesting roughly a 50 percent reduction in greenhouse gas emissions.

CMU researchers found that thinking that we will reduce greenhouse gas emissions by 50 percent in the electricity sector from a coal to gas switch is likely to be an overestimate. The research team found overall that we will only get a 7-15 percent reduction due to changes in grid operation in response to price changes in natural gas.

“As natural gas prices go down, it becomes cheaper to operate natural gas plants, and some of these plants start being operated more often. This results in some coal plants being operated less often. However, given certain technical constraints related to the operation of existing power plants, the displacement of coal-based generation is limited,” said Jaramillo, an assistant research professor in CMU’s Department of Engineering and Public Policy (EPP). She adds that reducing emissions of GHG by 50 percent using natural gas would require a significant retirement of coal plants and construction of new natural gas plants.

The researchers’ second paper explored the uncertainty in emissions that could be expected due to decisions to retire coal-fired power plants. Such decisions are expected and in some cases have been promised by energy companies due to economics as well as emerging new environmental policies. The study suggests that reductions in greenhouse gas emissions associated with limited retirement of coal plant will be minimal, but emissions of sulfur and nitrogen oxides would be substantial, up to 25 percent, in some areas. It is important to note that this paper focuses on up to 7 gigawatts of coal capacity being retired without building new power plants to replace them.

“We found that if expected coal plants retire, that alone will not bring us dramatic reductions in climate change inducing greenhouse gas emissions,” said Matthews, a professor in CMU’s Civil and Environmental Engineering and EPP departments.

“In addition, the benefits achieved from reducing emissions of sulfur and nitrogen oxides, while substantial in aggregate measures, will not be evenly distributed; and while some counties will see reductions in the emissions of these criteria air pollutants, some counties will see increases,” Jaramillo said.

“Without extending typical greenhouse gas emission LCA analysis of coal and natural gas to include use in the real world economically driven systems, you can come away with the wrong impression of the benefits commonly ascribed to the coal-to-natural gas transition” said Griffin, co-director of the Green Design Institute.