Simulating zonal scale shifts in the partitioning of surface and subsurface freshwater flow in response to increasing pCO₂

Abstract

Freshwater discharge is one main element of the hydrological cycle that physically and biogeochemically connects the atmosphere, land surface, and ocean and directly responds to changes in pCO₂. Nevertheless, while the effect of near-future global warming on total river runoff has been intensively studied, little attention has been given to longer-term impacts and thresholds of increasing pCO₂ on changes in the partitioning of surface and subsurface flow paths across broad climate zones. These flow paths and their regional responses have a significant role for vegetation, soils, and nutrient leaching and transport. We present climate simulations for modern, near-future (850 ppm), far-future (1880 ppm), and past Late Cretaceous (1880 ppm) pCO₂ levels. The results show large zonal mean differences and the displacement of flows from the surface to the subsurface depending on the respective pCO₂ level. At modern levels the ratio of deeper subsurface to near-surface flows for tropical and high northern latitudes is 1:4.0 and 1:0.5, respectively, reflecting the contrast between permeable tropical soils and the areas of frozen ground in high latitudes. There is a trend toward increased total flow in both climate zones at 850 ppm, modeled to be increases in the total flow of 34 and 51%, respectively, with both zones also showing modest increases in the proportion of subsurface flow. Beyond 850 ppm the simulations show a distinct divergence of hydrological trends between mid- to high northern latitudes and tropical zones. While total wetting reverses in the tropics beyond 850 ppm due to reduced precipitation, with average zonal total runoff decreasing by 46% compared to the 850 ppm simulation, the high northern latitude zone becomes slightly wetter with the average zonal total runoff increasing by a further 3%. The ratio of subsurface to surface flows in the tropics remains at a level similar to the present day, but in the high northern latitude zone the ratio increases significantly to 1:1.6 due to the loss of frozen ground. The results for the high pCO₂ simulations with the same uniform soil and vegetation cover as the Cretaceous are comparable to the results for the Cretaceous simulation, with higher fractions of subsurface flow of 1:5.4 and 1:5.6, respectively for the tropics, and 1:2.2 and 1:1.6, respectively for the high northern latitudes. We suggest that these fundamental similarities between our far future and Late Cretaceous models provide a framework of possible analogous consequences for (far-) future climate change, within which the integrated human impact over the next centuries could be assessed. The results from this modeling study are consistent with climate information from the sedimentary record which highlights the crucial role of terrestrial-marine interactions during past climate change. This study points to profound consequences for soil biogeochemical cycling, with different latitudinal expressions, passing of climate thresholds at elevated pCO₂ levels, and enhanced export of nutrients to the ocean at higher pCO₂.