A team at Osaka Metropolitan University, led by Associate Professor Masahito Ueyama, published the findings in Atmospheric Chemistry and Physics in October 2025, after doing mobile measurements (on car- and bicycle-mounted sensors) and eddy covariance (EC) measurements in Osaka and nearby Sakai.
They discovered that total methane (CH₄) emissions from these urban areas are far above those reported in local government emission inventories. In Osaka city, the emissions measured were about 18 times greater than those in official inventories; in Sakai, about 2.5 times greater.
They also separated out what sources are responsible. Natural gas leakages, biological sources (like sewage, compost, etc.), industrial/commercial sources (restaurants, factories), and even more “local special sources” turned out to matter more than inventories assumed.
Key Findings: What Was Overlooked
Here are some of the overlooked or underestimated sources in Osaka and what the study found about them:
- Natural Gas Emissions
Lahore systems—city gas pipelines, buildings and utility infrastructure—leaked more methane than official reports accounted for. In Osaka, natural gas accounted for about 64% of total CH₄ emissions; in Sakai it was around 47% based on mobile measurements. Bicycle-based measurements tended to report even higher natural gas proportions (~ 75%) in some areas. - Biological and “Soft” Sources
Biological sources like sewage treatment plants, compost piles, dairy farms, and reservoirs were identified as significant contributors. Additionally, some very local and unusual sources appeared: water-filled ditches around ancient burial mounds (kofun), sites of fermented food production, etc. These were not well-captured in the inventories. - Spatial Distribution & Leak Intensity
Many of the leaks or elevated methane concentrations were from small leakage indicators (LIs) rather than large point sources (e.g., massive factory vents). So rather than a few big offenders, many smaller leaks across a wide area contribute significantly. - Discrepancies by City and Density
More urbanized areas like Osaka city itself showed higher emissions than less densely urban Sakai, both in natural gas and biogenic emissions. Also, features like density of natural gas usage and the number and capacity of sewage treatment plants scale with emission. - Inventory Gaps and Underestimation
Inventories tend to undercount or omit many of these small or dispersed sources, either because they are hard to locate or because existing measurement methods miss them (for example, measuring emissions only from known point sources, or relying on estimation methods that assume many sources behave uniformly).
Measurement Methods: How They Did It
The study combined two strong methods to get more reliable numbers:
- Mobile measurements: cars and bicycles carrying methane and ethane sensors were driven through different parts of the cities. This allowed mapping of methane concentrations across streets, neighborhoods, industrial zones, etc. It helped identify many small leakage sources.
- Eddy covariance (EC) flux measurements: this method captures the net exchange of gases between a surface and the atmosphere over a larger “footprint”. It provides an integrated flux (i.e. emissions over area) that can calibrate and validate upscaled emission estimates from mobile data.
By using both, they could “scale up” the mobile measurement-derived emissions to approximate what Osaka’s total urban emissions look like. And the result: big gap between what those measurements show and what inventories list.
Implications: Why This Matters
These findings matter a lot for climate policy, urban planning, and greenhouse gas mitigation. Here are some key implications:
- Methane is a potent greenhouse gas: Over short periods (20 years), methane traps heat much more strongly than CO₂. So underestimating it means underestimating near-term warming risk.
- Inventories need updating: If local government inventories are missing major sources, then policies based on those inventories might not target the real hotspots. Mitigation efforts could be misdirected or less effective.
- Regulation of natural gas infrastructure: Since natural gas leakage is a large share, cities like Osaka might need better leak detection, pipeline maintenance, regulation of appliances/buildings, etc.
- Policy on biogenic sources: Sewage treatment, composting, food processing, etc. Policies may need to consider these “soft” sources more closely.
- Methods for measuring methane: This research shows the value of “on-the-ground” mobile sensors + EC methods. Using only reported/fixed sources or broad regulatory inventories can undercount hidden emissions. Similar hidden sources likely exist in many other cities.
Challenges and Uncertainties
While the study is strong, it also highlights where more work, verification, and caution are needed:
- Temporal coverage: Measurements happened over specific time periods. There may be seasonal variation (e.g. wetter vs drier months) not fully captured.
- Spatial coverage: While many areas were covered, some zones may still be off the grid or less accessible. Some point sources (like high smokestacks, distant facilities) may not get well detected by ground mobile sensors.
- Upscaling assumptions: Scaling from sample measurements to whole-city emissions always involves assumptions; they used EC fluxes to calibrate, which helps, but uncertainties remain.
- Detection limits: Small leaks may still fall below detection thresholds; also, distinguishing between sources (natural gas vs biogenic) via gas ratios (methane/ethane etc.) has uncertainties.
Despite these, the gap between measured emissions and inventory estimates is large enough that these uncertainties don’t explain it away. In other words, even accounting for uncertainties, Osaka’s methane emissions are underreported.
What Can Be Done: Mitigation and Policy Responses
Given the findings, several steps could be taken to better manage methane emissions in Osaka and in other cities that may have similar underreporting issues:
- Enhanced Monitoring and Mapping
- Expand mobile sensor surveys to more neighborhoods and times (including off-peak, night, wet weather, etc.).
- Use aerial or satellite instruments to detect large sources and cross-validate ground data.
- Incorporate data from eddy covariance and other “top-down” atmospheric methods.
- Inventory Revision and Transparency
- Local governments should revise their emission inventories incorporating the newly identified sources.
- Public reporting of methane emissions should include natural gas leaks, biogenic and agricultural sources, and industrial/commercial small sources.
- Leak Detection and Repair Programs
- Identify and fix leaks in gas pipelines, valves, meters, residential and commercial buildings.
- Strengthen regulation on installations using natural gas.
- Regulation of Biogenic Sources
- Tighten regulation or monitoring of sewage treatment plants (especially their anaerobic digestion processes, sludge storage etc.).
- Look at industrial/commercial food fermentation, composting, dairy farms, etc. to see where emissions can be reduced (e.g. cover storage, improve processing).
- Urban Planning and Infrastructure Improvement
- Improve waste management and sewage infrastructure so that methane leaks are minimized.
- Design water bodies or ditches (including those with historical/ancient features) to reduce standing water that can encourage methane production.
- Public Engagement and Awareness
- Inform the public about natural gas leaks, encourage reporting.
- Raise awareness about “fermented food industries” or other small sources that may not be obvious but contribute.
- Policy Instruments and Incentives
- Financial or regulatory incentives for companies to repair leaks or retrofit infrastructure.
- Support research and funding for better measurement technologies.
- Methane reduction targets integrated into broader climate and air quality policies.
Broader Context: Osaka Findings in Global Perspective
The Osaka findings fit within a growing body of research showing that many urban or national emission inventories undercount methane. Similar underestimates have been found for landfills, waste sectors, and urban areas in the U.S. (e.g. studies by Harvard etc.).
What Osaka reveals is that even in well-monitored, developed settings, small dispersed sources + natural gas leaks + some culturally specific sources (like fermented foods, local biogenic features) can add up meaningfully.
Conclusion
The study from Osaka shines a light on how much methane may be slipping through the cracks of current emission inventories. Natural gas leaks, biological sources, small commercial and industrial emitters, and unusual local contributions are all under-recognized.
Given methane’s potency, especially in near-term climate forcing, these blind spots matter. If Osaka underreports its emissions by nearly an order of magnitude, many other cities may be doing so too.
To meet climate goals and reduce warming, cities and governments will need to update inventories, strengthen measurement, correct for under-accounted sources, and implement mitigation strategies.
Only with a clearer, more complete picture can policy become effective — and we may then finally reduce one of the most powerful short-term contributors to global warming.
