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Federal Register / Vol. 86, No. 8 / Wednesday, January 13, 2021 / Rules and Regulations are less than this value i.e., 3 percent or less are necessarily insignificant without consideration of any other factors. The reasoning for choosing this threshold is presented later in this document.
The EPA acknowledges that, when interpreting other CAA provisions, the EPA has used different thresholds to define significant contribution, but it is appropriate to select a threshold based on the nature of the problem being addressed. For example, to address the problem of interstate transport under CAA section 111a2DiIwhich concerns criteria pollutants, i.e., pollutants that affect the NAAQSthe EPA selected a threshold of 1 percent based on analysis of air quality modeling specific to the criteria pollutant at issue. 76 FR 48208, 48236 August 8, 2011 Cross-State Air Pollution Rule CSAPR. For criteria pollutants, both the location and quantity of emissions are factors in determining their impact. In contrast, the impact of GHGs e.g., climate change is based on a cumulative global loading, and the location of emissions is not nearly as important a factor as it is for assessing local impacts associated with criteria pollutants. Because GHGs do not have the local near-term impacts that criteria pollutants tend to have, a larger value is appropriate to use in determining significance as it still addresses the health and welfare impacts of GHG emissions without specifically evaluating local near-term impacts, which is analytically unreasonable to do given the global nature of GHGs. While the 3 percent threshold will be applied against domestic emissions, source categories exceeding that threshold represent a much smaller fraction of global GHG
emissions.10
By determining a threshold, the EPA
is setting a clear indication of how source categories will be evaluated for significance based on GHG emissions.
For those source categories that are below the 3 percent threshold, the EPA
would make a determination through future rulemaking of insignificance.
This means that if a source category collectively emits 3 percent or less of the total U.S. GHG emissions, it will be 10 The EPA recognizes that in the 2016 Oil & Gas Rule, it determined that GHG emissions from the oil and natural gas source category contribute significantly to dangerous air pollution, in part, on the grounds that those emissions exceeded the total amount of emissions from various foreign countries.
81 FR 35824, 35840 June 3, 2016. The EPA
believes that its current approach of identifying a threshold for significance based on a percentage of U.S. emissions is better reasoned than the 2016 Oil & Gas Rules approach of drawing comparisons to the absolute emissions of other countries.

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considered to be insignificant. For those source categories that are above the threshold, a more detailed evaluation of other criteria can be used to make a determination of significance. This is described in section IV.D below. It is important for the EPA to make this clear indication as it allows source categories and the general public a level of transparency as to how the EPA will be evaluating source categories for significance. The threshold in this action will provide a degree of certainty regarding whether a source category will later be found significant or insignificant based on the threshold.11
After evaluating the two natural break points in GHG emissions, the EPA
determined that 3 percent of the U.S.
GHG emissions was the best threshold for determining significance. As noted above, there is currently only one source category above this threshold, EGUs, and the evaluation of significance for the EGU source category has been a topic explored and discussed by the Agency in great detail over the course of the last decade.12 Just below the 3
percent threshold are three source categories: Oil and Natural Gas, Petroleum Refineries, and IndustrialCommercial-Institutional Steam Generating Units i.e., Boilers. There are no other source categories with GHG
emissions between 1.5 percent and the 3 percent. By using a threshold of 3
percent of the U.S. GHG emissions i.e., only including EGUs above the threshold, the EPA will effectively be covering 43 percent of the U.S.
stationary source GHG emissions via regulation of a single source category. If the EPA were to instead set a threshold between the other identified breakpointbetween 1.5 percent and 2.5 percent of U.S. GHG emissionsthe EPA observes that this threshold would lead to a relatively modest increase in the stationary source U.S. GHG
emissions that would be regulated of an additional 13 percent for a total of 56
percent of U.S. stationary source GHG
emissions.13 In addition, regulation of the additional source categories that comprise 13 percent of U.S. emissions 11 The EPA does not currently have a comprehensive inventory of the U.S. GHG
emissions for all of the NSPS source categories. For the EPA to make determinations of significance for a source category, a more comprehensive emissions profile of a source category should be used. The EPA will make determinations of significance for other source categories in the future.
12 See 79 FR 34960 and 80 FR 64510.
13 Note that one of those next three largest source categories is oil and natural gas. In the recently finalized policy package, the EPA found that regulation of GHGs from this source category is unnecessary as it is currently being controlled by regulation of volatile organic compounds. See 85 FR
57018, 57030 September 14, 2020.

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would eliminate only a portion of those emissions. With an even lower threshold of significance set at 1.0
percent of U.S. GHG emissions, there would be significantly more source categories covered about 10 based on the EPA estimates above the threshold but likely would include an even more modest increase in stationary source GHGs that would cover 60 percent of U.S. stationary source GHGs. Under this framework, the EPA is basing a decision to apply a threshold of 3 percent on the relative contribution of regulating source categories that contribute significantly to the overall impact of climate change. To that end, the temperature impact associated with the hypothetical elimination of all source categories above a 3 percent threshold corresponds to a hypothetical global mean temperature reduction of 0.049
degrees Celsius C approximately 0.1
degree Fahrenheit, the calculated effect in 2100 of removing 1,780 million metric tons MMT of CO2 emissions each year from 2020 through 2100 from source categories above that threshold i.e., just EGUs. Eliminating the next largest source category i.e., Oil and Gas Processing and Production would only generate an additional hypothetical global mean temperature reduction of less than 0.01C and even smaller source categories correspondingly contribute less to global temperature.
The EPA is making the decision that the threshold for a significance determination for U.S. GHG emissions to be in the form of a percentage. A
percentage is a metric that measures the relative contribution to the whole and, in this action, the EPA believes that it is appropriate to measure and evaluate significant contribution of U.S. GHG
emissions as a relative contribution to the whole of GHG emissions in the U.S.
The EPA is determining that a threshold in the form of a percentage is both reasonable and more appropriate for making the significance determination in this rule based on a percents relative nature. This is important because the trajectory of U.S. GHG emissions is trending down. As overall emissions decrease over the course of time, a source categorys relative contribution to GHGs may not have changed or may have even increased based on GHG
reductions in other source categories and sectors. A relative percentage threshold recognizes that the EPA may later determine a source category is significant based on these circumstances, because a source categorys emissions may eventually exceed the threshold even though it is currently below the threshold.

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