Dehumidification over Tropical Continents Reduces Climate Sensitivity and Inhibits Snowball Earth Initiation

Abstract

The enigmatic Neoproterozoic geological record suggests the potential for a fully glaciated “Snowball Earth.” Low-latitude continental position has been invoked as a potential Snowball Earth trigger by raising surface albedo and reducing atmospheric CO2 concentrations through increased silicate weathering. Herein, climate response to reduction of total solar irradiance (TSI) is tested using four different land configurations (aquaplanet, modern, Neoproterozoic, and low latitude supercontinent) with uniform topography in the NCAR Community Atmosphere Model (CAM, version 3.1) general circulation model with a mixed-layer ocean. Despite a lower surface albedo at 100% TSI, the threshold for global glaciation decreases from 92% TSI in the aquaplanet configuration to 85% TSI with a low-latitude supercontinent. The difference in thresholds is principally due to the sensitivity of total specific humidity and therefore greenhouse forcing to reductions in TSI. Dehumidification of the troposphere over large tropical continents decreases greenhouse forcing and also increases direct heating by decreasing cloud cover. Continental heating intensifies the Walker circulation and the transport of dry air over the ocean, enhancing surface evaporation and marine tropospheric humidification, maintaining a high specific humidity and greenhouse effect over the ocean. Topography also provides an important control on Snowball Earth initiation. Modern topography in the modern continental arrangement lowers the initiation threshold by up to 2% TSI relative to a modern continental arrangement without topography. In the absence of potential silicate weathering feedbacks, large tropical landmasses raise the barrier to initiation of Snowball events. More generally, these simulations demonstrate the substantial influence of geography on climate sensitivity.