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.
The study, published in PNAS, 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 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, former U-M professor of climate and space sciences, and now director of the Atmospheric Chemistry Department at the Max Planck Institute for Chemistry.
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.
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.”
Data from University of Michigan