The stakes could hardly be higher. Methane is the second most significant anthropogenic greenhouse gas after carbon dioxide and is responsible for roughly one third of the rise in global mean surface air temperature between 1750 and 2019. Because it has a short atmospheric lifetime of about nine years but a global warming potential nearly 82 times that of carbon dioxide over a 20-year period, cutting methane emissions is one of the most powerful levers available for slowing near-term climate change. More than 150 countries have now committed, through the Global Methane Pledge and their Nationally Determined Contributions, to reduce methane emissions across all sectors by at least 30 percent below 2020 levels by 2030. Every credible pathway to limiting warming to 1.5 degrees Celsius depends on achieving that reduction.

Yet cities, where more than half of humanity already lives and where the proportion is projected to reach 70 percent by 2050, remain stubbornly difficult to audit. Urban methane comes from a tangle of sources: natural gas consumption, leaks from aging pipeline networks, landfills and wastewater treatment systems. Field studies have repeatedly found that official bottom-up inventories, which rely on activity data and emission factors, can underestimate urban methane emissions by a factor of two to three. Aircraft surveys, ground-based mobile monitors and tower networks can deliver accurate snapshots of a city’s emissions, but they cannot provide the continuous, repeatable monitoring needed to track progress year after year, and their spatial sensitivity is often limited.

Satellites promise to close that gap. The TROPOspheric Monitoring Instrument, or TROPOMI, flies aboard the polar sun-synchronous Sentinel-5 Precursor satellite, launched in October 2017 at an altitude of 824 kilometers with a consistent 13:30 local overpass time. The instrument retrieves the methane column by detecting solar backscattered light in the shortwave infrared absorption band at 2.3 micrometers, achieving daily global coverage at a spatial resolution of 7 by 5.5 square kilometers at nadir since August 2019. Previous studies have used TROPOMI to constrain urban methane inventories across 61 cities worldwide and to conduct a preliminary national analysis of urban emissions across North America, but a transparent, widely applicable framework with detailed error accounting has remained elusive.

The Manchester team, led by Huihui Long together with Maria Tsivlidou, Hugo Ricketts and Grant Allen, built their method on the concept of mass balance, refining the established source pixel approach. First, they remapped the heterogeneous Level-2 satellite pixels onto a regular 0.1 by 0.1 degree grid, treating each observation footprint as a surface polygon and calculating the fractional area of overlap with each grid cell, weighted by pixel area and retrieval error. This regularization allows orbit-level emission estimates that are directly comparable with gridded bottom-up inventories over the same spatial domain. Strict quality filters followed: only cloud-free retrievals with quality assurance values above 0.5, solar zenith angles below 70 degrees, smooth topography, low aerosol optical thickness and retrieval precisions better than 10 parts per billion were retained.

Two innovations distinguish the new framework. The first concerns wind. Conventional source pixel methods rely on 10-meter near-surface winds bundled with the satellite retrievals, but methane emitted at the surface is ventilated by winds throughout the planetary boundary layer, the turbulent lowest kilometer or so of the atmosphere where surface emissions mix and are advected across urban scales. The team therefore computed pressure-weighted average wind speed and direction across all ERA5 reanalysis pressure levels within the boundary layer, using hourly-resolved meteorology. The difference matters: over Los Angeles, the near-surface winds pointed from the north-northeast to east-northeast at speeds below 6 meters per second, while the boundary-layer-weighted winds blew predominantly from the west-southwest to south-southwest at up to about 10 meters per second. Since emission rate is conceptually proportional to mean boundary-layer wind speed, and wind direction determines where the upwind background lies, these discrepancies directly reshape the calculated emissions.

The second innovation is an upwind-based approach to defining the background methane concentration. Instead of assuming that the air surrounding a city is clean, the method defines a background region of equal area located immediately adjacent to the city along the boundary-layer-pressure-weighted mean upwind direction, with both boxes oriented so their sides lie perpendicular to the wind. Emissions in the background are implicitly accounted for and subtracted, so the background region need not be free of methane sources. The methane enhancement over the city is then the difference between the mean column mixing ratio over the source region and that over the upwind background, and the emission rate follows from a mass balance equation combining that enhancement, the mean boundary-layer wind, surface pressure and the dimensions of the source box. Coverage thresholds, requiring at least 25 percent valid data over the source region and at least 10 background observations, guard against statistically unreliable single-orbit estimates.

