Improved Constraints on Global Methane Emissions and Sinks Using δ ¹³C-CH₄.

Area of Science:

• Atmospheric Chemistry
• Climate Science
• Earth System Science

Background:

• Global atmospheric methane (CH4) concentrations have risen significantly since 2006.
• The precise drivers of this increase remain debated, with hypotheses including changes in emissions and atmospheric sinks.

Purpose of the Study:

• To investigate the primary drivers of the post-2006 global atmospheric methane increase.
• To test competing emission and sink hypotheses using a unified model framework.

Main Methods:

• Simulated CH4 and isotopic (δ13C-CH4) fields using the TM5 tracer transport model for 1984-2016.
• Compared model outputs with observational data to evaluate proposed emission and sink scenarios.
• Assessed the impact of tropospheric chlorine (Cl) and wetland emissions on CH4 budgets.

Main Results:

• Fossil fuel (FF) CH4 emission trends did not align with observed δ13C-CH4, suggesting FF emissions are not the dominant post-2006 driver.
• A decrease in hydroxyl radicals (OH) could not explain the observed CH4 increase or isotopic trends.
• Tropospheric Cl and dynamic wetland emissions significantly influenced CH4 partitioning and fossil emission estimates.

Conclusions:

• The post-2006 methane increase is likely driven by factors other than solely increased fossil fuel emissions.
• Uncertainties in CH4 sinks, such as tropospheric chlorine, and the representation of wetland emissions significantly impact budget calculations.