Geologic Carbon Cycle Constraints on the Sensitivity of Runoff to Atmospheric CO2

There remain substantial uncertainties regarding how important components of the hydrological cycleincluding evapotranspiration (ET) and runoffwill change in response to higher CO2. Recent observational estimates indicate that ET is increasing faster than the increase in precipitation, resulting in substantially lower runoff as atmospheric CO2 rises. However, the effects of CO2 fertilization on plant water use remain difficult to estimate and thus even more difficult to model and project into the future. Further, geologic records of the hydrological response to past carbon cycle perturbations indicate potentially large increases in runoff with higher CO2 and associated warming. Here, we demonstrate that the fact that the Earth has remained habitable for most of its history sets a hard lower-bound on the sensitivity of runoff to atmospheric CO2. In short, the recovery of the Earth system from carbon cycle perturbations is thought to be driven by enhanced silicate weathering, which transfers atmospheric CO2 to the oceans as alkalinity via runoff. Though a number of factors mediate weathering rates, total runoff determines the total flux of silicate-derived cations and hence the removal flux of excess CO2 following a carbon cycle perturbation. Using a carbon cycle model that parameterizes weathering as a function of rock reactivity, erosion, runoff, temperature, and soil CO2, we show that recovery from a carbon cycle perturbation is only possible if the lower-bound for the runoff sensitivity is approximately 1%/K. Using geological proxy data for the Paleocene-Eocene Thermal Maximum, we find that to match the record of marine 13C requires that the runoff sensitivity be greater than 0%/K. We further place these geologic constraints in context with estimates of the runoff sensitivity simulated by an ensemble of stand-alone land-surface and coupled Earth system models. These results suggest that recent data indicating large (~-10 to -15%/K) global declines in runoff either represent short-term transients or that current measurements are missing a large fraction of global runoff from land to sea.