Applied to the three megacities, the method produced three-year mean emissions of 7.78 plus or minus 4.84 tonnes per hour for London, 47.19 tonnes per hour for Los Angeles, and 42.94 tonnes per hour for New York. London showed only a modest urban enhancement, averaging between 1.47 and 1.98 parts per billion across the study years, which made the estimate highly sensitive to background variability; roughly 80 percent of the total emission uncertainty there was attributable to the methane enhancement, and an anomalously low October in 2021, when background concentrations exceeded urban ones, dragged the annual figure down until it was excluded as an outlier. Los Angeles, by contrast, displayed a pronounced and persistent hotspot, with emissions rising from 26.21 tonnes per hour in 2021 to 62.77 tonnes per hour in 2023, though the large uncertainties mean a previously reported declining trend cannot be ruled out. In New York, emissions ranged from 30.85 to 44.77 tonnes per hour, with a more balanced uncertainty structure in which wind variability contributed about 40 percent.

The comparison with official inventories is sobering. Over identical spatial domains, EDGAR reported a mean of 20.07 tonnes per hour for Los Angeles, where landfills account for more than 90 percent of inventory emissions, and just 4.38 tonnes per hour for New York, roughly 11 percent of the satellite-derived figure. The UK’s National Atmospheric Emission Inventory underestimated London’s emissions by up to 45 percent in 2022, while the US EPA inventory fell short by approximately 56 percent for Los Angeles and 84 percent for New York. Encouragingly, when the team rescaled estimates from six previous top-down studies to the same urban domains, most fell within their uncertainty bounds, suggesting the new method is consistent with independent measurement-led approaches while being far simpler to repeat.

The authors are candid about the limitations. Cloud cover, coastal albedo heterogeneity and aerosol scattering eliminated the majority of overpasses, leaving as few as five valid orbits over New York in 2021 and almost no usable summer data for Los Angeles, where persistent marine boundary-layer clouds dominate. Such uneven sampling can introduce seasonal and inter-annual biases that must be weighed when comparing annualized emissions with inventories. Nevertheless, the team argues that the trade-off is necessary to preserve statistical reliability, and that the framework’s transparent uncertainty budgeting, combining retrieval, albedo, aerosol and wind uncertainties in quadrature, offers a repeatable path forward. With the Global Methane Pledge’s 2030 deadline approaching, the ability to audit cities from space, anywhere on Earth including poorly observed regions such as India and China where most of the global population resides, may prove essential for verifying whether stated methane targets are actually being met.

Subject of Research: Satellite-based quantification of urban methane emissions using TROPOMI observations and an improved source pixel method

Article Title: Satellite-based global monitoring of urban-scale methane emissions

Article References: Long, H., Tsivlidou, M., Ricketts, H., & Allen, G. (2026). Satellite-based global monitoring of urban-scale methane emissions. Atmospheric Measurement Techniques, 19(18), 6171-6191. https://doi.org/10.5194/amt-19-6171-2026

Image Credits: AI Generated

DOI: 10.5194/amt-19-6171-2026

Keywords: methane, TROPOMI, satellite monitoring, urban emissions, greenhouse gases, London, Los Angeles, New York, planetary boundary layer, EDGAR inventory, climate change, Global Methane Pledge

Tracing methane’s provenance often involves analyzing the isotopic composition of its constituent carbon and hydrogen atoms. These isotopes serve as molecular fingerprints, enabling scientists to differentiate methane derived from biological activities, fossil fuel extraction, or other biogeochemical processes. However, the nuances of how microbial enzymatic activity influences these isotopic ratios have not been fully elucidated—until now.

In a groundbreaking study led by researchers at the University of California, Berkeley, a team has demonstrated for the first time how variations in the expression of a crucial microbial enzyme—methyl-coenzyme M reductase (MCR)—significantly influence the isotope signature of the methane produced by methanogenic archaea. This research bridges molecular biology with isotope geochemistry, offering novel insights that could radically enhance our ability to map methane flows and thus target reduction efforts with greater precision.

Jonathan Gropp, a UC Berkeley postdoctoral fellow and first author on the study, notes that while global carbon dioxide budgets have become increasingly refined through well-established traceability methods, methane remains fraught with uncertainty. “When we integrate all carbon dioxide sources and sinks, the overall budget aligns closely with atmospheric measurements. But methane fluxes exhibit broad margins of error, sometimes off by tens of percent, which hampers our understanding of their changing contributions and dynamics over time,” Gropp explained.

