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Exemplar |
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Because of fossil fuel emissions, methane steadily increased in the air in the 1980s until it reached an equilibrium when the rate of methane entering the atmosphere was similar to the rate of removal by oxidation. Atmospheric methane began to resurge in 2007, primarily because of emissions driven by wetlands, agriculture, and landfills. One part per billion (ppb) of atmospheric methane is equivalent to 2.8 million tonnes entering the atmosphere. M. HERSHER/SCIENCE
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Ciais, P et al. Why methane surged in the atmosphere during the early 2020s. Science 391 (2025). 10.1126/science.adx8262 |
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Solving the mystery of a methane surge |
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Jesse Smith, Senior Editor, Science |
Methane is the second most important radiative forcing agent among all the anthropogenic trace greenhouse gases, with a climate warming effect second only to carbon dioxide. Therefore, it is important to understand why its atmospheric concentration changes. Its global atmospheric abundance is growing steadily but not monotonically, and the question of what factors are responsible for the variability the methane growth rate is a key one.
The long-term secular rise of atmospheric methane concentrations is, unsurprisingly, due largely to anthropogenic methane emissions from agriculture, the oil and gas industry, and waste management, which together contribute more than half of all global methane emissions. The shorter-term ups and downs that occur at a subdecadal timescale are more difficult to understand, though. Many factors have the potential to influence atmospheric methane concentrations, including changes in emissions from agriculture, wetlands, permafrost, wildfires and the oil and gas industry, which themselves depend on drivers such as climate change, rainfall changes caused by El Niño/La Niña, and economic factors, all of whose impacts can be difficult to attribute and untangle from each
other.
The approach used by Ciais et al. is one way to try to do that
. What the authors did was to look at the atmospheric methane record for the period since 2010, with particular attention on the latest wobble in a wobbly history: the puzzling surge that occurred after the beginning of 2019 and continued until the end of 2020. They concluded that the primary reason for the rapid increase in the growth rate was not what was going into the atmosphere but what was coming out. As is true for any given constituent of any dynamic reservoir, the atmospheric abundance of methane reflects the balance of input and output—the water level in a bathtub depends on how fast water is added from the faucet as well as how fast it is going down the drain.
In this case, they found that methane surged mostly because its consumption by hydroxyl radicals (OH) declined due to a drop in atmospheric OH concentration. OH is the primary atmospheric oxidant, responsible for the destruction of most organic air pollution like methane, so if its abundance decreases then its effectiveness as an atmospheric “detergent” also diminishes. Atmospheric OH concentrations are very difficult to measure directly, however, so the authors determined them using atmospheric chemistry models and determinations of the atmospheric abundances of key OH precursors, which (like methane) are easily measured, as well as by using estimates of the emissions of those precursors. Using that information, they showed that between 2020 and 2022, the
concentration of atmospheric OH decreased enough to account for more than 80% of the methane concentration growth rate change, while emissions from wetlands and inland waters, plus contributions from agriculture and waste management, explained the balance.
The subsequent atmospheric methane decrease that occurred in 2022 and 2023 was due to the opposite combination: an increase in OH and a decrease in emissions. The authors also provide a good regional breakdown of the magnitudes and processes that caused these methane changes, and a model-grid-box-scale picture of methane emission anomalies for the different major sources. So, one study at a time, we are understanding what factors are driving variations in atmospheric methane concentration, and how to quantify them better. |
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