The pioneering work of Manabe led to the hypothesis that in a high CO2 world the interiors of mid-latitude continental regions would undergo substantial hydrologic changes, including reductions in summer soil moisture. We revisit this issue using the latest version of the Geophysical Fluid Dynamics Labs (GFDL) atmosphere-land model coupled to a slab ocean. The new model differs from previous GFDL models, notably in terms of the inclusion of enhanced vertical resolution, an explicit boundary layer, and a diurnal cycle of insolation.

In the earlier GFDL models pioneered by Manabe, the simulated response to a doubling of atmospheric CO2 included an increase in wintertime rainfall over most mid-latitude continental regions, an earlier snowmelt season and onset of springtime evaporation, and an increase of potential evaporation. These factors led to large-scale increases in soil moisture in winter and decreases in summer in mid-latitudes in doubled-CO2 experiments. The new model shows similar results, and the processes discussed above are important in this model as well. In addition, we find that changes in atmospheric circulation play an important role in the structure of regional hydrologic changes.

Additional experiments have been run to investigate the factors responsible for the atmospheric circulation changes. We find that the circulation changes are primarily driven by tropical sea surface temperature changes. In addition, land-surface atmosphere feedbacks can play an important role. These results highlight some of the important issues in projections of CO2-induced regional hydrologic changes.