Cutting methane emissions is often seen as low-hanging fruit in the effort to curb climate change because of the short lifespan of methane gas in the atmosphere and its high warming potential. And recently, as researchers look to curtail methane emissions, they’ve been finding the amount to be much higher than once thought. That seems to be the case wherever they look: in agriculture, in oil and gas extraction and production, and now in wastewater treatment.

Two Princeton researchers recently examined methane emissions from wastewater treatment – one study using direct measurement of emissions, the other analyzing data from existing studies — and both reached the same conclusion that these emissions are almost double what was previously thought.

Atmospheric chemist Mark Zondlo performed on-the-ground methane emissions measurements at 63 wastewater treatment plants in the United States. Z. Jason Ren used machine learning methods to analyze data with new precision from previous methane monitoring studies globally.

Both researchers calculated, independently, that methane emissions from human wastewater treatment are responsible for 500 million metric tons more CO₂ than earlier studies had found. That’s roughly equal to a year’s worth of emissions from over 1 million cars.

“As cities continue to urbanize and develop net-zero plans, they can’t ignore the wastewater treatment sector,” Zondlo said.

This piece originally appeared in the March 2023 issue of The Charge, Princeton's environmental newsletter.