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Federal Register / Vol. 86, No. 8 / Wednesday, January 13, 2021 / Rules and Regulations
results of a number of peer reviewed publications in the past decade: e.g., Matthews et al. found that the temperature change is roughly proportional to the total quantity of CO2
emissions over a wide range of potential scenarios.8
A threshold of GHG emissions from the source category compared to the rest of the U.S. GHG emissions i.e., the percent of total U.S. GHG emissions can be used to demonstrate significance.
Emissions can be large enough from a source category that the evaluation of GHG emissions in isolation is sufficient for making a finding of significance for the source category. Conversely, the EPA believes that some source categories are sufficiently small in GHG
emissions that a finding of insignificance can be made by only evaluating the GHG emissions from the source category. For many source categories, the evaluation of GHG
emissions alone will be sufficient for determining whether there is significant contribution.
It should be noted that under section 111b1A, the EPA is required to make a significance finding on a category-by-category basis. That provision requires the Administrator to list a category of sources for regulation if he determines that it causes or contributes significantly to dangerous air pollution. Section 111b1A emphasis. added. As a result, the text of 111b1A compels
or is at least best read to require the EPA
to make the significance determination for a particular source category on the basis of the emissions or other relevant attributes of that particular source category. In contrast, the EPA may not combine source categories that individually would not meet the significance criteria and determine that, when combined, the source categories do meet the significance criteria.9
2. Using a Threshold in Significance Determination Under this framework, the EPA is determining a threshold for the evaluation of significance of GHG
emissions from source categories. The use of a clear threshold provides certainty regarding the EPAs process and allows the regulated entities to have insight into how the EPA will make determinations on significance for their respective source category. The threshold introduced in this rulemaking is a reflection of the EPAs best understanding of the landscape of the U.S. GHG emissions from stationary sources. The EPA is introducing a methodology to evaluate significance with respect to the U.S. GHG emissions that can be applied for any source category, and that application of the methodology is only being directly applied to the EGU source category in this action as further introduction of this approach. It is important to note that a significance determination for the
U.S. GHG emissions will be needed before the EPA may regulate any other source category under CAA section 111b for GHG emissions.
As Table 1, below, makes clear, there are at least two natural breakpoints between groups of emitting source categories. The first natural breakpoint is between EGUs and all other source categories. EGUs stand out as by far the largest stationary source of the U.S.
GHG emissions, emitting over 25
percent of all the U.S. GHG emissions.
Based on available data, the next largest source category, Oil and Natural Gas, emits just under 3 percent of U.S. GHG
emissions. Two other source categories, Boilers and Petroleum Refineries, also fall between 2.5 percent and 3.0 percent of U.S. emissions. Between 1.5 percent and 2.5 percent of U.S. GHG emissions there is another natural breakpoint and all of the remaining source categories fall below 1.5 percent of the U.S. GHG
emissions. Note that source category emissions in Table 1 are an estimate of what the Agency currently understands about the emissions from CAA section 111 source categories. If the EPA were to do a rulemaking and a significance determination for a specific source category, the EPA would do a thorough analysis of the available and attributable GHG emissions data to ensure appropriate determinations and assessments.

TABLE 1EXAMINATION OF GHG EMISSIONS FROM LARGE STATIONARY SOURCES OF GHG EMISSIONS

% of total U.S. GHG
emissions
Emissions in that range MMT CO2e

Source categories affected at different thresholds
Above 25%

>1670 MMT

3% to 25%
2.5% to 3.0%
2.0% to 2.5%
1.5% to 2.0%
1.0% to 1.5%

200 MMT1670 MMT
167200 MMT
134167 MMT
100134 MMT
67100 MMT

EGUs 1778 MMT/27% of total US GHG Emissions, 3.6% of Global emissions.
No categories identified
Oil/Gas Production and Processing; Refineries; Boilers
No categories identified
No categories identified
Landfills; I Iron and Steel

Percent of U.S.
GHG emissions from stationary sources covered at given threshold %
43
43
56
56
56
60

MMT CO2e = Million metric tons of carbon dioxide equivalent Note that the oil and gas production and processing GHG emissions are very close to the 3% value and thus there is a possibility that this source category may be above the threshold in the near term.
I Note that the Landfills source category has already been regulated under CAA section 111 and the level of the emissions in Table 1. reflects reductions in GHG emissions as a result of that regulation as a co-benefit.

The EPA is introducing a threshold of 3 percent of U.S. GHG emissions to
evaluate a source categorys emissions to determine significance for purposes
of CAA section 111b. The EPA is also determining that source categories that
8 H. Damon Matthews, Nathan P. Gillett, Peter A.
Stott & Kirsten Zickfeld, The Proportionality of Global Warming to Cumulative Carbon Emissions.
Nature 459, 829832 2009, available at https
www.nature.com/articles/nature08047.

9 By the same token, as the EPA explained in the 2020 Oil & Gas Rule, there are limits to the EPAs ability to expand a source category to include other sources. As the EPA stated in that rule, the authority to revise the scope of a source category must be exercised within reasonable boundaries and cannot be employed in a way that results in an
unreasonable expansion of an existing source category. . . . The EPA is not authorized to expand the scope of a listed source category to cover a new set of sources that are not sufficiently related to the sources in the pre-existing category. . . 85 FR 57027.

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