A much larger amount of methane than official estimates suggest could be escaping during natural gas use in several major Chinese cities.
The study focuses on the Yangtze River Delta, a vast urban region in eastern China. Chinese natural gas consumption has almost tripled in a decade, particularly in cities. Yet methane losses associated with this consumption remain difficult to measure directly. The inventories used to account for emissions may therefore miss some of these releases.
Panoramic night view of Shanghai’s business district.
Credit: Mstyslav Chernov / Wikimedia Commons — CC BY-SA 3.0
To overcome this difficulty, the researchers did not measure methane alone. They tracked ethane, another gas found alongside it in natural gas. Unlike methane, which also comes from rice paddies, landfills, and wastewater, the ethane observed here makes it easier to identify the footprint of fossil fuels.
The team analyzed ten years of atmospheric measurements, from 2012 to 2021. It then used a computer model reproducing the transport and transformation of gases in the atmosphere. The principle is simple: the researchers adjust natural gas emissions in the model until they reproduce the ethane variations actually measured.
The result gives an estimated average leakage rate of 3.5%, with a range of 2.5% to 4.3%. This is far above the 0.2% value commonly used in the Chinese inventories cited by the study. This percentage concerns losses linked to the consumption chain studied, not a leak measured at a particular facility.
In concrete terms, this rate would correspond to an average of 0.68 million tonnes of methane emitted each year in the region between 2012 and 2021. The estimated range is from 0.48 to 0.83 million tonnes per year. According to the authors, these emissions are significantly underestimated, or even absent, in several existing inventories.
Why does this difference matter? Methane is the main constituent of natural gas and efficiently traps heat in the atmosphere. A leak therefore causes more than just a loss of fuel. It directly releases a powerful greenhouse gas.
The study does not, however, involve counting every leak in streets or buildings. The rate is inferred from atmospheric measurements at the regional scale and from simulations. This method makes it possible to account for dispersed emissions, but it provides an overall estimate. The stated range reflects the uncertainties associated with this approach.
These results now pave the way for urban inventories based more heavily on real atmospheric measurements. They also make it possible to target future campaigns aimed at locating losses in distribution networks and among users, in order to determine where the methane currently accounted for at the regional level is being released.