Satellites estimate more methane from three cities than inventories report

Measurements from London, Los Angeles and New York were higher than official estimates, though they varied by city and year.

Editorial illustration — not from the study.

Researchers developed a method that uses observations from the TROPOMI satellite, wind conditions and methane levels upwind of a city to estimate the city’s net methane emissions. They tested it on London, Los Angeles and New York over three years, while accounting for changes in the air layer near the ground.

The resulting estimates were generally higher than existing emissions inventories, with substantial differences among cities and from year to year. The estimates were broadly consistent with earlier ground- and aircraft-based studies when their uncertainty ranges were compared.

What the satellites estimated

From 2021 to 2023, the satellite-based estimates ranged from 5.99 to 11.90 tonnes of methane per hour in London, 26.21 to 62.77 tonnes per hour in Los Angeles, and 30.85 to 44.77 tonnes per hour in New York. The study reports that inventories underestimated emissions across all three cities. The satellite estimates were about 1.5–3.0 times the inventory estimates for London, 1.3–3.1 times for Los Angeles, and 7.0–10.2 times for New York. The estimates also showed significant differences between cities and between years.

Why city methane checks matter

Cities are important sources of methane, a climate-forcing gas, but their emissions are difficult to measure and verify routinely. A satellite-based approach could support comparisons between reported inventories and independent estimates, provide ongoing city-scale monitoring, and help assess whether stated methane-reduction targets are being met.

Evidence and limits

This was a journal study describing and testing a satellite-based mass-balance method. The initial assessment covered only three megacities—London, Los Angeles and New York—and the results varied substantially by city and year. The estimates include uncertainty from the method, and the abstract presents the approach as having potential for wider monitoring rather than as a complete verification of emissions for all cities or targets.

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