Mitigating Aliso Canyon / Porter Ranch methane emissions

The California Air Resources Board (CARB) released a draft mitigation program for the Aliso Canyon (a.k.a Porter Ranch) gas leak, seeking to address the climate impacts of one of the largest natural gas leaks in U.S. history. According to the report's preliminary estimates, the incident resulted in about 100,000 tons of methane leaking into the atmosphere.

There's a lot to talk about here, but I was drawn to one important detail in the draft program. 

CARB is asking SoCalGas to prioritize mitigation of an equivalent mass of methane emissions, rather than allowing the company to spread its efforts over a basket of warming-equivalent greenhouse gases. To support the focus on methane mitigation, CARB has proposed using a 20-year global warming potential (GWP) to convert methane into its carbon dioxide equivalent (CO2e) for any non-methane mitigation. This is a big deal because the IPCC's current estimate for the 20-year GWP for methane is 84, whereas the 100-year GWP is 28. As a result, if SoCalGas wanted to mitigate carbon dioxide instead of methane, the 20-year GWP would triple the amount required. 

As far as I know, this would be the first time that a significant climate mitigation policy operated on a 20-year GWP horizon. All of the major international and national policy regimes have used 100-year GWPs, reflecting the long-term nature of the climate challenge. But with more and more focus on the rate of warming, as well as on the contribution of so-called short-lived climate pollutants (SLCPs) like methane and black carbon on overall radiative forcing (W/m2), we may see additional interest in shorter time horizons. 

One last thought. It's little ambiguous whether SoCalGas would need to apply 20-year GWPs to convert non-methane, non-CO2 gases into CO2e; parts of CARB's draft report suggest this would be the case, but I don't think it's explicit. In other words, if SoCalGas wanted to reduce HFC emissions as part of its mitigation obligations, it's not clear whether they should convert HFC emissions to CO2e using 20- or 100-year GWPs, only that the amount of methane to be offset by non-methane mitigation will be converted to CO2e using a 20-year GWP. I hope CARB will clarify this minor detail in its final report.