Inconsistent national reports undercount wastewater emissions

Policy brief Climate policy https://doi.org/10.1038/s41558-026-02555-7 Inconsistent national reports undercount wastewater emissions Cuihong Song, David Ponder, Wei Peng & Zhiyong Jason Ren National greenhouse gas inventories systematically undercount methane and nitrous oxide emissions from wastewater due to outdated methods and incomplete coverage. Addressing these discrepancies is essential to strengthen transparency in global climate efforts and improving the effectiveness of national mitigation strategies. 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the emissions from wastewater systems are often under-represented in national inventory reports (NIRs) primarily due to outdated accounting methodologies and a failure to capture the full scope of emission sources, ranging from decentralized systems to centralized treatment facilities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fail to cover key wastewater pathways (Fig. 1a,b), such as decentralized sanitation and effluent discharge, or rely on outdated methodologies that understate emissions.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 waste water emissions by several-fold. For example, accounting for all sources boosts Spain’s CH4 emissions by ~450%, and Finland’s N2O nature climate change Check for updates emissions by ~550%. These findings imply that national climate pledges may be built on undervalued baselines. 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. Recommendations for policy • Expand inventory coverage to include all wastewater pathways, including septic systems, effluent discharges and decentralized systems. • Harmonize reporting methods to ensure countries use consistent, up-to-date emission factors and accounting practices. • Improve data collection and transparency in the wastewater sector, especially in developing countries, to reduce uncertainties and build trust in reported numbers. • Incorporate accurate wastewater emissions in climate mitigation plans, as under-reporting this sector could undermine national mitigation efforts and opportunities. Cuihong Song 1,2,5, David Ponder 3, Wei Peng 2,4 & Zhiyong Jason Ren 1,2 1 Department of Civil and Environmental Engineering, Princeton University, Princeton, NJ, USA. 2Andlinger Center for Energy and the Environment, Princeton University, Princeton, NJ, USA. 3The US Water Alliance, Federal Way, WA, USA. 4School of Public and International Affairs, Princeton University, Princeton, NJ, USA. 5Present address: Department of Civil and Environmental Engineering, University of New Hampshire, Durham, NH, USA. e-mail: Policy brief c a Emission gap (% of reported) Australia Belarus Bulgaria Canada Denmark 40% or 50% 60% or 75% 80% 100% NA b 12% 255% 68% Reported total 276 MMT Czechia 25% 4% N2O gap 55 MMT Belgium CH4 wastewater sources included (% of actual) 51% CH4 gap 18 MMT Austria 79% 4% Finland 164% France 56% Germany 29% Greece 196% Hungary 4% Ireland 160% Italy 130% Japan 15% Netherlands 4% New Zealand 81% Norway 26% Poland 9% Portugal 0% Romania 62% Russia 57% Spain 259% Sweden 25% Switzerland 0% Turkey 110% Ukraine 25% UK 245% USA N2O wastewater sources included (% of actual) 5% 2% Brazil 30% China 10% 20% or 25% Egypt 31% 40% or 50% India 29% 60% or 75% Indonesia 11% Malaysia 84% 100% NA Mexico 64% South Africa 61% 0 20 40 60 Wastewater emissions (MMT CO2-eq) Fig. 1 | Omission of wastewater pathways in the latest NIRs and resultant emission gaps. 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. Published online: xx xx xxxx 3. El Abbadi, S. H. et al. Benchmarking greenhouse gas emissions from US wastewater treatment for targeted reduction. Nat. Water 3, 1133–1143 (2025). This study reveals that US wastewater treatment emissions are markedly higher than current estimates and underscores the need to mitigate configuration-specific hotspots, especially fugitive CH4 and nutrient-removal-driven N2O emissions. Further reading 1. Luers, A. et al. Make greenhouse-gas accounting reliable — build interoperable systems. Nature 607, 653–656 (2022). This paper highlights four foundational requirements for reliable greenhouse gas accounting: improved data, inter operable systems, enhanced trust mechanisms and sustainable financing. 2. Banja, (...truncated)