It is elementary: a large part of greenhouse gas emissions are attributable to power generation, especially to fossil fuel combustion. According to the Environmental Protection Agency (EPA), electricity production accounted for 30 percent of the U. S.’s total GHG emissions in 2014, marking the largest share . About two-thirds of the produced electricity comes from fossil fuel combustion, mostly coal and natural gas . It can be implied that employing less carbon-intensive technologies such as renewables and nuclear—although the latter can be politically charged—would lead to less GHG emissions.
But exactly how much of the emissions are being generated from each energy technology? How would the portfolio change as a nation’s energy mix evolves? Quantifying the emissions from a smokestack involves a concept of the emission factor, which is defined as “the average emission rate of a given GHG for a given source, relative to units of activity ”. There is a guidance that provides detailed emission factors for various fuel types, including different types of coal, natural gas, etc., so that businesses can prepare an inventory and report emissions.
However, identifying the amount of emissions per kWh accurately is still challenging . Moreover, the amount of emissions must be considered for all stages of a technology and its fuel lifecycle . This idea of “lifecycle emissions” can be applied to all types of energy technology. Take nuclear power lifecycle for instance. It starts with uranium mining and milling, conversion and enrichment. Following are plant construction and operation, reprocessing of fuel, and finally, decommissioning . Emissions are estimated for each stage and summed to derive a total estimate. This approach can serve as an important indicator for decision makers, even though the estimates for lifecycle emissions largely vary depending on methodologies.
Just like the energy mix graphs that are so commonly seen, a GHG emissions chart can be yielded by applying the lifecycle estimates to the U. S.’s power generation data of 2015, provided by the EIA. While the electricity production by energy sources on the left side are quite familiar, the righthand graphic tells more directly about GHG emissions. Fossil fuels, especially coal, are more dominant, while renewable energy sources are blended into one marginal slice. Some renewables like hydro and wind are quite visible today, but their impacts in terms of GHG emissions are negligible as shown in the righthand chart.
What if, then, all of the kWh generated by fossil fuel combustion is shifted to “cleaner” sources, such as nuclear and renewables? This is a somewhat extreme scenario, but if we assume that the electricity previously generated by coal, natural gas and petroleum is now produced by nuclear and various renewables—equal addition assumed for convenience—, then the pie chart would look like the following:
Now, the renewables are present again, and the nuclear option looks like a huge polluter. But guess what—the total emission is now less than 7 percent of today’s figure.
 U. S. EPA. Sources of Greenhouse Gas Emissions, Retrieved from https://www.epa.gov/ghgemissions/sources-greenhouse-gas-emissions
 U. S. EIA, March 29, 2016. Electricity Explained: Electricity in the United States, Basics. Retrieved from http://www.eia.gov/energyexplained/index.cfm?page=electricity_in_the_united_states
 UNFCCC. Retrieved from http://unfccc.int/ghg_data/online_help/definitions/items/3817.php
 Weisser, D., 2007. A guide to life-cycle greenhouse gas emissions from electric supply technologies. Energy. Vol. 32
 Sovacool, B. K., 2008. Valuing the greenhouse gas emissions from nuclear power: A critical survey. Energy Policy. Vol. 36