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Federal Register / Vol. 86, No. 8 / Wednesday, January 13, 2021 / Rules and Regulations
Accordingly, a percentage threshold allows the EPA, over time, to always focus on the source categories with the potential to have the greatest impact.
The framework on which EPA bases its decision today is, therefore, amenable to future use, which augurs in favor of the frameworks use to make todays finding.
The EPA is introducing in this action that a threshold in the form of a percentage is both reasonable and more appropriate for making a significance determination for GHGs based on a percents relative nature. A tonnage threshold is a static metric that would not change over time. As previously described, the trajectory of U.S. GHG
emissions is trending down. As emissions decrease over the course of time, it is likely that source categories that were once above any static threshold will fall below such a threshold. Even though a source category may reduce overall U.S. GHG
emissions, that 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.
Additionally, if emissions do decrease over time, the use of a tonnage threshold potentially results in no source category meeting the criteria for significance, even if collectively the U.S.GHG
emissions continue to pose a danger to public health or welfare.
It should be noted that the U.S. GHG
emissions of the EGU source category are more than an order of magnitude larger than the emissions threshold in the framework, representing 43 percent of U.S. stationary source GHG
emissions. The EPA believes that it is possible for source categories with GHG
emissions substantially larger than the threshold to be deemed significant on the basis of the primary criterion alone i.e., magnitude of emissions and without consideration of the secondary criteria described elsewhere in this notice.
3. Tiers of Source Categories Based on GHG Emissions As noted previously, the primary criterion in evaluating the significance of a source category is, again, the relative magnitude of the U.S. GHG
emissions. The EPA believes that NSPS
source categories may be grouped into three tiers on the basis of magnitude of the U.S. GHG emissions, as follows:
1 Source category with GHG emissions substantially above the threshold. This source category has emissions of a large enough magnitude that a determination of significance can be made on the basis of the magnitude of emissions alone. As discussed
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later in this document, this tier is comprised solely of the EGUs source category; in other words, EGUs do not require consideration of the secondary criteria in order to determine significance.
2 Source categories with an intermediate magnitude of the U.S. GHG emissions i.e., those with emissions above the threshold but less than the quantity emitted by the EGU
source category. For source categories with emissions above the threshold, evaluation of the magnitude of the U.S. GHG emissions is inconclusive. Rather, a significance determination requires an examination of the source categorys magnitude of emissions combined with a more detailed look at the secondary criteria discussed elsewhere in this document.
3 Source categories with a small magnitude of GHG emissions i.e., those with emissions below the threshold. Source categories with a small magnitude of emissions will be deemed insignificant based on evaluation of the primary criterion alone, without detailed consideration of any secondary criteria.

D. Secondary Criteria for Determining Significance As described above, the EPA is determining that the U.S. GHG
emissions from a source category are the primary and most important criterion for making a determination of significance for a source category.
However, there may be instances where the U.S. GHG emissions from a source category do not give a comprehensive enough picture to make a determination of significance. The threshold that the EPA has described above in Section IV.B would provide a clear indication that the U.S. GHG emissions from source categories below that threshold are necessarily insignificant. However, under this framework, for any source category that is above that threshold, there are other source-category specific considerations that should be evaluated in addition to GHG emissions when making a determination of significance.14 For that reason, the EPA
will consider other, secondary, criteria in the evaluation of significance for certain source categories. These other criteria are described in the subsequent subsections. It is important for the EPA
to consider secondary criteria in the evaluation of significance for certain source categories because the criteria provide unique context to the source category beyond the information provided by the magnitude of the source categorys GHG emissions.
14 Although there is no source category other than EGUs above the 3% threshold, because the threshold is a percentage and as previously described, other source categories may move into this tier as overall GHG emissions decrease and other source category emissions increase.

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1. Evaluation and Context of GHG
Emissions Under the introduced framework, the evaluation of the magnitude of the U.S.
GHG emissions from a source category is a substantial indicator of whether a source category is significant, but in the specific instance of source categories that have greater GHG emissions than the threshold, an evaluation based on the magnitude of the U.S. GHG
emissions may be inconclusive. Within the introduced framework, there are other emissions-based metrics that must be evaluated to clarify and make a significance determination for these source categories.
a. Source Category Trends An important criterion that can help illuminate and contextualize a significance determination is an evaluation of the trends in emissions and number of designated facilities within a source category. Primarily, the EPA is evaluating whether a source category is on a trajectory of the U.S.
GHG emission decline. If the source category, as a whole, is decreasing its GHG emissions, an explanation for why it is on the decline may aid in making a significance determination. In one scenario, if the source category is decreasing emissions because the source category is declining in production or other output e.g., due to decreasing demand for goods or other market conditions, due to relocation overseas, or due to the cumulative effect of regulations, it may lend towards an insignificance determination as the emissions are already declining and expected to continue to decline even without further regulation. In a separate scenario, if a source categorys GHG
emissions are declining due to increased efficiency and updated technology, it may lend towards a determination of significance. This would allow the EPA
the ability to regulate the source category in order to ensure that efficiency and technology improvements become standard across the source category through both an NSPS 111b regulation for new, modified and reconstructed sources and an emission guidelines 111d regulation for existing sources.
In a scenario in which the EPA were to find a source category to be growing in either emissions or number of designated facilities or both, it could lend towards that source category being found to be significant. This would allow EPA to regulate and mitigate emissions from new, modified and/or reconstructed designated facilities
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