Abstract
The atmospheric methane trend is not fully understood. Here we investigate the role of the main sink, the main natural source, and anthropogenic emissions on the methane growth rate over the last three decades using numerical models and emission inventories. We find that the long-term trend is driven by increased anthropogenic methane emissions, while wetland emissions show large variability and can modify the trend. The anthropogenic influence on hydroxyl radical, through nitrogen oxides and carbon monoxide emissions, has modified the trend over the last decades and contributed to the atmospheric methane stabilization from 2000 to 2007. The hydroxyl radical increase prior to this stabilization period might have contributed to the decline in the isotopic ratio after 2007 due to the time dependent isotopic response of hydroxyl radical. Emission reductions due to COVID-19 restrictions via the influence on hydroxyl radical, possibly contributed to approximately two thirds of the increase in methane growth from 2019 to 2020.
Generated Summary
This research investigates the factors influencing atmospheric methane concentrations, focusing on the period since 1990. The study employs numerical models and emission inventories to analyze the roles of the main methane sink, natural sources, and anthropogenic emissions in driving the observed methane growth rate. The researchers aim to understand the long-term trends and the impact of various factors, including anthropogenic emissions, wetland emissions, and changes in the hydroxyl radical (OH) concentration, on methane dynamics. The methodology involves the use of a chemistry transport model (OsloCTM3) to assess the trend in the OH sink, alongside the community land model CLM5.0 to calculate natural methane fluxes. Furthermore, a box model is utilized to evaluate the influence of these components on the atmospheric methane growth rate. The study also examines the impact of emission reductions during the COVID-19 pandemic on methane growth and explores the influence of methane on the isotopic ratio. The study considers a wide range of emission inventories and scenarios to account for uncertainties in anthropogenic emissions. The analysis seeks to consistently assess the time evolution and relative roles of methane sink, natural sources, and anthropogenic sources.
Key Findings & Statistics
- The global mean methane concentration reached 1908 parts-per-billion (ppb) in 2021, which is >160% higher than pre-industrial levels (year 1750).
- Large annual increases in methane, equivalent to those observed in 2020 and 2021 (15.2 and 17.8 ppb, respectively), have not been seen since the 1980s.
- Anthropogenic sources contribute to about 60%, and natural sources (mainly wetlands) contribute to about 40% of the total sources of methane.
- The box model calculations using the CEDS-2017 emission inventory and a Shared Socioeconomic Pathway (SSP) scenario give atmospheric growth rates closer to observed values after the stabilization period.
- The OsloCTM3 simulations show a decrease in OH (and an increase in methane lifetime) from 1850 to 1990 of ~3%.
- From 1990, OH rapidly increased up to 2007 with an increase in OH of 5.8% in the simulation with fixed meteorology.
- From 2019 to 2020 the methane trend increased by 3.7 ppb yr-1 due to COVID emission reductions of NOx and CO, possibly contributing to the 5.5 ppb yr-1 larger observed growth rate in 2020 compared to 2019.
- Results show that about 30% of the strong observed decrease in δ13CCH4 of 0.2% between 2008 and 2014 can be explained by OH from OsloCTM3 and AerChemMIP driven by CEDS emissions.
- The contribution due to waste emissions is particularly uncertain due to large uncertainty in its global isotopic signature (yellow shading in Fig. 6b).
- The CEDS-2021 emission inventory shows a much smaller increase in both fossil and livestock emissions compared to EDGARv7.
- The RCP8.5 emission scenario was followed post 2000, while the CEDS-2017 follows a scenario with small increase post 2015.
- For the year 2020, the CEDS21 emission for year 2019 is scaled by estimated emission reduction due to policies to combat the COVID19 pandemic (see Method).
- In 2020, the atmospheric methane trend increased by 5.5 ppb yr-1.
Other Important Findings
- The long-term trend in atmospheric methane is primarily driven by increased anthropogenic methane emissions.
- Wetland emissions exhibit significant variability and can modify the overall methane trend.
- Anthropogenic influence on hydroxyl radical, through nitrogen oxides and carbon monoxide emissions, has modified the trend over the last decades.
- Hydroxyl radical increase before the stabilization period (2000-2007) may have contributed to the decline in the isotopic ratio after 2007.
- Emission reductions due to COVID-19 restrictions likely contributed to approximately two-thirds of the increase in methane growth from 2019 to 2020.
- The increase in OH is slightly less between 2000 and 2007 using the most recent CEDS emission inventory (CEDS21) compared to the CEDS17 simulation.
- After 2000 the lifetime is adjusted based on the CEDS21 + COVID simulation and before year 2000 the lifetime is adjusted based on the CEDS17 simulations, as the trend in the emissions used in these simulations are similar before year 2000.
- The general pattern in the time evolution is similar for the CLM model driven by different meteorological datasets, where the global net-emission increased from 1992 to 2000, then increased again from the end of the stabilization period up to 2011 and decreased thereafter.
- The contribution to the trend is negative throughout the stabilization period and becomes less negative in the period with renewed growth in atmospheric methane.
- The main contributor to the atmospheric methane trend is anthropogenic methane emissions, but changes in OH and wetland emission can modify the trend.
- The models show that an earlier increase in the oxidation capacity of the atmosphere can decrease the isotopic ratio in later years.
Limitations Noted in the Document
- The error bars from the GMB in Fig. 2 are mainly based on the range of different estimates, excluding uncertainty in emission factors and activity data.
- There are no uncertainty estimates for historical emissions and only two different emission paths prior to 1990 exist.
- The CLM results are in the lower range of the Global Methane Budget (GMB) emissions due to lower emissions in the tropics.
- The different meteorological data used to drive the model give slightly different wetland fluxes and span a larger range for net emissions.
- The study uses one global mean value for the isotopic signature for each emission sector.
- The box model is based on global and annual means and does not account for the highly variable OH concentrations in both time and space.
- The box model does not take into account complex interactions of chemistry, dynamics and spatial and temporal variability in emissions.
- Uncertainties in global anthropogenic emissions of NOx and CO are large.
Conclusion
The research presented offers valuable insights into the dynamics of atmospheric methane, emphasizing the multifaceted nature of its sources and sinks. The key finding is that long-term trends are primarily driven by anthropogenic emissions, but the trends can be modified by natural sources and changes in the atmospheric sink. The stabilization of methane concentrations from 2000 to 2007 is attributed to the anthropogenic influence on the hydroxyl radical. The study suggests that changes in the hydroxyl radical, particularly its increase prior to the stabilization period, influenced the subsequent decline in the isotopic ratio of methane. The impact of the COVID-19 pandemic, leading to emission reductions, likely contributed to the increase in methane growth from 2019 to 2020. Further research is needed to understand the role of different emission inventories. Overall, the research underscores the complex interplay of human activities and natural processes in regulating atmospheric methane levels. The dominance of anthropogenic emissions in driving long-term trends, the role of wetland emissions, and the influence of OH all highlight the need for comprehensive strategies. By quantifying these factors, the research contributes to a more complete understanding of past methane trends and aids in future mitigation efforts. This is critical to reversing the trend, as the atmospheric growth in methane in 2020 and 2021 is higher than in the early 1980s.