Satellite Data Reveals Faster Rise in Urban Methane Emissions

A new study using satellite data indicates that methane emissions from major cities worldwide increased by 6% between 2019 and 2023. The observed increase is higher than estimates derived from traditional accounting methods. The research highlights a significant discrepancy between top-down atmospheric measurements and the bottom-up inventories used by many cities to track and manage their greenhouse gas emissions.

Study Findings and Discrepancy

A research team from the University of Michigan analyzed data from the TROPOMI satellite instrument, measuring methane concentrations over 92 major global cities.

• For 72 cities with sufficient data, methane emissions in 2023 were measured to be 6% higher than in 2019 and 10% higher than in 2020.
• In contrast, accounting-based estimates—which compile reported data from known sources like landfills and gas infrastructure—suggested urban methane emissions rose between 1.7% and 3.7% since 2020.
• Emissions trends varied by region. The data indicated a tendency for decreased methane emissions in European cities during the study period, while emissions increased in other regions.

"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."
— Eric Kort, corresponding author of the study

Implications for City Climate Targets

The study included over half of the cities in the C40 network, a group of municipalities committed to achieving net-zero emissions by 2050.

• Total methane emissions across the studied C40 cities in 2023 were 10% higher than 2020 levels.
• Researchers estimate that, based on the satellite data, C40 cities will need to account for an additional 2 teragrams of methane emissions annually. This amount represents approximately 30% of their stated emissions reduction targets for the gas.
• The study suggests urban methane emissions accounted for an estimated 10% of all human-caused methane emissions in 2023. This volume was nearly four times higher than emissions from oil and gas "ultra emitters," a category of exceptionally high-point sources.

Research Methodology and Context

• The measurements were taken by the TROPOMI (TROPOspheric Monitoring Instrument) aboard the European Copernicus Sentinel-5 Precursor satellite, launched in 2017.
• The instrument measures sunlight reflected by the Earth's atmosphere to determine concentrations of specific gases. Its spatial resolution is sufficient to identify emissions from individual cities but is too coarse to pinpoint exact sources, such as a specific landfill or facility, within a city.
• Methane is a potent greenhouse gas. Over a 20-year period, it is estimated to be approximately 80 times more potent at warming the planet than carbon dioxide. In urban areas, it can be emitted from sources including natural gas infrastructure, landfills, and wastewater treatment plants.

"Cities have the motivation and power to reduce greenhouse gas emissions and therefore, present significant opportunities for impactful emissions reduction. 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."
— Erica Whiting, the study's first author

Looking Forward

Kort noted that future higher-resolution satellite measurements are being explored to better identify contributions from specific localized sources.

The study was led by University of Michigan Engineering and funded by NASA and the National Institute of Standards and Technology. Previous research by the same group using aerial measurements had identified gaps in methane accounting at oil and gas production sites and in several large U.S. cities.