Prioritize rapidly scalable methane reductions in efforts to mitigate climate change

Methane emission reductions are crucial for addressing climate change. It offers short-term benefits as it holds high short-term reductions in radiative forcing. Efforts towards the reduction of methane emissions are already underway. In this study, we compared and analyzed the mitigation benefits of cutting large amounts of methane emissions from the oil and gas sector on short-time scales with reducing an equivalent amount of carbon dioxide using carbon capture and storage (CCS). Characteristics of CCS are that it would require substantial infrastructure development and that it incorporates deployment delays. Results illustrate that prioritizing quickly deployable methane emission reduction alternatives that necessitate minimal construction is an efficient approach to achieve near-term climate change relief.


Introduction
In November 2022, the U.S. Environmental Protection Agency (US EPA) proposed regulations that would reduce methane (CH 4 ) 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 CH 4 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 CH 4 emissions from the oil and gas sector are already underway.The Global Methane Pledge and commitments at COP26 targeted 30% CH 4 emission reductions by 2030 (DOS, 2021).More recently, COP27 highlighted the importance of reducing CH 4 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 CH 4 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 CH 4 (Solomon et al. 2010;Dreyfus et al. 2022;Singh et al. 2022).Reducing CH 4 emissions could bring benefits in the near-term when compared to reducing emissions of longer-lived climate forcers like CO 2 (Abernethy et al. 2021;Cain et al. 2022;Ming et al. 2022;Abernethy and Jackson 2022).Besides rapid deployment, CH 4 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 CH 4 emissions from the global energy sector on short time scales with reducing an equivalent amount (on a global warming potential basis) of CO 2 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 CH 4 emission reduction technologies.

Methane and carbon dioxide reduction scenarios
Oil and gas CH 4 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 CH 4 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 CO 2 , we converted CH 4 to CO 2 emissions using a fossil CH 4 GWP 20 (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 CO 2 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 CO 2 emitted per unit of throughput in Tg of CO 2 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.
CH 4 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 Step-change reduction at year 5 1.20% of annual operations emissions spread evenly across years 0-5 80% CH 4 Five-year linear reduction starting at year 0 -CO 2 Step-change reduction to 30% of business-as-usual emissions at year 5, and linear decrease over the next 10 years Period of rapid build out in years 0-5 incurs 1.20% of annual operations emissions spread evenly across these years.Relatively slower build-out assumed for years 5-15 with 1.43% of annual operations emissions spread evenly across those years 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, CH 4 mitigation technologies applicable to the oil and gas sector are off-theshelf with minimal lead time for construction.We, therefore, assumed installation could begin immediately.
The CH 4 and CO 2 mitigation scenarios in Table 1 were compared on the basis of relative global-average surface temperature increase avoided with respect to the businessas-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.

Rapidly scalable CH 4 reduction technologies outperform longer term options to cut CO 2 emissions
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 CH 4 reduction scenarios.This phenomenon reflects the short life of CH 4 in the atmosphere (half-life of 11.8 years) (IPCC, 2022).Given CH 4 's short atmospheric lifetime, ultimately, the reduced CH 4 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 CO 2 reduction scenarios is initially lower than in CH 4 reduction scenarios, eventually it catches up.After a crossover point, CO 2 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 CH 4 or 8250 Tg CO 2 ).CH 4 reduction technologies are rapidly scalable and benefits associated with CH 4 reductions are evident beginning in year three.In contrast, the waiting period and emissions associated with construction of CCS facilities delay benefits from CO 2 emissions reductions until year eight.Accordingly, there is a five-year longer wait for any relief from climate change compared to its equivalent CH 4 scenario.Avoided temperature increases in the CH 4 scenario, however, begins to plateau after year 20.At this point, the temperature change avoided by CH 4 mitigation is 28 mK while the CO 2 mitigation scenario reaches only 20 mK.After about 31 years, the benefits of mitigating CO 2 exceed those of mitigating CH 4 .At timescales beyond this point, the benefits of reducing CO 2 will outweigh those of reducing CH 4 by a large extent.
The trends in Fig. 1b, which reflect an aggressive 80% reduction in business-as-usual CH 4 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 CH 4 and CO 2 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 CH 4 and CO 2 ) 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 CH 4 reduced (80% versus 30%) causes benefits to accrue for longer.The benefits of reducing CO 2 emissions with CCS outstrip those of reducing CH 4 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%.

Relief from climate change comes sooner with scalable technology
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 lightduty 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 CH 4 emissions in achieving short term climate goals, it is important to note that reducing CH 4 and CO 2 emissions does not require an either-or choice.CO 2 reductions provide long term benefits while CH 4 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.

Fig. 1
Fig. 1 Surface temperature change avoided relative to business-asusual levels over a 40-year time horizon.Dashed vertical lines indicate the year at which ∆T avoided is equivalent for CH 4 and CO 2 reduction scenarios

Table 1
Scenarios for emissions reductions of CH 4 and equivalent amount of CO 2 from business-as-usual † levels † Business-as-usual levels: 100 CH 4 Tg/year; 8250 CO 2 Tg/year