Satellite measurements suggest that scientific and government reports don't capture how methane emissions are rising, which could hamper mitigation efforts
Study: Space-based observation of global increase in urban methane emissions from 2019-2023 (DOI: 10.1073/pnas.2504211123)
Key takeaways
- Satellite measurements show that urban methane emissions around the globe have risen 6% since 2019, and emissions from C40 cities partaking in climate pledges are rising at a similar rate as non-participating cities.
- The observed growth in methane emissions is not captured by accounting-based emissions estimates, suggesting that some methane sources are missing or underestimated.
- C40 cities will have to account for nearly 2 additional teragrams of methane emissions-about 30% of their emissions reduction target.
Urban emissions of methane-a potent greenhouse gas-are rising faster than "bottom-up" accounting estimates anticipated, according to a study led by University of Michigan Engineering and funded by NASA and the National Institute of Standards and Technology.
The discrepancy was found with satellite measurements of methane over 92 major cities around the world. For 72 of the cities, there were sufficient data to track changes in methane emissions between 2019 and 2023. Overall, global urban methane emissions in 2023 were 6% higher than 2019 levels and 10% higher than 2020 levels, although they tended to decrease in European cities.
In contrast, accounting methods-which tally emission estimates of individual methane sources-suggest that urban methane emissions have only risen between 1.7% and 3.7% since 2020.
The study included over half of the C40 network, a group of 97 cities around the world aiming to reach net-zero emissions by 2050. Total methane emissions across all the studied C40 cities in 2023 were also 10% higher than 2020 levels, and the cities will have to contend with an extra 2 teragrams of methane emissions per year, which is about 30% of their emission reduction targets. The gap between official estimates and satellite measurements warn that city policies designed with accounting estimates may not reduce methane emissions as desired.
"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 published in Proceedings of the National Academy of Sciences. He advised the study's lead author as a U-M professor of climate and space sciences and engineering, and is now director of the Atmospheric Chemistry Department at the Max Planck Institute for Chemistry.
Checking emission books
The study continues Kort's work identifying gaps in accounting of methane, which can enter the atmosphere from old or leaky natural gas infrastructure, landfills and wastewater treatment plants, and is 80 times more potent at warming the planet than carbon dioxide over a 20-year period.
With measurements from airplane surveys, Kort's research group has shown that flaring at oil and gas production sites leaks 5 times more methane than previously estimated, and that the true climate impact of offshore oil and gas production is double the official estimates. The findings helped make flares an emissions-reduction target in the Inflation Reduction Act, leading to a $30 million Department of Energy call for new technology to reduce leaks from gas flaring.
In 2019, similar aerial measurements suggested that several large cities across the U.S. were also emitting more methane than previously thought. The new study showed that this is a global problem.
"Cities have the motivation and power to reduce greenhouse gas emissions and therefore, present significant opportunities for impactful emissions reduction," said Erica Whiting, U-M doctoral student in climate and space sciences and engineering and 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 researchers' global satellite measurements suggest that urban emissions accounted for 10% of all human methane emissions in 2023, and city methane emissions overall were nearly four times higher than the oil and gas "ultra emitters" that have been the focus of previous studies and emission policies.
Monitoring methane from space
The new findings come from the TROPOMI instrument, which was launched aboard the European Copernicus Sentinel-5 Precursor satellite in 2017 to track atmospheric pollution and climate change. TROPOMI measures the amount of sunlight reflected by the atmosphere back into space. It separately measures many wavelengths of light, each of which provides information on the concentration of a particular gas or pollutant, and it has sufficient spatial resolution to pinpoint individual cities.
TROPOMI's resolution is too coarse to identify where exactly unreported methane is coming from within the city, however. The researchers think that higher-resolution measurements could help cities update their accounts and emission policies.
"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."
