A new paper just published in the PNAS identifies different culprits.
The growth rate of the atmospheric abundance of methane (CH) reached a record high of 15.4 ppb yr between 2020 and 2022, but the mechanisms driving the accelerated CH growth have so far been unclear. In this work, we use measurements of the C:C ratio of CH (expressed as δC) from NOAA's Global Greenhouse Gas Reference Network and a box model to investigate potential drivers for the rapid CH growth. These measurements show that the record-high CH growth in 2020-2022 was accompanied by a sharp decline in δC, indicating that the increase in CH 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.
https://www.pnas.org/doi/10.1073/pnas.2411212121
Methane (CH) is the second-most abundant anthropogenic greenhouse gas and has global warming potential (GWP) of 28 over 100 y (1); as a result, CH has consequential near-term radiative effects and is a prominent target for mitigation (2). Following a short pause in growth from 1999 to 2006, both the abundance and growth rate of atmospheric methane have been increasing (3). During 2020-2022, the observed CH growth rate reached a record high since NOAA measurements began in 1983, averaging 15.4 ± 0.6 ppb yr (4). Understanding the mechanisms driving this accelerated growth is essential for predicting its future climate impact and providing scientific support for climate mitigation strategies (2).
The carbon isotopic composition of atmospheric CH (δC) is a powerful tool for tracking the sources and sinks of atmospheric CH. Different CH sources have distinctive δC values: Microbial CH emissions (wetlands, livestock, landfills, etc.) have lower δC values (global mean of -62‰) than pyrogenic (biomass and biofuel burning, global mean of -24‰) and fossil fuel CH emissions (global mean of -45‰) (5). Various sinks of atmospheric CH also have distinctive isotopic effects. Therefore, combined observations of atmospheric CH mole fraction and δC can provide unique constraints on the changes of global CH sources and sinks during the post-2006 rapid CH growth.
The National Oceanic and Atmospheric Administration's Global Monitoring Laboratory (NOAA/GML) has been carefully monitoring the global CH burden through the Global Greenhouse Gas Reference Network (GGGRN) for over four decades. The collaboration between NOAA/GML and the Institute of Arctic and Alpine Research (INSTAAR) at the University of Colorado Boulder has enabled δC measurements from the GGGRN since 1998, currently measuring weekly or biweekly from 22 globally distributed background sites (6). The dataset has been widely used for studying the evolution of global CH sources and sinks (7-9). Here, we report our most recent observations of atmospheric CH mole fractions and δC values through the end of 2022 and then use a box model to examine and quantify the contributions of potential drivers of the record-high CH growth rate.
The rest of the paper can be found here.
This is unlikely to mollify climate zealots as agriculture is listed among the potential causes. With biological activity exploding across the world, due to CO2 fertilization, I personally wonder how much of this is increasing wetland productivity as well as flourishing soil microbes.