Reducing methane emissions from human activities is a critical element in the global fight against climate change. Although a growing number of world cities are making ambitious pledges to tackle the problem, a recent study published in the Proceedings of the National Academy of Sciences (PNAS) reveals alarming results. Using an innovative, space-based observation approach, researchers found that between 2019 and 2023, methane emissions from major cities worldwide did not decrease; instead, they showed a distinct upward trend. These devastating figures highlight the severe shortcomings of traditional, ground-based inventories and question the practical effectiveness of existing climate policies established by global networks, including the C40 cities.
Megacities as Global Hotspots for Methane Emissions
Today, more than half of the global population lives in urban areas, and these cities are responsible for nearly 70 percent of the world’s anthropogenic (human-caused) greenhouse gas emissions. Methane (CH4) is a highly potent greenhouse gas; while its atmospheric lifetime is relatively short, its impact on global warming is immense. Consequently, curbing methane emissions is a key strategic lever in limiting warming on decadal timescales.
The urban environment hosts a highly complex array of methane sources: landfills, wastewater handling facilities, and fossil fuel infrastructure—such as natural gas distribution and end-use—all contribute substantially to the global emission budget. To gain a comprehensive understanding of this issue, a research team including Erica Whiting, Genevieve Plant, and Eric A. Kort analyzed a massive dataset. The study quantified the emissions of 92 global cities (including their broader metropolitan areas), which collectively represent a population of 1.18 billion people.
Satellite Tracer Technology: In Pursuit of Real Data
Historically, the exact magnitude and source of urban methane emissions have been poorly understood. Traditional “bottom-up” inventories typically underestimate real emissions. Furthermore, previous ground-based or airborne measurements were geographically and temporally limited, often focusing exclusively on specific cities in North America or Western Europe over short periods.
To fill this observational gap, the researchers employed a space-based “tracer-tracer” approach utilizing the TROPOMI (TROPOspheric Monitoring Instrument) on board the European Space Agency’s Sentinel-5 Precursor satellite. The method leverages the concentration ratio between two trace gases: methane and carbon monoxide (CO). While they are not necessarily co-emitted from the exact same surface sources, both are emitted within the urban domain and are impacted by the same meteorology. Because CO emissions are relatively well quantified and reported, this ratio allows scientists to reliably infer actual CH4 emissions using satellite data, without relying on highly complex atmospheric transport models.
Alarming Figures: 31.2 Teragrams of Methane Annually
The quantitative results of the research clearly demonstrate the massive volume of these emissions. The aggregate methane emissions of the 92 studied cities reached a staggering 31.2 teragrams (Tg CH4/y) in 2023 (with a 95% confidence interval ranging from 22.3 to 40.4 Tg).
This volume is equivalent to approximately 10 percent of the entire global anthropogenic methane budget. To put the severity of this data into perspective, this urban emission value is 3.75 times the total contribution of the recently reported and highly scrutinized oil and gas “Ultra-Emitters.”
Is the C40 Climate Network Failing? Growth Instead of Decline
One of the primary drivers of global urban climate policy is the C40 network, founded in 2005, which currently comprises 97 member cities. These cities have pledged to significantly cut their greenhouse gas emissions. The C40-supported UN Race to Zero campaign specifically calls for a reduction in methane emissions of at least 34 percent by 2030.
The researchers tracked the emissions of 72 of these cities (51 C40 cities and 21 non-C40 cities) continuously from 2019 to 2023. The results showed a weak decline in 2020 (likely due to pandemic-related lockdowns), followed immediately by steady, relentless growth. Over the four-year period, the aggregate methane emission from these 72 cities increased by 2.3 Tg.
Satellite observations stand in stark contrast to ambitious political promises. According to the measurements, methane emissions in C40 cities—which have pledged that 34% reduction—actually grew by a significant 10 percent (95% CI: 2%, 17%) between 2020 and 2023. This growth rate is statistically similar to the 12 percent (95% CI: -1.5%, 25%) increase observed in non-C40 cities outside the climate network.
Space-Based Observations as the Foundation for Future Climate Policy
This scientific outcome paints a troubling picture: current official emission inventories completely fail to capture this observed growth. This suggests that urban emissions have been poorly characterized and that local mitigation policy interventions are likely not optimally designed or implemented.
The clear message of the PNAS study is that for the C40 network and other major cities to achieve their 34% reduction target by 2030, steep and rapid interventions will be required. At the same time, the study proves that space-based TROPOMI observations offer an accurate and effective tool for ongoing monitoring. In the future, this technology will play a crucial role in determining whether a city’s climate protection measures are actually delivering tangible environmental benefits, or if success only exists on outdated statistical spreadsheets.
Reference and Official Source:
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Original scientific publication (PNAS): Space-based observation of global increase in urban methane emissions from 2019–2023
