Global warming potentials in the draft Aliso Canyon Methane Leak Climate Impacts Mitigation Program

Danny Cullenward and Michael Mastrandrea of Near Zero, together with Stanford University PhD candidates Emily Grubert and Aaron Strong, sent a comment to the California Air Resources Board (ARB) on ARB’s Draft Mitigation Program for the Southern California Gas Company (SoCalGas) methane leak in Aliso Canyon.

Our remarks focus on scientific issues that arise in the course of using 20-year global warming potentials (GWPs) to convert non-CO2 gases into their carbon dioxide equivalents (CO2e).

As the Draft Program observes, Governor Brown’s January 2016 Proclamation directs ARB to “fully mitigate” the leaked methane emissions from Aliso Canyon. In turn, ARB’s Draft Program recommends that Southern California Gas focus its mitigation efforts on methane emissions in California. It also contemplates mitigation of other greenhouse gases, including non-methane short-lived climate pollutants (SLCPs) and carbon dioxide.

In order to ensure equivalence between the impact of the original leak and mitigation effects across a portfolio of greenhouse gases, ARB’s Draft Program uses a standard metric: the GWPs published by the Intergovernmental Panel on Climate Change (IPCC). To the best of our knowledge, the Draft Program, if finalized and applied to SoCalGas, would constitute the first time a legally binding climate mitigation policy selects 20-year GWPs. In contrast, most climate mitigation policies apply 100-year GWPs to calculate CO2e.

The application of a GWP time horizon involves both normative and scientific judgments. In its most recent report, for example, the IPCC recognized that the choice of time horizon involves value judgments that cannot be established by scientific analysis alone. Furthermore, we note that the Draft Program’s selection of 20-year GWPs follows the approach taken in ARB’s Draft SLCP Reduction Strategy, which outlined a variety of mitigation options for SLCPs and expressed their impacts using 20-year GWPs.5 The Draft Program also reflects discussion in the 2014 Updated Scoping Plan regarding the potential use of 20-year GWPs for SLCPs.

Nevertheless, ARB’s selection of 20-year GWPs in the Draft Program raises important technical issues that we believe that Board staff should monitor in order to maintain consistency within and between California’s climate mitigation policies. In particular, we identify three issues in the Draft Program that we hope that ARB staff will clarify in the final version and a fourth area we believe ARB should monitor for consistency with other climate mitigation policies:

Recommendation #1: Clarify the application of GWPs for nonmethane greenhouse gases.

Recommendation #2: Cite the final version of the IPCC GWPs, not the pre-publication draft.

Recommendation #3: Make an explicit selection of GWPs with or without climate-carbon feedbacks.

Recommendation #4: Monitor interactions between mitigation policies that apply different GWP time horizons.

Disclaimer: this letter is written on behalf of its signatories in their individual capacities and does not represent the view of any affiliated organization or employer.