image: Global and hemispheric OH concentration relative to the 2018-2019 mean inferred from atmospheric observations of CO, MCF, and Methane.
Credit: ©Science China Press
A young team of researchers has found consistent observational evidence that global atmospheric oxidation capacity declined significantly in 2020, marking one of the most substantial perturbations to the oxidizing power of the troposphere in recent decades. The study, led by Wei Chen and Yuzhong Zhang from Westlake University and published in National Science Review, integrates multiple independent observational constraints and atmospheric modeling to pinpoint the causes of this decline and quantify its impact on the global methane budget.
Atmospheric oxidation, largely driven by hydroxyl radicals (OH), regulates the lifetimes of greenhouse gases such as methane and pollutants like carbon monoxide and nitrogen oxides. The researchers used a novel method based on satellite observations of carbon monoxide (CO) to infer OH concentrations across latitudes, revealing a 4.0±0.9% drop in global OH levels in 2020 relative to the 2018–2019 average. This result was independently supported by surface observations of methyl chloroform and satellite measurements of methane, indicating a robust multi-species signal of reduced atmospheric oxidation.
Interestingly, the new CO-based method, supported independently by the methane-based approach, reveals that OH concentrations declined in both hemispheres in 2020—by 2.4 ± 1.2% in the Northern Hemisphere and a larger 5.7 ± 1.2% in the Southern Hemisphere. This hemispheric pattern challenges the prevailing view that OH changes were driven solely by reduced anthropogenic emissions during COVID-19 lockdowns, which were largely concentrated in the Northern Hemisphere.
To further investigate the drivers of OH decline, the authors conducted simulations using the GEOS-Chem chemical transport model. Their results suggest that the reduction in OH in the Northern Hemisphere was largely due to decreased nitrogen oxide (NOx) emissions from human activities during lockdowns, while in the Southern Hemisphere, partly due to extreme wildfires in Australia in early 2020 released large amounts of reactive carbon compounds into the atmosphere. These different mechanisms are further supported by opposite anomalies in satellite-observed tropospheric ozone between the two hemispheres.
The decline in OH had a significant impact on methane, the second most important anthropogenic greenhouse gas. In 2020, global atmospheric methane concentrations surged by 14.8 ppb a-1—one of the largest annual increases ever recorded by NOAA. Analysis based on CO-inferred OH changes indicates that significant reductions in OH occurred in both hemispheres, helping to explain much of the observed acceleration in both global and hemispheric methane growth. Using a methane inversion framework, the study finds that this surge corresponds to an excess burden of 27 Tg a-1, of which approximately 18 Tg a-1 (or 52±19%) can be attributed to reduced OH concentrations, with the rest primarily due to increased emissions from tropical and high-latitude wetlands. This result, based on a detailed budget analysis, reconciles discrepancies in previous studies on the 2020 methane rise.
“Our findings suggest that chemistry-climate interactions are becoming increasingly important in regulating global methane budgets,” says corresponding author Yuzhong Zhang. “As anthropogenic emissions decline and biomass burning increases under climate change, variations in atmospheric oxidation will play a growing role in determining the trajectory of methane and other reactive gases.”
This study not only highlights the critical role of atmospheric chemistry in influencing climate through methane but also underscores the importance of integrating diverse observational constraints to enhance our understanding of global atmospheric processes.
###
See the article:
Converging evidence for reduced global atmospheric oxidation in 2020
Journal
National Science Review