Strong sources of CO2 in upper estuaries become sinks of CO2 in large river plumes

► The water-to-air exchange of CO2 from 106 estuaries worldwide was examined. ► Upper estuaries are strong sources of CO2, whose fluxes fall oceanward. ► Water-to-air fluxes are highest in fall and lowest in winter. ► Globally, estuaries release around 0.26 Pg C/y to the atmosphere. ► Large-river plumes are commonly sinks of CO2.