Discrepancies in national inventories reveal a large emissions gap in the wastewater sector

The wastewater sector is now increasingly targeting fugitive CH4 and N2O emissions. National inventory reports (NIRs) are central to tracking these non-CO2 emissions, yet the extent to which NIRs capture sector-wide wastewater emissions remain unclear due to inconsistent accounting methodologies, complex systems and large fluctuations across regions and time. Here we conduct a global analysis of wastewater GHG accounting in NIRs, which reveals widespread omissions of key wastewater pathways and methodological discrepancies. These shortcomings systematically underestimate sectoral emissions and undermine cross-country comparability. For 38 countries studied, we estimate an unreported gap of 52.0–73.2 million metric tons (MMT) of CO2-equivalent (CO2e) per year, largely from omitted pathways and underestimation at centralized wastewater facilities. Extrapolated globally, approximately 94–150 MMT CO2e yr−1 are under-reported, though precise quantification remains challenging due to the lack of detailed activity data. These findings underscore the need for more comprehensive and harmonized accounting approaches in future Intergovernmental Panel on Climate Change guideline revisions to strengthen wastewater GHG inventories. The wastewater sector is a major source of non-CO2 emissions, but accurate emissions accounting remains challenging, despite guidelines for basic inventories provided by the IPCC. This research reveals that national inventories widely omit key sources and apply inconsistent methods, which lead to greatly under-reported emissions.