Reconstructions of a Snowball Earth: A Data/Model Perspective
DESCRIPTION: The overlap between the rise of complex metazoan life and the "Snowball Earth" glaciations of the Cryogenian period in the Neoproterozoic Era, ca. 720-635 Ma (Ma = million years ago), raises tantalizing questions about the influence of climatic processes on the evolution of life. For this investigation, we explore the factors contributing to the Cryogenian glaciations, especially their onset and termination – the environmental events most likely to influence speciation or trigger extinctions. We are utilizing a General Circulation Model (GCM) suitable for examining just this type of extreme environmental scenario: the Resolving Orbital and Climate Keys of Earth and Extraterrestrial Environments GCM: a.k.a. ROCKE-3D, initially referred to in our proposal as the NASA Generalized Rocky ExoPlanet (GREP) model. ROCKE-3D uses the GISS ModelE2-R GCM (Schmidt et al., 2014) as a core, which is the fully coupled ocean-atmosphere model used for the most recent CMIP (Climate Modeling Intercomparison Project) efforts, which in turn contribute to IPCC assessments of future climate change. As part of this and other related efforts at NASA GISS and GSFC, ROCKE-3D continues to be modified for use with an array of rocky planet configurations, now with atmospheric compositions unlike modern Earth.
OUTCOMES: A Neoproterozoic Sturtian glacial experiment (ca. 715 Ma) using a fully coupled ocean- atmosphere version of the GISS GCM retains substantial ice-free area in the tropics (just over 50% of Earth’s surface) despite significant solar insolation reduction (94% modern) and low CO2 (40 ppmv). Persistent sea ice is also on average just 5 meters thick, far thinner than the 1-km-thick “sea glaciers” that have been proposed by other researchers for this time period. Our finding disputes the common narrative that the Earth was nearly or completely frozen over at a time critical to the evolution of complex multicellular life, and is in fact in accordance with paleobiological research that does not support the occurrence of mass extinctions or other evolutionary bottlenecks at this time.
• The stable ice-free tropics and open ocean of the Sturtian glacial experiment means that Earth at this time can be considered at least regionally habitable, despite having a global mean annual temperature (MAT) of -12 ̊C to -14 ̊C – noticeably colder than the 0 ̊C global MAT often assumed as the temperature limit for allowing the presence of liquid water on the surface of a habitable planet.
• For the Neoproterozoic Sturtian glacial condition (ca. 715 Ma), both solar heating in the tropics, plus surface wind stresses associated with the descending branch of the Hadley cells at approximately 30 ̊ latitude, combine to keep tropical oceans both relatively warm (annual average sea surface temperatures reaching as high as 11 ̊C) and free of permanent sea ice cover. Ice floes are swept away from the edges of the persistent sea ice front and eventually melt in the warmer tropical waters – a climate scenario perhaps better decribed as a “slushball” state.