Central to the new research is the enzymatic process mediated by MCR, a protein complex that directly catalyzes the final step in microbial methane production. The team employed CRISPR gene-editing techniques to modulate MCR activity within Methanosarcina acetivorans, a representative methanogen capable of utilizing a variety of substrates—including acetate and methanol—to produce methane. This methodological innovation allowed the scientists to experimentally mimic environmental conditions where substrate scarcity limits enzymatic activity, thus providing unprecedented insight into how microbes respond at a molecular level.

Dipti Nayak, assistant professor of molecular and cell biology at UC Berkeley and co-author of the study, emphasized the novelty of integrating molecular manipulation with isotopic measurements. “This work is the first to marry molecular biology tools like CRISPR with isotope biogeochemistry to decode how methanogen biology shapes methane’s isotopic fingerprint,” she said. The findings challenge prevailing assumptions that the isotope signature of methane is dictated solely by the organism’s carbon source.

Isotopes—variants of elements differing in neutron number—play a crucial role in tracing environmental processes. For instance, the relative abundances of carbon-12 and carbon-13, along with hydrogen isotopes hydrogen-1 and deuterium (hydrogen-2), in methane molecules can vary depending on biological pathways and environmental contexts. Traditionally, scientists have interpreted these isotopic patterns as static markers tied to the substrate. However, this new work reveals a dynamic interplay involving cellular enzyme activity directly influencing isotopic ratios.

Geochemist Daniel Stolper, co-author and associate professor of earth and planetary science at UC Berkeley, elucidated the experimental observations. The decreased expression of MCR induced a cascade where other enzymes inside the methanogen’s metabolic network began operating concurrently in forward and reverse directions. This cycling facilitates isotopic exchanges, notably the incorporation of hydrogen atoms derived from intracellular water into the methane molecule, thereby altering the isotope signature away from the expected substrate-derived fingerprint.

Such enzyme-mediated isotope exchange affirms that in natural settings, where methanogens face variable nutrient limitation and environmental stresses, isotopic fingerprints become more complex than laboratory-derived standards have accounted for. “This variability implies that the contribution of methane from acetate-utilizing microbes may have been underappreciated in global methane budgets,” Gropp suggested. Recognizing these microbial physiological responses may refine models that apportion methane sources and improve climate mitigation strategies.

Beyond ecological implications, the research holds promising applications in biotechnology. By adjusting the expression of MCR and potentially rerouting electron flow within methanogens, scientists could suppress methane production in favor of producing alternative, more environmentally benign bioproducts. Nayak envisions engineering methanogens as biological factories that redirect carbon and electrons away from methane emission, thereby addressing climate change through synthetic biology.

The study not only pioneers a methodological advance by employing CRISPR gene editing to manipulate enzyme networks in archaea—a domain less accessible to genetic tools—but also opens exciting avenues for exploring other isotope systems within microbial biochemistry and geobiology. Stolper expressed optimism that coupling molecular biology with isotope geochemistry will yield a transformative understanding of how biological processes mediate Earth’s chemical cycles.

Published in the prestigious journal Science on August 14, 2025, the research underscores the importance of interdisciplinary approaches to untangle the complexities of methane biogeochemistry. By revealing how gene expression variations in methanogens influence isotopic signatures, this work equips scientists with enhanced diagnostic tools critical for tracking methane emissions and informing policies addressing global warming.

This scientific breakthrough heralds a paradigm shift in methane source attribution and highlights the urgent need to incorporate microbial physiological variability into global methane flux inventories. As methane concentration continues its upward trajectory and accelerates climate change impacts, such nuanced insights into microbial methane formation represent vital progress toward sustainable environmental management.

Subject of Research: Methane production by methanogenic archaea and how modulation of methyl-coenzyme M reductase enzyme expression affects the isotopic composition of microbial methane.

Article Title: Modulation of methyl-coenzyme M reductase expression alters the isotopic composition of microbial methane

News Publication Date: 14-Aug-2025

Web References:
DOI: 10.1126/science.adu2098

Image Credits: Alienor Baskevitch/UC Berkeley

Keywords: methane, methanogens, isotope geochemistry, methyl-coenzyme M reductase, CRISPR gene editing, greenhouse gases, microbial metabolism, climate change, stable isotopes, microbial biochemistry, environmental microbiology