Hong Kong Team Solves Hidden Climate Puzzle Under Cities

After two decades of detective work, scientists have uncovered a massive blind spot in the fight against climate change hiding right beneath our feet.

A research team led by Professor Yuan Zhiguo at City University of Hong Kong discovered that global sewer networks emit between 1.18 and 1.95 million tons of methane every year. Until now, climate experts assumed sewers produced essentially zero methane because wastewater moves through pipes too quickly for the gas to form.

They were wrong. Sewage contains rich organic matter, and sewer pipes create oxygen-free conditions perfect for methane production. Methane traps 80 times more heat than carbon dioxide in the short term, making it the second-biggest driver of global warming.

The discovery adds up to 38% to the estimated carbon footprint of wastewater management worldwide. That's like finding an entire hidden city's worth of emissions that no country was tracking or trying to reduce.

Professor Yuan's team spent 20 years developing the SeweX model, which simulates the chemical and biological processes happening inside sewer pipes. They collected real-world data from 21 cities across Australia, the United States, China, and Belgium using custom sensors to validate their findings.

The breakthrough came when researchers discovered methane generation closely relates to the wetted pipe surface. With simple data like pipe size, slope, water flow, and temperature, cities can now estimate their sewer emissions for the first time.

The Ripple Effect

This discovery gives cities around the world a powerful new tool to fight climate change. As urban areas expand their sewer networks, methane emissions from these systems will grow unless addressed.

Countries can now include sewer emissions in their official greenhouse gas inventories, transforming an invisible problem into a solvable one. The research opens fresh opportunities for engineers to design sewers that minimize methane production from the start.

The timing couldn't be better. The Climate and Clean Air Coalition reports that human-caused methane accounts for nearly 45% of current warming. Cutting methane offers one of the fastest ways to slow climate change because it breaks down much quicker than carbon dioxide.

Professor Yuan sees hope in the numbers. "As urban sewers continue to expand, their potential methane emissions will also increase," he explained. "Including them in the greenhouse gas accounting system will help improve national inventories and provide a new entry point for emissions reduction."

The study, published in Nature Water, proves that solving climate change sometimes means looking in unexpected places. Twenty years of patient research just gave humanity a new weapon against warming, hidden all along in the pipes beneath our cities.