An international team of researchers from Australia’s Commonwealth Scientific and Industrial Research Organisation (CSIRO), the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), France’s Climate and Environment Sciences Laboratory and other research centers has found a new way to estimate global methane emissions with higher accuracy. To that end, the scientists combined satellite observations of atmospheric methane concentrations with ground-based measurements of its isotopic variants, the so-called isotopologues. This allowed them not only to see where the gas comes from but also to understand its origin, be it wetland-, agriculture- or fossil fuel-related.
Methane is a key greenhouse gas accounting for about 30% of global warming since the beginning of the industrial era. It breaks down rapidly in the atmosphere, within 12 years on average, during which time it warms the climate 80 times more than carbon dioxide. Between 2020 and 2022, its concentration in the atmosphere rose at a record rate of 15–18 parts per billion per year, twice as fast as in the previous decade. Until now, experts have debated the causes of this surge: a decrease in hydroxyl radicals (the chief cleaner of the atmosphere), rising natural emissions from tropical wetlands or increased emissions from agriculture and fossil fuel production.
To find out the causes, the researchers have used a complex reverse transport model (4D-Var), which takes into account the movement of air masses, making it possible to identify the location and quantity of the gas emitted from the observed concentration.
The key innovation of the study is the use of two isotopic labels: carbon (δ13C) and hydrogen (δD). Methane from different sources has distinctive signatures: gas formed in wetlands as a result of microbial activity is lighter in isotopic composition, while methane from biomass combustion or leaks during oil and gas production is heavier. By measuring these ratios in air samples from ground stations and comparing them with satellite data from the TROPOspheric Monitoring Instrument (Tropomi, a spectrometer mounted on the European Space Agency’s Sentinel-5P satellite), the researchers managed not only to estimate the total volume of emissions but also to separate them by origin. This allowed them to distinguish the signatures of different sources.
Calculations showed that global methane emissions in 2019–2021 were higher than previous inventory estimates, totaling some 623 million tons per year. The use of isotopic data increased this estimate by about 21 million tons compared to calculations based solely on methane concentration. Moreover, the geography of emissions turned out to be different than previously assumed: estimates increased by 26 million tons per year in China, by 7 million tons in India and by 5 million tons in Central Africa. In contrast, estimates fell by 5 million tons for the Amazon basin and by a total of some 12 million tons for the rest of the world.
One of the most revealing conclusions has to do with China. It was previously believed that the closure of coal mines is reducing the contribution of fossil fuels to emissions, with an increasing contribution from rice cultivation. However, isotopic analysis has revealed a more complex picture. The heavy isotopic composition of methane indicates a significant contribution from fossil sources, i.e., production and transportation of coal, oil and gas. Meanwhile, agriculture turned out to be an underestimated source in India, with wetlands and oil-and-gas activities underestimated in Central Africa.
There are also fascinating conclusions about seasonality. While conventional estimates show peak emissions in August, the new model accounting for isotopes shifts the peaks to September. This indicates longer-term methane emissions from tropical wetlands and later agricultural cycles in Southeast Asia. The scientists also found that seasonal emission fluctuations in China are weaker than previously thought. This further supports the idea that a significant portion of emissions is related to year-round activity in the fuel-and-energy sector.
