Why methane surged in the atmosphere during the early 2020s

Area of Science:

• Atmospheric chemistry and biogeochemical cycles.
• Climate change science and greenhouse gas monitoring.

Background:

• Atmospheric methane (CH4) growth rate exhibited significant fluctuations after 2019.
• Understanding the drivers of these CH4 variations is crucial for climate change mitigation.

Purpose of the Study:

• To identify and quantify the factors contributing to the observed changes in the atmospheric methane growth rate between 2019 and 2023.
• To differentiate the impacts of hydroxyl radical (OH) concentrations and emissions from wetlands and inland waters.

Main Methods:

• Utilized multiple atmospheric inversion models.
• Constrained models with observation- and model-based hydroxyl radical (OH) fields.
• Analyzed atmospheric CH4 data to determine year-on-year variations.

Main Results:

• A decline in OH radicals from 2020-2021, followed by a recovery in 2022-2023, accounted for 83% of CH4 growth rate variations.
• Wetland and inland water emissions increased between 2019 and 2020-2022, then decreased in 2023.
• Emission changes were primarily observed in northern tropical wetlands (Africa, Asia), with declines in South America and increases in the Arctic.

Conclusions:

• Changes in hydroxyl radical (OH) concentrations were the dominant driver of atmospheric methane growth rate variations.
• Wetland emissions, particularly in tropical regions, significantly influenced methane levels.
• Attributing CH4 growth rate changes requires considering both chemical and emission-based factors.