Abstract
We analyse historical (1850–2014) atmospheric hydroxyl (OH) and methane lifetime data from Coupled Model Intercomparison Project Phase 6 (CMIP6)/Aerosols and Chemistry Model Intercomparison Project (AerChemMIP) simulations. Tropospheric OH changed little from 1850 up to around 1980, then increased by around 9% up to 2014, with an associated reduction in methane lifetime. The model-derived OH trends from 1980 to 2005 are broadly consistent with trends estimated by several studies that infer OH from inversions of methyl chloroform and associated measurements; most inversion studies indicate decreases in OH since 2005. However, the model results fall within observational uncertainty ranges. The upward trend in modelled OH since 1980 was mainly driven by changes in anthropogenic near-term climate forcer emissions (increases in anthropogenic nitrogen oxides and decreases in CO). Increases in halocarbon emissions since 1950 have made a small contribution to the increase in OH, whilst increases in aerosol-related emissions have slightly reduced OH. Halocarbon emissions have dramatically reduced the stratospheric methane lifetime by about 15%–40%; most previous studies assumed a fixed stratospheric lifetime. Whilst the main driver of atmospheric methane increases since 1850 is emissions of methane itself, increased ozone precursor emissions have significantly modulated (in general reduced) methane trends. Halocarbon and aerosol emissions are found to have relatively small contributions to methane trends. These experiments do not isolate the effects of climate change on OH and methane evolution; however, we calculate residual terms that are due to the combined effects of climate change and non-linear interactions between drivers. These residual terms indicate that non-linear interactions are important and differ between the two methodologies we use for quantifying OH and methane drivers. All these factors need to be considered in order to fully explain OH and methane trends since 1850; these factors will also be important for future trends.
Generated Summary
This study analyzes historical (1850–2014) atmospheric hydroxyl (OH) radical and methane lifetime data from Coupled Model Intercomparison Project Phase 6 (CMIP6) simulations. The research employs a multi-model approach, utilizing three state-of-the-art Earth system models: Geophysical Fluid Dynamics Laboratory Earth System Model version 4 (GFDL-ESM4), Community Earth System Model version 2 Whole Atmosphere Community Climate Model (CESM2-WACCM), and the United Kingdom Earth System Model version 1.0, with low-resolution (N96) atmosphere and low-resolution (1°) ocean (UKESM1-0-LL). The study investigates pre-industrial (PI; 1850s) and present-day (PD) zonal mean fields of modelled OH and related species, along with historical time series of global tropospheric OH and corresponding methane loss rates and lifetimes. Sensitivity experiments are conducted to isolate the effects of specific drivers, such as anthropogenic emissions and halocarbon mole fractions. The primary objective is to understand the drivers of changes in OH and methane lifetime since 1850. The analysis includes the examination of the impact of climate change and non-linear interactions between drivers on OH and methane evolution. The study compares model-derived OH trends with those estimated by observational data, such as from inversion models using methyl chloroform (MCF) measurements and methane isotopes.
Key Findings & Statistics
- Tropospheric OH changed little from 1850 up to around 1980, then increased by approximately 9% up to 2014, resulting in a reduction in methane lifetime.
- The model-derived OH trends from 1980 to 2005 are consistent with trends estimated by studies that infer OH from inversions of methyl chloroform and associated measurements.
- The upward trend in modelled OH since 1980 was mainly driven by changes in anthropogenic near-term climate forcer emissions, including increases in anthropogenic nitrogen oxides and decreases in CO.
- Increases in halocarbon emissions since 1950 have made a small contribution to the increase in OH.
- Aerosol-related emissions have slightly reduced OH.
- Halocarbon emissions dramatically reduced the stratospheric methane lifetime by about 15%–40%.
- The multi-model mean whole-atmosphere PD chemical lifetime in histSST is 8.4 ± 0.3 years, lower than the mean PI lifetime of 9.5 ± 0.5 years.
- The methane-OH feedback factor (f) yields a value of 1.25 for CESM2-WACCM and 1.23 for GFDL-ESM4.
- Observed PI and PD methane levels are 808 and 1794 ppb, respectively.
- Holding NTCFs at PI levels increases PD methane by 16%-33%.
- The impact of halocarbon emissions has been to reduce PD methane by 7%-15%.
- Holding methane emissions at PI levels would have led to PD methane levels of 516 ppbv, which is 36% (292 ppbv) lower than PI mole fractions.
Other Important Findings
- The model simulations show typical inter-annual variability in global OH of about ±2%-3%.
- The models show a strong increase in OH of about 9% from 1980 to 2014.
- The inferred trends from different inversion methods show a wide range of trends but are generally upwards from 1980 to 2005.
- From 2005 onwards, the inversions generally indicate downward trends, whereas the models suggest a continued slight upwards trend.
- The zonal mean PD-PI change in OH reveals local increases of over 50% in zonal mean tropospheric OH, particularly over polluted NH mid-latitudes, and a local decrease of over 10% in the Southern Hemisphere (SH) middle to upper troposphere at around 20° S.
- The impact of increases in NTCF emissions since 1850 up to PD was to generally increase tropospheric OH by 10%–50% in the zonal mean and 13%-22% across the whole troposphere.
- The overall impact of changed emissions of NTCFs has been to reduce the methane lifetime.
- Emissions of halocarbons since 1950 have led to polar stratospheric ozone depletion, mainly in the SH, which has increased stratospheric OH levels but also increased tropospheric OH.
- The residual term represents the contribution of climate change to the OH anomaly, along with any contributions from non-linear interactions between components.
Limitations Noted in the Document
- The study’s sensitivity experiments do not isolate the effects of climate change on OH and methane evolution.
- The residual component for OH is positive, while the residual component for equilibrium methane is also positive, which suggests that non-linear interactions show different impacts in the two methodologies.
- The study does not separate the relative impacts of CO and NMVOC emissions, which will have tended to reduce OH.
- The derived values of f are probably slightly smaller because the histSST-piNTCF runs also include increases in temperature and humidity.
- The residual term may not be a good indicator of climate change effects alone.
- The study’s inability to isolate the effects of different ozone precursors limits the ability to fully understand the drivers of OH changes.
Conclusion
The research findings emphasize the crucial role of anthropogenic emissions in shaping global OH and methane trends. The study’s analysis indicates that changes in anthropogenic near-term climate forcer emissions, particularly the increases in NOx and decreases in CO, are the primary drivers of the upward trend in modelled OH since 1980. The results demonstrate that while methane emissions are the main driver of atmospheric methane increases since 1850, changes in ozone precursor emissions have modulated methane trends. Halocarbon and aerosol emissions were found to have relatively small contributions to methane trends. The study underscores the importance of considering all these factors to fully explain OH and methane trends since 1850. The study highlights the complexity of the factors influencing OH and methane lifetimes. The impact of halocarbon emissions on stratospheric methane lifetime is dramatic, reducing it by up to about 40% between 1960 and 1990. Moreover, the study indicates that the residual term, which represents the contribution of climate change and non-linear interactions, plays a significant role in OH anomalies. The study points out that the non-linear interactions are important and differ between the two methodologies. This means that perfect quantitative attribution cannot be achieved, and attribution of the residual term to climate change effects is rather uncertain. This research contributes to the understanding of the complex interplay of factors influencing atmospheric chemistry and highlights the need for considering the multifaceted impacts of anthropogenic activities and climate change.