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Atmospheric methane levels have been rising markedly since the preindustrial era, punctuated only by a brief period of relative stabilization from 1999-2006. The reasons for this brief lull, and the subsequent continued growth since 2007, have been an ongoing topic of research and debate in the scientific community for nearly two decades.

A new study published in AGU Advances takes a different approach to unraveling the methane mystery, by focusing not just on methane's shifting sources, but more comprehensively on the chemistry of the atmosphere itself – specifically, the role of the hydroxyl radical (OH), a highly reactive molecule that readily converts methane to carbon dioxide and water.

"Methane trends in the atmosphere cannot be understood by looking at emissions alone," explained Jian He, lead author of the study and a research scientist with CU Boulder CIRES working at NOAA's Chemical Sciences Laboratory. "More than 90% of methane is removed by reacting with OH, so changes in OH will fundamentally affect how methane accumulates in the atmosphere."

By examining methane through the lens of OH chemistry, the researchers found a somewhat different picture than some previous studies: the rise in atmospheric methane since 2007 is likely due to a combination of increasing emissions from energy production, agriculture, and waste, which have been large enough to outweigh its removal from estimated increases in OH.

The invisible sink

Methane is a potent greenhouse gas that is emitted globally from a variety of both natural and human-caused sources, including wildfires, fossil fuel extraction, and microbial processes in wetlands, waste, and agriculture (e.g., cows, rice paddies). The amount of methane in the atmosphere is determined by a balance between these sources and the sinks that destroy it.

OH is an extremely short-lived molecule that is generated rapidly in the atmosphere when sunlight interacts with ozone in the presence of water vapor. Its high reactivity towards many other molecules in the atmosphere, including methane, means that it exists for just fractions of a second.

Since the level of OH in the air depends on sunlight, water vapor, temperature, and the mixture of other chemicals in a particular area, OH is highly variable both spatially (e.g., across latitudes, regions, and climate zones) and temporally (on diurnal and seasonal cycles). As a result, global OH values and their trends cannot be directly measured and instead must be derived from proxy measurements or simulated with global chemical models.

A chemical approach

Previous methane studies that focused primarily on observations of methane emissions in a "top-down" modeling approach normally prescribe OH concentrations, either based on monthly climatology or a specified trend from chemical transport models. The new research takes a different approach by explicitly simulating methane's dominant sink, allowing OH levels to fluctuate in response to changing climate, atmospheric chemistry, and changing emissions, including of methane itself.

He and colleagues employed a full global chemistry-climate model developed by NOAA's Geophysical Fluid Dynamics Laboratory (GFDL), focusing on the time period from 1980 to 2017. Spatially-resolved methane emissions from different source sectors (energy, biomass burning, agriculture, waste, wetlands) are implemented from emissions inventories and then optimized to be consistent with global atmospheric measurements of methane (CH₄).

The team also incorporated the chemistry of methane's stable carbon isotope, ¹³CH₄, in the model and used its observations to constrain their analysis. Different methane sources leave distinct isotopic fingerprints, providing valuable clues to help disentagle overlapping sources (e.g., biomass burning emits methane that is more ¹³C-enriched, whereas microbial processes produce ¹³C-depleted methane). Atmospheric reaction with OH also has an effect on the isotopic ratio, as OH reacts preferentially with the lighter isotope, ¹²CH₄.

"If OH changes are prescribed incorrectly, or if a prescribed total methane sink value is used instead of simulating OH or methane sinks explicitly, may draw the wrong conclusions about the importance of different methane sources," He explains.

A complex puzzle

The model results show that human activities remain the dominant driver of methane's long-term rise since the 1980s. Agricultural practices, waste management, and fossil fuel extraction all have contributed to increasing emissions.

Crucially, the results also show a long-term increase in OH levels since the 1980s, including after 2006 – a finding with major implications for how methane trends are interpreted.

Since OH depletes methane, the post-2006 rise in atmospheric methane measurements implies even larger methane emission increases. In other words, as the atmospheric OH sink of methane increases, the estimated emissions from agriculture, waste, and the energy sector must increase at a higher rate to overcome the enhanced cleanup. These findings also help explain another piece of this puzzle: the shift of atmospheric methane toward more ¹³C-depleted values since about 2007. Such a shift has been interpreted in some studies as evidence that fossil fuel emissions have remained flat while microbial sources alone, like wetlands or livestock, have caused the recent methane growth.

The new study confirms that increases in ¹³C-depleted microbial emissions from agriculture and waste emissions – especially in the tropics – are indeed a major factor, but finds that emissions from the energy sector – coal mining, oil, and natural gas – also rose after 2006. These fossil fuel emissions are isotopically heavier (more ¹³C), and their increase partially offsets the lighter isotopic signal from agricultural and waste sources. At the same time, emissions from ¹³C-enriched biomass burning in the tropics declined.

Changes in OH, specifically a stronger OH sink, also influences isotopic trends, complicating source-based interpretations. According to the authors, the differing conclusions about fossil fuel emissions may be due to differences in assumptions about atmospheric sinks.

The changes in OH also emphasize an important feedback between methane and OH: higher methane levels suppress OH, which in turn allows methane to persist longer in the atmosphere. The researchers found that this feedback has strengthened over recent decades, thereby also contributing to the accumulation of methane in the atmosphere.

Not the final word

Importantly, the study emphasizes that unraveling the global methane budget is inherently difficult. Different methodologies can lead to different conclusions, particularly regarding the spatial distribution of emissions and isotopic signatures. Further, different modeling frameworks – box models, inverse models, and chemistry–climate models – make different assumptions and can therefore reach different conclusions from the same observational data.

"There is no single ‘correct' answer yet," said coauthor Vaishali Naik from NOAA GFDL. "Different methodologies and measurements illuminate different parts of the problem."

The work highlights the importance of continued observations, not only of methane concentrations but also of isotopes and atmospheric oxidants. Without better constraints on OH and other sinks, even sophisticated top-down approaches may misattribute methane trends.

Ultimately, the message is one of caution and complexity. Methane's rise is likely not the result of a single source or process. It reflects a dynamic interplay between emissions, atmospheric chemistry, and feedbacks that amplify or dampen change.