Reduced methane growth rate explained by decreased Northern Hemisphere microbial sources

Area of Science:

• Atmospheric chemistry and climate science.
• Biogeochemical cycles and greenhouse gas emissions.

Background:

• Atmospheric methane (CH4) levels rose through the 20th century, then slowed before increasing again.
• Previous studies cited fossil fuels, wetlands, agriculture, and hydroxyl (OH) sinks as potential causes for the slowdown.

Purpose of the Study:

• To investigate the primary drivers of the late-twentieth-century slowdown in atmospheric methane growth rates.
• To differentiate contributions from various methane sources and sinks using isotopic analysis.

Main Methods:

• Analysis of synchronous time series of atmospheric CH4 mixing ratios and carbon isotope ratios ((13)C/(12)C).
• Utilized a two-box atmospheric model to simulate CH4 evolution.
• Compared observed isotopic changes with modeled scenarios of source and sink variations.

Main Results:

• Reduced microbial sources in the Northern Hemisphere best explain the observed CH4 growth rate changes.
• Isotopic data exclude reduced fossil fuel emissions as the main cause of the slowdown.
• Northern Hemisphere agricultural emissions, especially from rice cultivation in Asia, account for approximately 51% of the emission decrease, linked to fertilizer use and water management.

Conclusions:

• The late-20th century atmospheric methane slowdown was primarily driven by decreased microbial emissions in the Northern Hemisphere.
• Changes in Asian rice agriculture significantly contributed to this reduction.
• Further research into microbial methane sources and agricultural practices is warranted.