Prioritize rapidly scalable methane reductions in efforts to mitigate climate change

In November 2022, the U.S. Environmental Protection Agency (US EPA) proposed regulations that would reduce methane (CH₄) emissions from oil and gas supply chains in the U.S. by 87%, compared to 2005 levels (EPA, 2022). If emission reductions of this magnitude were deployed globally, they would result in reductions that could abate 80 Tg of CH₄ per year (IEA, 2022). Technologies capable of reducing these emissions already exist. In many cases, they are economically attractive because they reduce loss and leakage of salable product.

Efforts to reduce CH₄ emissions from the oil and gas sector are already underway. The Global Methane Pledge and commitments at COP26 targeted 30% CH₄ emission reductions by 2030 (DOS, 2021). More recently, COP27 highlighted the importance of reducing CH₄ emission from oil and gas value chains for achieving net zero by 2050. In the US, the Inflation Reduction Act (IRA) 2022 incorporated fees for entities along the oil and gas supply chain that report CH₄ emissions above specified thresholds, starting in 2024 (H.R.5376, 2022). These efforts are part of a broader trend that emphasizes emission reductions of short-lived climate forcers like CH₄ (Solomon et al. 2010; Dreyfus et al. 2022; Singh et al. 2022). Reducing CH₄ emissions could bring benefits in the near-term when compared to reducing emissions of longer-lived climate forcers like CO₂ (Abernethy et al. 2021; Cain et al. 2022; Ming et al. 2022; Abernethy and Jackson 2022). Besides rapid deployment, CH₄ reductions in the energy sector have the additional advantage of high short-term reductions in radiative forcing.

In this paper, we compare the climate change mitigation benefits of cutting large amounts of CH₄ emissions from the global energy sector on short time scales with reducing an equivalent amount (on a global warming potential basis) of CO₂ using carbon capture and storage (CCS), a technology that would require significant infrastructure development. While both of these approaches can be pursued simultaneously, our analysis illustrates the short-term benefits of exploiting available, economically viable, and scalable CH₄ emission reduction technologies.

Oil and gas CH₄ emissions are currently reported as approximately 80 Tg per year, although they may be 25 to 40% greater than this estimate (Hmiel et al. 2020). Our analysis, therefore, assumes a business-as-usual emissions level of 100 Tg of CH₄ per year. We then evaluated the changes in radiative forcing and consequent global-average surface temperature change from reducing these emissions by 30%, which is in line with current global targets. A second scenario assumes 80% emission reductions to highlight the advantages of pushing past current targets for achieving reductions similar to those proposed by the US EPA (EPA, 2022). To evaluate emission reduction scenarios that decrease an equivalent amount of CO₂, we converted CH₄ to CO₂ emissions using a fossil CH₄ GWP₂₀ (global warming potential at year 20) of 82.5 as reported in the Sixth Assessment Report (IPCC, 2022). Table 1 describes the proposed scenarios.

Using CCS to reduce CO₂ emissions requires manufacturing of large-scale equipment, and construction of foundations, pipelines and auxiliary systems. Such activities generate additional GHG emissions that might vary depending on particular characteristics and these systems have a range of emission estimates reported in the literature. Manufacturing and construction emissions for CCS are reported to be between 0.07 and 0.33 Tg of CO₂ emitted per unit of throughput in Tg of CO₂ sequestered per year (Koornneef et al. 2008; Cuellar-Franca and Azapagic, 2015). We adopted the average of this range to develop the scenarios in Table 1, which include infrastructure emissions and construction times (Townsend and Gillespie 2020). These scenarios take account of the changes over time in emissions associated with manufacturing the steel and materials required to build CCS facilities.

CH₄ emission reduction technology options are less infrastructure-intensive than CCS. Nonetheless, we assessed whether we would need to include GHG emissions from infrastructure build out to deploy them. These options include replacing pneumatic pumps with electrical pumps, replacing pneumatic devices with mechanical controllers, and replacing high-bleed or high-emitting pneumatic devices with intermittent or low-bleed devices (Methane Guiding Principles 2022). We used the Economic Input–Output Life Cycle Assessment (EIO-LCA) model (Green Design Institute, 2022) and reported construction costs (Methane Guiding Principles 2022) to evaluate construction emissions associated with these transitions. Construction emissions are approximately 0.01% (installing electric pumps) to 0.5% (accelerated installation of low-bleed devices) of one year’s worth of emissions reductions. In our analysis, these emissions are treated as negligible. Furthermore, CH₄ mitigation technologies applicable to the oil and gas sector are off-the-shelf with minimal lead time for construction. We, therefore, assumed installation could begin immediately.

