Rapid shift in methane carbon isotopes suggests microbial emissions drove record high atmospheric methane growth in 2020-2022.

Abstract

The growth rate of the atmospheric abundance of methane (CH₄) reached a record high of 15.4 ppb yr^-1 between 2020 and 2022, but the mechanisms driving the accelerated CH₄ growth have so far been unclear. In this work, we use measurements of the ¹³C:¹²C ratio of CH₄ (expressed as δ¹³C_CH4) from NOAA's Global Greenhouse Gas Reference Network and a box model to investigate potential drivers for the rapid CH₄ growth. These measurements show that the record-high CH₄ growth in 2020-2022 was accompanied by a sharp decline in δ¹³C_CH4, indicating that the increase in CH₄ abundance was mainly driven by increased emissions from microbial sources such as wetlands, waste, and agriculture. We use our box model to reject increasing fossil fuel emissions or decreasing hydroxyl radical sink as the dominant driver for increasing global methane abundance.