Global Wastewater Emissions Are 19-27% Higher Than Nations Report. Here's Why.

When nations report their greenhouse gas emissions to international bodies, they rely on accounting methods that tell them how much methane and nitrous oxide escapes from their sewage treatment plants and sanitation systems. For dozens of countries, those methods are two decades out of date - and the gap between what they count and what actually enters the atmosphere is substantial.

A Princeton University study, published February 11 in Nature Climate Change, quantified that gap for the first time across a broad international sample. Looking at 38 countries on five continents - 26 economically developed, 12 emerging economies - the researchers found that nations consistently underestimate their wastewater emissions by 19 to 27 percent. The accumulated shortfall amounts to 94 to 150 million metric tons of carbon dioxide equivalent per year missing from global emissions inventories.

The Source of the Problem: Stuck in 2006

The primary cause is straightforward: most countries still use the Intergovernmental Panel on Climate Change's 2006 guidelines to calculate wastewater emissions, even though the IPCC published a substantially improved set of methods in 2019. The 2019 refinements incorporate advances in scientific understanding of how methane and nitrous oxide form and escape from different types of wastewater systems. Countries that have not updated their approach are, in effect, working with calculations that do not reflect 15 years of improved measurement science.

A second cause is less technical and more categorical: many countries simply omit entire categories of sanitation from their inventories. Pit latrines, septic systems, and untreated sewage discharges - which serve large portions of the population in emerging economies - are frequently excluded from national reports. These systems emit greenhouse gases, but because they fall outside formal treatment infrastructure, they often fall outside accounting frameworks too.

"If you don't know exactly how much emissions you have, then it's really difficult to make effective policies or technologies or methods to reduce the emissions," said Z. Jason Ren, professor of civil and environmental engineering and a faculty member at the Andlinger Center for Energy and the Environment at Princeton, who led the research.

Why Wastewater Emissions Are Often Underappreciated

The wastewater sector does not attract the same attention as transportation, power generation, or heavy industry in emissions reduction discussions. Yet on a carbon dioxide-equivalent basis, sector-wide emissions from wastewater systems are comparable in magnitude to aviation or commercial shipping - two sectors that receive extensive policy focus.

The gases involved are particularly potent. Methane has a global warming potential roughly 28 to 36 times that of carbon dioxide over a 100-year horizon. Nitrous oxide is approximately 273 times more potent. Wastewater systems are among the largest emitters of both. The combination means that even percentage-point improvements in emission factors can produce meaningful reductions in effective warming contribution.

There is also a structural reason why progress in this sector has lagged: wastewater infrastructure is long-lived. A municipal treatment plant built today might remain in service in 2075 or beyond. Decisions about treatment technology, process design, and operational practices made in the near term will lock in emission profiles for decades. Ren frames this as both a challenge and an opportunity - choices made now carry unusually long consequences.

The Methodology and Its Scope

The researchers examined the latest national inventory reports from each of the 38 countries and compared them to determine what had been omitted. They applied modern evaluation techniques to large facilities - sewage treatment plants and untreated discharge points - to estimate actual emissions. They then used the updated 2019 IPCC methods as a benchmark to determine what the numbers would look like under current scientific guidance.

The resulting gap of 94 to 150 million metric tons of CO2-equivalent per year represents a conservative estimate, since it addresses known methodological and categorical gaps rather than attempting to capture every potential source of underreporting. Countries with larger populations served by informal sanitation systems likely have larger underestimates than those with more complete coverage of formal treatment infrastructure.

Ren accompanied the research article with a policy paper urging the IPCC and national policymakers to update their wastewater emissions standards. The fix is, in principle, available: the 2019 IPCC methods exist and can be adopted. What has been missing is consistent pressure and clear guidance for countries to make the switch.

"If you don't have an accurate accounting, it's hard to make good and right decisions, yet such decisions can have long lasting impacts," Ren said.

An Underexplored Path to Emissions Reduction

Because wastewater engineering has historically focused on public health rather than climate, the tools for cutting methane and nitrous oxide from these systems have received far less investment and research than carbon dioxide reduction in energy or transportation. That relative neglect means there may be cost-effective opportunities that have not yet been seriously explored.

Accurate accounting is the prerequisite. Countries cannot design effective policies or evaluate technologies without knowing their baseline. The 19 to 27 percent underestimate identified by the Princeton team represents not just a data problem but a policy problem - one that compounds with every year inventories remain uncorrected.