Riverine carbonate alkalinity (HCO3- and CO32-) sourced from chemical weathering represents a significant sink for atmospheric CO2. Alkalinity flux from Arctic rivers is partly determined by precipitation, permafrost extent, groundwater flow paths, and surface vegetation, all of which are changing under a warming climate. Here we show that over the past three and half decades, the export of alkalinity from the Yenisei and Ob' Rivers increased from 225 to 642 Geq yr-1 (+185%) and from 201 to 470 Geq yr-1 (+134%); an average rate of 11.90 and 7.28 Geq yr-1, respectively. These increases may have resulted from a suite of changes related to climate change and anthropogenic activity, including higher temperatures, increased precipitation, permafrost thaw, changes to hydrologic flow paths, shifts in vegetation, and decreased acid deposition. Regardless of the direct causes, these trends have broad implications for the rate of carbon sequestration on land and delivery of buffering capacity to freshwater ecosystems and the Arctic Ocean.