Methane is the greenhouse gas most responsible for global warming behind carbon dioxide. The largest source of human-caused methane emissions is from oil and gas production, while most of the rest – almost a fifth of global methane emissions – comes from rotting landfills.
For the study, Joannes Maasakkers at the Netherlands Institute for Space Research and a team used satellites to identify the landfills behind much of the methane emissions in four cities: Buenos Aires in Argentina, Lahore in Pakistan, and Mumbai and Delhi in India.
The researchers identified the cities as methane hotspots using an instrument on board the Copernicus Sentinel-5P satellite that monitors methane emissions across the planet each day.
Because the satellite has a broad field of view, the team could point two higher-resolution satellites, called GHGSat-C1 and C2, at the cities.
In Buenos Aires, the team found that a large landfill called Norte III accounted for half the city's methane emissions. Almost 90 per cent of the methane came from an uncovered part of the landfill.
Meanwhile, a landfill in Mumbai accounted for 26 per cent of the city's methane emissions. The largest landfill sources in Lahore and Delhi were responsible for 12 and 6 per cent of methane emissions respectively.
"The emissions from individual facilities are very significant," said Maasakkers. "These observations can tell us where the large methane emissions are and where mitigation action can be taken."
The research team has pointed to obvious places to focus emission-reduction efforts, including covering landfills and collecting methane, and other steps like reducing the amount of organic waste could reduce methane emissions from landfills by sixfold.
Landfill waste – responsible for about 11 per cent of global methane emissions – is expected to increase about 70 per cent by 2050 as the global population continues to climb, according to the World Bank.
Because methane is 80 times more powerful than CO2 over a 20-year period, reducing "methane emissions now… can have a quick impact on climate change," Maasakkers said.
In the past, researchers made estimates of landfill emissions based on landfill volume and assumed rates of decay.
Satellite technologies are a boost to scientists, said Jean Bogner, a University of Illinois environmental scientist not involved in the research.
This novel approach helps to "adequately capture site-specific emissions, which for landfills can vary by orders of magnitude" depending on everything from soil conditions to whether mitigation measures are in place.