The CH₄ and CO₂ mitigation scenarios in Table 1 were compared on the basis of relative global-average surface temperature increase avoided with respect to the business-as-usual scenario. The higher the value is, the better the mitigation effort. We used global surface temperature change models reported in the literature in our calculations (IPCC, 2022; Abernethy & Jackson 2022; Gasser et al 2017). The interested reader can find more information on the scenarios and modeling approach in the Supporting Information.

Figure 1 displays the effects of mitigation efforts over a 40-year time horizon. In interpreting this illustration, we focus on two primary results. First, the change in surface temperature increase avoided eventually levels off in the CH₄ reduction scenarios. This phenomenon reflects the short life of CH₄ in the atmosphere (half-life of 11.8 years) (IPCC, 2022). Given CH₄’s short atmospheric lifetime, ultimately, the reduced CH₄ in the atmosphere leads to a reduction equivalent to technology-based emission reductions and the temperature change plateaus. Second, although the avoided surface temperature rise in CO₂ reduction scenarios is initially lower than in CH₄ reduction scenarios, eventually it catches up. After a crossover point, CO₂ emission reduction scenarios offer greater benefits. The dashed vertical lines in Fig. 1 mark this crossover point, which is on the order of decades.

Figure 1a compares the strategies that achieve 30% reduction in business-as-usual annual levels (100 Tg CH₄ or 8250 Tg CO₂). CH₄ reduction technologies are rapidly scalable and benefits associated with CH₄ reductions are evident beginning in year three. In contrast, the waiting period and emissions associated with construction of CCS facilities delay benefits from CO₂ emissions reductions until year eight. Accordingly, there is a five-year longer wait for any relief from climate change compared to its equivalent CH₄ scenario. Avoided temperature increases in the CH₄ scenario, however, begins to plateau after year 20. At this point, the temperature change avoided by CH₄ mitigation is 28 mK while the CO₂ mitigation scenario reaches only 20 mK. After about 31 years, the benefits of mitigating CO₂ exceed those of mitigating CH₄. At timescales beyond this point, the benefits of reducing CO₂ will outweigh those of reducing CH₄ by a large extent.

The trends in Fig. 1b, which reflect an aggressive 80% reduction in business-as-usual CH₄ emissions, are similar to those in Fig. 1a. However, global surface temperature change is more quickly avoided. For example, the first 20 mK of avoided surface temperature rise for CH₄ and CO₂ occur between years 14 to 20 when the emission reduction target is 30%, and between years 8 to 15 when it is 80%. The gap in the time required to achieve this benefit (between CH₄ and CO₂) is six and seven years, respectively. Avoided surface temperature rise takes 17 years longer to plateau in the 80% reduction scenario as compared to the 30% reduction scenario. The greater amount of CH₄ reduced (80% versus 30%) causes benefits to accrue for longer. The benefits of reducing CO₂ emissions with CCS outstrip those of reducing CH₄ in the oil and gas sector in year 35 at 80% reductions. At this point, the avoided surface temperature rise is 97 mK, 62 mK greater than when the reduction target is 30%.

Our analysis emphasizes two main points. First, construction and associated emissions delay the benefits of climate change mitigation technologies. Prioritizing quickly-deployable options that require limited construction helps to bring relief sooner. Second, GHG mitigation options that require longer to scale up can take decades to overtake quickly scalable solutions. The results we present add urgency to pursuit of rapidly scalable technologies.

One of the primary drivers of our analysis is the long lead times required for emissions from CCS to materialize. CCS is not unique in this regard. For example, transitioning from internal combustion engines to electric vehicles in the light-duty fleet will take decades. This transtition entails manufacturing and installation of charging infrastructure along with building factories to produce enough lithium-ion batteries to meet demand.

While our analysis emphasizes the importance of reducing CH₄ emissions in achieving short term climate goals, it is important to note that reducing CH₄ and CO₂ emissions does not require an either-or choice. CO₂ reductions provide long term benefits while CH₄ reductions provide near-term reductions in warming that will be important in mitigating the current impacts (Weiskopf et al. 2020; Sarkodie et al. 2022) of a changing climate.