BASED ON C. Song et al. Nature Climate Change https://doi.org/10.1038/s41558-025-02540-6 (2026).

The policy problem

Wastewater systems are important sources of methane (CH4) and nitrous oxide (N2O), both are potent greenhouse gases. Accurate accounting of these emissions is essential to support effective mitigation and policy action. Yet, the emissions from wastewater systems are often under-represented in national inventory reports (NIRs) primarily due to outdated accounting methodologies and a failure to capture the full scope of emission sources, ranging from decentralized systems to centralized treatment facilities and discharge of treated and untreated wastewater. Furthermore, the use of different accounting methods and inconsistent omission of key emission sources among countries make it difficult to compare data across nations and lead to widespread under-reporting. These discrepancies are especially problematic under the Paris Agreement, where national inventories form the basis of countries’ climate pledges, as well as the Global Stocktake that monitors the mitigation efforts. Strengthening the completeness and consistency of these inventories is therefore crucial for credible climate targets and informed policy decisions.

The findings

This analysis reveals widespread gaps in how countries report wastewater emissions (Fig. 1). Many national reports fail to cover key wastewater pathways (Fig. 1a,b), such as decentralized sanitation and effluent discharge, or rely on outdated methodologies that understate emissions. For the 38 countries studied, total CH4 and N2O emissions from wastewater are actually 19–27% higher than what those countries officially report (Fig. 1c). This unreported ‘emissions gap’ amounts to roughly 52–73 million metric tons of CO2-equivalent (MMT CO2-eq) per year, or 94–150 MMT CO2-eq per year if extrapolated globally. In some cases, correcting these gaps would raise a country’s wastewater emissions by several-fold. For example, accounting for all sources boosts Spain’s CH4 emissions by ~450%, and Finland’s N2O emissions by ~550%. These findings imply that national climate pledges may be built on undervalued baselines.

Fig. 1: Omission of wastewater pathways in the latest NIRs and resultant emission gaps.
figure 1

a,b, Percentage of wastewater pathways included in CH4 (a) and N2O (b) emission estimates. c, Absolute and relative emission gaps compared with reported values. NA, not available. Basemaps in a and b from Natural Earth. Figure adapted from C. Song et al. Nat. Clim. Change https://doi.org/10.1038/s41558-025-02540-6 (2026), Springer Nature Limited.

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The study

We conducted a comprehensive review of how countries calculate and report wastewater emissions in their official inventories. We analysed the latest NIRs of 38 countries, including 30 developed (Annex I) and 8 developing (non-Annex I) nations across 5 continents. We compared the wastewater emission sources each country includes (covering five main categories: latrines, septic systems, centralized treatment plants, treated effluent and untreated wastewater discharges) and examined the methodologies and emission factors used. The results identified that sources remain uncounted and large methodological discrepancies exist. To quantify the impact, we estimated emissions using more complete literature coverage and the latest field measurements of CH4 and N2O from wastewater systems. By adding in the often omitted sources and updating outdated factors, we calculated how much each nation’s reported emissions would increase. The result is a clear evidence base showing that current reporting practices systematically undercount wastewater emissions.