Abstract

Identifying the drivers of changing continental runoff is key to understanding current and predicting future hydrological responses to climate change. Potential drivers of runoff change include changes in precipitation and evaporation due to climate warming, vegetation physiological responses to elevated atmospheric CO2 concentrations, increases in lower-atmospheric aerosols, and anthropogenic land cover change. In this study, we present a series of simulations using an intermediate-complexity climate and carbon cycle model, to assess the contribution of each of these drivers to historical and future continental runoff changes. We present results for global runoff, in addition to northern latitude runoff that discharges into the Arctic and North Atlantic Oceans, so as to identify any potential contribution of increased continental freshwater discharge to changes in North Atlantic deep-water formation. Between 1800 and 2100, the model simulated a 26% increase in global runoff, and a 32% runoff increase in the northern latitude region. This increase was driven by a combination of increased precipitation from climate warming, and decreased evapotranspiration due to the physiological response of vegetation to elevated CO2. When isolated, climate warming (and associated changes in precipitation) increased runoff by 16% globally, and by 27% at northern latitudes. Vegetation responses to elevated CO2 led to a 13% increase in global runoff, and a 12% increase in the northern latitude region. These changes in runoff, however, did not affect the strength of the Atlantic Meridional Overturning Circulation, which was affected by surface ocean warming rather than by runoff-induced salinity changes. This study indicates that vegetation physiological responses to elevated CO¬2 may be contributing to changes in continental runoff at a level similar to that of the direct effect of climate warming.