Influence of a dynamic ocean on Permian ice sheet growth

Abstract:

Previous studies using an atmospheric general circulation model have shown that extensive ice sheets can grow on the Gondwanan supercontinent during the late Paleozoic (~340-250 Ma) under modern orbital parameters and CO₂ levels at or below two times pre-industrial values (CO₂ ≤560 ppm). Proxy records for the late Paleozoic suggest a wide range of potential CO₂ values, but the range of values that allows for the growth of large ice sheets is much smaller. A prescribed slab ocean with diffusive heat transport (using modern heat fluxes) was used for those experiments, but lacks the ability to accurately capture the ocean dynamics of the Paleozoic. Here, within the NCAR Community Earth System Model framework, we use a fully dynamic ocean model to explore Permian ice sheet growth under more realistic conditions. Fully coupled CESM simulations for two CO₂ levels (280 & 560 ppm) are run until the ocean equilibrates (~1500-2000 years). Climatologies from those simulations are then used to drive a three-dimensional dynamic ice sheet model in an asynchronous coupling, in which the ice sheet model is run until the ice sheets equilibrate. The ice sheet geometry and height are then used to update the coupled ocean-atmosphere model, which is run for ~20 years, until the climate system re-equilibrates. Preliminary results show that ice sheet growth at the higher CO₂ value is small in comparison with ice sheet growth in the lower CO₂ environment, which is consistent with previous late Paleozoic modeling studies. The latitudinal temperature gradient is also steeper at 280 ppm CO₂ than at 560 ppm, though the dynamic ocean buffers some of these changes relative to what they would be for a slab ocean.