Globally elevated greenhouse gas emissions from polluted urban rivers

Cities are at the heart of global anthropogenic greenhouse gas (GHG) emissions, with rivers embedded in urban landscapes as a potentially large yet uncharacterized GHG source. Urban rivers emit GHGs due to excess carbon and nitrogen inputs from urban environments and their watersheds. Here relying on a compiled urban river GHG dataset and robust modelling, we estimated that globally urban rivers emitted annually 1.1, 42.3 and 0.021 Tg CH4, CO2 and N2O, totalling 78.1 ± 3.5 Tg CO2-equivalent (CO2-eq) emissions. Predicted GHG emissions were nearly twofold those from non-urban rivers (~815 versus 414 mmol CO2-eq m−2 d−1) and similar to scope-1 urban emissions in intensity (1,058 mmol CO2-eq m−2 d−1), with particularly higher CH4 and N2O emissions linked to widespread eutrophication and altered carbon and nutrient cycling in urban rivers. Globally, the emissions varied with national income levels with the highest emissions happening in lower–middle-income countries where river pollution control is deficient. These findings highlight the importance of pollution controls in mitigating urban river GHG emissions and ensuring urban sustainability.