Timelines for mitigating the methane impacts of natural gas expansion

Continued natural gas expansion is expected to reduce CO2 emissions from electricity in several large economies, but reductions of CO2 may not be sufficient to meet climate policy goals. While CO2 arises from large point... [ view full abstract ]

Continued natural gas expansion is expected to reduce CO₂ emissions from electricity in several large economies, but reductions of CO₂ may not be sufficient to meet climate policy goals. While CO₂ arises from large point sources with small uncertainties, mitigating methane (CH₄) emissions from the natural gas supply chain can be more challenging. Recognizing these differences is important to meet targets to reduce CO₂-equivalent emissions of all greenhouse gases under the Paris Agreement and beyond, and to avoid commitments to mitigation rates that may be difficult to achieve on decadal timescales.

In this paper we examine the effect of expected reductions of CO₂ emissions from the U.S. electricity sector by 2030, likely achieved through a displacement of coal by natural gas, on CH₄ emissions from natural gas and coal used for electricity. Using a variety of emissions scenarios and a set of emissions equivalency metrics, we compare expected CO₂-equivalent emissions against a 2030 CO₂-equivalent target. This target is a conservative estimate of emissions reductions in the power sector corresponding to economy-wide reductions intended in U.S. climate policy commitments between 2025 and 2050.  We find that CO₂-equivalent emissions without additional methane mitigation may substantially exceed the target and identify two different mitigation options. If CO₂ declines as expected today, for instance under the U.S. EPA’s Clean Power Plan, CH₄ leakage from the natural gas supply chain needs to decline at faster rates than observed in U.S. data since the 1990s to meet the 2030 CO₂-equivalent target. This is regardless of the metric used to convert CH₄ to CO₂-equivalent emissions. If CO₂ declines faster, a CH₄ clean-up can be delayed. However, necessary changes in the fuel mix to achieve deep CO₂ cuts can be substantial. Deeper CO₂ cuts reduce the U.S. electricity sector's contribution to warming between 2030 and 2050 more persistently, while temperature reductions due to CH₄ mitigation are lost more quickly. Further, achieving deep CH₄ cuts will require novel strategies to reduce methane leakage uncertainties and target emitters effectively.