Domestic wastewater is an overlooked source and quantity in global river dissolved carbon.

Riverine dissolved carbon (DC) plays a crucial role in global carbon cycle. Yet, the contribution of wastewater to global riverine DC remains unquantified. Here, we quantify the impact of treated and untreated domestic wastewater on dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) loadings at nation and river basin scales. We show that, globally, domestic wastewater discharges ~21.4 Tg DC annually-6.42 Tg DIC and 1.17 Tg DOC from treated, and 9.64 Tg DIC and 4.21 Tg DOC from untreated, collectively accounting for 3.13 ± 0.46% of the global riverine DC export. Contributions are highest in densely populated regions with high treatment capacity. Effluent DC levels are influenced by treatment technology, temperature, precipitation, economic growth and urban development. This underscores a need to incorporate the wastewater derived carbon into global carbon budget assessment, as well as to monitor and reduce carbon in wastewater effluents.