Cenozoic stable isotope constraints on the Eurasian continental interior hydroclimate response to high CO2

The response of the terrestrial hydrologic cycle to higher atmospheric CO2 remains poorly constrained, largely due to difficulty in predicting how land surface processes may modify individual hydroclimate parameters such as precipitation (P), evapotranspiration (ET), and runoff (q). To interrogate how the terrestrial hydrologic cycle may change with warming and higher CO2, we utilize the Cenozoic geologic record of terrestrial stable oxygen isotopes (818O) as recorded in authigenic minerals, which reflect precipitation 818O (818Op). Values of 818Op are sensitive to changes in terrestrial hydroclimate, including to the ratio of P/ET and to precipitable water. In short, decreasing P/ET or higher precipitable water produces shallower 818Op gradients inland, whereas increasing P/ ET or lower precipitable water results in steeper 818Op continental gradients. We compile nearly 15,000 samples of authigenic carbonate and tooth enamel across Eurasia-the largest continental landmass-that span the Cenozoic Era to reconstruct the past spatial distribution and zonal (i.e. east-to-west) gradients of 818Op. We find that, in epochs with higher CO2, zonal 818Op gradients increase, suggesting an increase in P/ET across Eurasia despite the expected increase in precipitable water in greenhouse climates. We compare these results to 818Op gradients simulated by an isotope-enabled climate model (iCESM) forced with both pre-industrial (PI) and 4xPI CO2 mixing ratios. Simulated 818Op gradients shallow as CO2 rises, in contrast to the reconstructed 818Op gradients. This data-model mismatch suggests either that Earth system feedbacks not represented in our iCESM simulations, such as changes in Northern Hemisphere ice extent and vegetation, may have fundamentally changed Northern Hemisphere hydroclimate or that land-surface processes under high CO2 climates may be misrepresented in iCESM.