An aerial view of a landfill site, a typical source of urban methane emissions.
Urban emissions of methane — a potent greenhouse gas — are rising faster than “bottom-up” accounting estimates anticipated, according to new satellite imagery, suggesting a significant blind spot in global mitigation efforts.
The discrepancy was identified using satellite measurements over 92 major cities worldwide, with sufficient data from 72 cities to track changes between 2019 and 2023. The findings show that global urban methane emissions in 2023 were 6% higher than 2019 levels and 10% higher than 2020 levels, although emissions declined in some European cities.
The study, published in Proceedings of the National Academy of Sciences and led by the University of Michigan Engineering, was funded by NASA and the National Institute of Standards and Technology.
By contrast, conventional accounting methods — which estimate emissions by aggregating individual methane sources — suggest urban emissions have increased by only 1.7% to 3.7% since 2020.
The research included more than half of the cities in the C40 Cities Climate Leadership Group, which are working towards net-zero emissions by 2050. Across those cities, methane emissions in 2023 were also 10% higher than 2020 levels, equating to an additional 2 teragrams per year — around 30% of their targeted reductions.
Researchers warn that this gap between observed emissions and official inventories could undermine the effectiveness of city-level climate policies.
“In order to reduce greenhouse gas emissions and set good emissions policy, cities need to know how much they are emitting and what those sources are. But there is quite a bit of uncertainty with that for methane,” said Eric Kort, corresponding author of the study.
Accounting gap raises policy concerns
Methane, which can be released from leaking gas infrastructure, landfills and wastewater systems, is around 80 times more potent than carbon dioxide over a 20-year period.
The study builds on previous work by Kort and colleagues highlighting underestimation of methane emissions across sectors. Earlier research found that flaring at oil and gas sites releases five times more methane than previously estimated, while offshore production may have double the reported climate impact.
Previous aerial studies in 2019 also suggested that several major US cities were emitting more methane than official figures indicated — a trend the latest research shows is global.
“Cities have the motivation and power to reduce greenhouse gas emissions and therefore, present significant opportunities for impactful emissions reduction,” said Erica Whiting, the study’s first author. “However, there was not previously a method to quantify and monitor urban methane emissions around the globe and, therefore, no observation-based method to evaluate emission reduction strategies.”
The analysis suggests that urban areas accounted for around 10% of global human-caused methane emissions in 2023, with city emissions nearly four times higher than so-called oil and gas “ultra emitters” that have been the focus of previous mitigation efforts.
Satellite monitoring reveals hidden emissions
The findings are based on data from the TROPOMI instrument aboard the Copernicus Sentinel-5 Precursor satellite, launched in 2017 to monitor atmospheric pollution.
TROPOMI measures sunlight reflected from the Earth’s atmosphere across multiple wavelengths, allowing scientists to determine concentrations of gases such as methane at a city scale.
However, its resolution is not sufficient to pinpoint specific emission sources within urban areas. Researchers say higher-resolution satellite systems could help identify major contributors such as individual landfills or industrial facilities.
“We, and others in the field, are looking into higher-resolution satellite measurements so that we can tease apart the contribution of large localized sources,” Kort said. “Those satellites can’t necessarily tell you the whole city’s emissions, but they could tell you what individual landfills or facilities are doing.”
The findings highlight the need for improved monitoring tools to ensure urban climate policies are based on accurate emissions data, particularly as cities play an increasingly central role in global decarbonisation efforts.
