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★ Greenhouse Gas Emissions Threshold - The Case For 'Net Zero'

CEA Foundation President, Ralph Silberstein explains why GHG thresholds set in the draft environmental report are no longer relevant. To determine the level of "significance" related to emissions from the proposed Idaho-Maryland Mine, Nevada County chose a threshold of 10,000 metric tons per year, but legal experts at the respected environmental law firm, Shute, Mihaly, and Weinberger LLP, make the case for why the County needs to consider "net zero" as the threshold instead.

This opinion piece was originally published in The Union.


Ralph Silberstein: Idaho-Maryland Mine greenhouse gas impacts and the draft environmental impact report

The draft environmental impact report for the Idaho-Maryland Mine project includes an assessment of the environmental impacts from greenhouse gas emissions. These emissions are from carbon dioxide and other greenhouse gases such as methane, which are the major contributors to climate change.

To determine the level of impact from the emissions, Nevada County used a threshold. Anything over the threshold would be classified as “significant” and would require additional actions, or “mitigations,” to reduce the severity of the impacts.

However, as stated in the draft report, neither the Northern Sierra Air Quality Management District nor Nevada County “has adopted numerical thresholds of significance for greenhouse gas emissions that would apply to the project.”[1]

Instead, Nevada County chose 10,000 metric tons per year as the threshold. The county determined this threshold by looking at air districts for Placer County, Sacramento Metropolitan Area, the Bay Area and Southern California.

The draft report calculated that emissions from mine operations would be just under the 10,000 metric tons threshold for the bulk of the mining operations.[2] The largest contributor to greenhouse gases would come from PG&E electricity, followed by emissions from trucks hauling mine waste rock and tailings off site.

Consequently, the conclusion was that the emissions would be “less than significant,” and no mitigations were proposed. (Note that construction greenhouse gas emissions are calculated separately, are listed as significant, and mitigations are proposed.)


Each California air district has unique air quality conditions with standards that are established by an inventory of those conditions.

As explained by the Baseline Engineering Consultants’ report addressing the draft environmental impact report, the thresholds that Nevada County chose to use were established by other districts prior to the 2016 California SB32 statewide update of greenhouse gas reduction targets.

Those changes reset the reduction targets to reduce emissions to 40% below 1990 levels by 2030.[3] Thus the thresholds used by the other districts referenced by Nevada County are no longer relevant. In fact, at least one of the districts is undergoing review to reset their threshold to conform to the current California SB32.

Nevada County’s use of the 10,000 metric ton threshold for the draft report was not adequate in the first place due to the different conditions in the various air districts. In addition, due to the changes in state targets, that 10,000 metric ton threshold is certainly not valid now. As summarized by Shute, Mihaly and Weinberger LLP in comments prepared on behalf of CEA Foundation:

“The DEIR must use a significance threshold that will achieve California’s new statewide greenhouse gas reductions goals over the proposed 80-year lifetime of the project. In light of those state goals and the severity of climate change, any operational emissions over existing conditions should be considered significant.

“Since 2010, scientific research has made clear that any additional emissions will contribute to the serious and growing climate crisis. See Intergovernmental Panel on Climate Change, Climate Change 2021: The Physical Science Basis, Summary for Policymakers (October 2021), attached as Exhibit I.

“Recognizing this reality, in 2018 Gov. Brown signed Executive Order 55-18, calling for the state to achieve carbon neutrality as soon as possible and no later than 2045, and to achieve and maintain net-negative emissions thereafter. See DEIR at 4.3-29.

“Given these facts, the DEIR should establish a net zero threshold for new greenhouse gas emissions from the project. See e.g., CARB 2017 Climate Change Scoping Plan, attached as Exhibit J, at 101 (’Achieving no net additional increase in greenhouse gas emissions, resulting in no contribution to greenhouse gas impacts, is an appropriate overall objective for new development.’) The DEIR does not explain why the project should not be judged by a significance threshold requiring no net increase in greenhouse gas emissions.”[4]

The draft environmental impact report states greenhouse gas emissions from mine operations would be just under 10,000 metric tons per year. This number was chosen as a threshold based on other air districts in California.

Neither our Air Quality Management District nor Nevada County have adopted numerical thresholds of significance for greenhouse gas emissions that would apply to the project.

The draft environmental impact report does not discuss the justification for adopting the 10,000 metric tons per year threshold, nor does it provide substantial evidence for applying this threshold to the project to demonstrate how it will achieve a fair share of the statewide greenhouse gas reductions goals for 2030 and beyond.

Ralph Silberstein is the president of the Community Environmental Advocates Foundation.


[2] Ibid, Table 4.3-23,DEIR Section 4.3, pgs 93-94

[4] Shute, Mihaly, & Weinberger LLP, Comments to the Idaho-Maryland Mine Project Draft Environmental Impact Report, pg 45 (download the comment from this page).


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