Energy and Heat Transport Constraints On Tropical Climates of the Sturtian Snowball Earth
Abstract:It remains uncertain whether the maximum extent of Neoproterozoic Snowball Earth glaciations involves total sea ice coverage or significant open ocean. Models disagree, and the geologic record is inconclusive, but a resolution to this key question has important ramifications for how climate processes function at extremes, as well as for the distribution of habitable space for nascent multicellular life.
Here we report results of new Sturtian (ca 715 Ma) Snowball Earth simulations that explore the response to three primary climate forcings often cited as contributing to Neoproterozoic cold climates: a continent distribution in low to mid-latitudes, a reduction in solar input (-6.19%), and lowered atmospheric CO2 (40 ppm). The simulations use the latest GISS ModelE2-R, with a coupled dynamic ocean using a 2° X 2.5° atmosphere and 1° x 1.25° ocean resolution. The GCM includes dynamic sea ice, an improved ocean mixing scheme, and continental runoff directed via a river drainage scheme.
The GCM responds rapidly to the extreme forcings, and within 300 years the sea ice front reaches the subtropics, but after 1000 years the sea ice extent remains stable near 30° latitude, despite global average surface air temperatures dropping to -12°C. Tropical sea surface temperatures are above 5°C and nearly half the ocean surface area remains ice-free. This result differs from published results using CCSM4, but is consistent with previous NASA GCMs. Examining the tropical energy fluxes and heat transports from both atmosphere and oceans shows that for the GISS model, increases in tropical sea ice would require higher surface albedos, stronger poleward heat transports, or a reduced greenhouse effect. Atmospheric composition and cloud forcing are likely sources of this major difference in response amongst the newest versions of these IPCC models. Tropical cloud forcing must be a major suspect, since it also differs dramatically among IPCC models for future climate. We continue to stress that our “slushball” solution can readily accommodate the geological record of Neoproterozoic glacial activity, as well as the ecological space needed for the development of complex life.