Marine nitrous oxide emissions: An unknown liability for the international water sector

•IPCC methodology for indirect marine nitrous oxide (N2O) emissions does not exist.•The water sector has an unknown N2O emissions liability from marine sewage disposal.•We model global sewage-nitrogen (N) emissions to coastal oceans during 1970–2050.•Emission factors for marine N2O will enable water sector N2O emissions accounting.•Industry benefits will include future revenue streams and better N emissions policy.

Reliable estimates of anthropogenic greenhouse gas (GHG) emissions are essential for setting effective climate policy at both the sector and national level. Current IPCC Guidelines for calculating nitrous oxide (N2O) emissions from sewage management are both highly uncertain and incomplete; a major methodological gap relates to the calculation of indirect N2O emissions from sewage disposed to marine environments. Here we apply a novel approach to estimate past and future global sewage-nitrogen emissions to coastal oceans and the potential marine N2O emissions linked to this nitrogen source. Then, by estimating the future cost associated with this largely uncharacterized emission source, we demonstrate the industry significance of developing a methodology for estimating N2O emissions from marine receiving environments. The capacity to accurately estimate, monitor and report GHG emissions has important consequences for informing future policy decisions regarding both mitigation and adaptation. A robust N2O emissions estimation methodology for sewage-nitrogen disposed to coastal oceans will allow the international water sector to more accurately and comprehensively inventory its N2O emissions. This will in turn allow for proper accounting of related future emissions liabilities while also enabling the sector to capitalize on any future economic returns linked to this source – providing much-needed capital to support the sector's future infrastructure and climate change adaptation challenges.

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