Extreme Climate Change: The late Neoproterozoic “Snowball Earth”

What is the snowball Earth? Severe glaciation that may have encompassed the entire Earth Two glaciations in the Neoproterozoic, ~ 725Ma (Sturtian) and 600Ma (Varanger) (Hoffman et al., 1998) Each episode lasted 4 to 30 million years (Hoffman et al., 1998) May have acted as an evolutionary “bottle neck” just prior to Cambrian radiation Chandler and Sohl, 2000

Earth' Climate Through time (Including 30% increase in Solar Luminosity) Adapted from James Lovelock, 1979) Kirchvink,

Evidence for a snowball Earth Geologic Evidence Low-latitude glaciogenic deposits: diamictites, dropstones, glacially striated bedrock, varves Banded Iron Formations (BIF) Globally extensive “cap” carbonates Hoffman and Schrag, 2002 Kirchvink,

Evidence for a snowball Earth Geochemical Carbon isotope excursions, pre- glacial /postglacial positive/negative Climatological Energy Balance Models (EBMs), Budyko (1969), Sellers (1969), Caldeira and Kasting, (1992) Global Climate Models (GCMs), some can simulate a snowball (Baum and Crowley, 2001, 2003; Jenkins and Smith 1999), some cannot (Poulsen et al., 2001, 2003; Hyde et al., 2000)

Estimated Atmospheric CO 2 (Adapted from J.F. Kasting, Science 259, p. 923, 1993) CO2 limit from Lack of siderite In paleosols (Rye et al., 1995) Kirchvink,

The Idealized Snowball to Greenhouse Cycle An abundance of continents at low-latitudes break up New rift margins provide new carbon sinks and fresh weathering surfaces and atmospheric CO2 is reduced Increased albedo from low-latitude continents and reduction in greenhouse gasses enhances glaciation, reduces weathering further reducing CO2 Glaciers and sea-ice advance to a low mid-latitudes and a run-away ice/albedo feedback is initiated leading to equatorial glaciation, 100 to 1000 yrs (Pollard and Kasting, 2004) (delta C-13 excursions, glaciogenic deposits) Ocean and atmosphere are isolated and greenhouse gasses build up (banded iron formation) Greenhouse gasses reach a threshold level initiating a extreme greenhouse and the glaciers/sea-ice rapidly retreat High CO2 atmosphere and warm tropical waters enhance precipitation of CaCO3 (cap-carbonates)

The Idealized Snowball to Greenhouse Cycle 1. low-latitude continental break-up 2. CO2 reduction, glaciation 3. Runaway ice/albedo feedback 4. Ocean/Atmosphere isolation CO2 build up 5. Extreme Greenhouse 7. low-latitude continental break-up

Climatic Forcing Factors to get a Snowball Earth Necessary factors: Reduce solar luminosity (estimated 6% lower during Neoproterozoic) Reduce CO2 (debatable as to how much, Chandler and Sohl, 2000) Favorable continental configuration (low-latitude continents) Other influencing factors: Atmospheric dynamics ( cloud effects, Hadley Cell, boundary layer ) Ocean dynamics ( thermal, salinity, and wind-driven circulation) Ice dynamics ( ice/snow accumulation, ice-flow, thermal diffusivity)

Necessary Factors Reduced Solar Luminosity 94% of current, ~ 6% less Simple 1-d energy balance models suggest reduced luminosity as means of initiation global glaciation (Budyko, 1969, Sellers, 1969) LAB 7!!!! Most models use ~6% reduction or 1285 Wm-2 Slow rate of change cannot drive high-frequency climate fluctuations (Chandler and Sohl, 2000) Lower luminosity in early Earth so this alone cannot account for equatorial glaciation, other forcing factors must be considered 94% of current, ~ 6% less

Necessary Factors Reduced Atmospheric CO2 pCO2 controlled by volcanic degassing and mid-ocean rifting output and consumption by silicate weathering If considered in simple EBMs pCO2 must be reduced to achieve low- latitude glaciation 140ppmv (1/2 pre-industrial values) commonly used to simulate conditions around snowball Hoffman and Schrag, 2002 Donnadieu et al., 2004 Baum and Crowley, 2001

Necessary Factors Continental Configuration Low-latitude continents enhance equatorial albedo, reduced shortwave radiation Land plants not evolved by the Neoproterozoic so a desert albedo ~0.5 used for continents Enhanced silicate weathering in tropics, reduction in CO2 Paleotopography enhances continental area, Grenvillian (~1 billion yBP) and Pan- African (~650 myBP) orogenies Sturtian Varanger

Necessary Factors Continental Configuration Poulsen et al., 2002 Examples of plate configurations used in models. Hyde et al., 2000 Donnadieu et al., 2004

Other Factors Atmospheric Dynamics Clouds- Under represented in GCMs, too little water to generate significant cloud cover, over ice clouds have negligible albedo effects and act to enhance radiative forcing but greenhouse effect only 10Wm-2 (Pierrehumbert, 2004), possibility of CO2 clouds? Hadley Cell circulation – transports heat to sea-ice margin working against ice/albedo feedback. If ice is at lower latitudes, Hadley Cell enhances feedback, becomes more intense and collapses prior to full glaciation (Pierrehumbert, 2004) Winter hemisphere isothermal-radiative equilibrium, Convection occurs in southern hemisphere but low tropopause limits greenhouse effects

Other Factors Atmospheric Dynamics January July T Surface winds- important heat transfer

Other Factors Atmospheric Dynamics Hydrologic Cycle- precipitation ~ 1cm/yr, high accumulation near 16N-S due to moisture convergence at upward branch of Hadley cell, greatly impact surface albedo even over sea ice Diurnal variations- unstable boundary layer during day and night stably stratfied- enhanced SH loss Pierrehumbert, 2005

Other Factors Ocean Dynamics Important factor to simulate snowball Simplified and nondynamical-50m slab model typically used-no currents and diffusive heat transport = snowball Coupled Ocean/Atmosphere models show that increased heat transport stabilizes ice-line away from equator Wind driven ocean circulation increases ocean heat transport compensates for strong SH loss caused by high latitude temperature decreases- stabilizes ice-line away from equator Deep ocean models unreliable because of unknown continental configuration, bathymetry and deep ocean circulation Chandler and Sohl, 2000 Poulsen and Jacob, 2004 Poulesn et al., 2001 Ocean Surface Energy Budget

Other Factors Ocean Dynamics January July Strong cross equatorial heat flow Suggestion that a complete snowball may cause a stagnant, stratified ocean Poulsen and Jacob, 2004

Other Factors Snow/Ice Dynamics Differences in sea ice thickness can influence oceanic heating and have implications for sustaining photosynthetic life Estimates are variable – usually capped at 1km over oceans in models Ice thickness determined by 1) heat flux from the ocean to the base of the ice 2) latent heat of freezing to the base of the ice 3) absorbed solar energy at surface 1-10mm/yr freezing at base (tropics) Latent heat loss Wm-2 at base (tropics) (Warrant et al., 2002) Baum and Crowley, 2001

Differences in albedo of sea-ice (~.5, glacial ice (.2-.4), and snow cover (.6-.9) influence absorbed radiation Other ice factors- at < -23C salts crystallize and increase sea-ice to.75 frost-increase albedo, dust- decrease albedo, dried algae- decrease albedo, but block possibility for photosynthesis- opposing Gaia, (Warren et al., 2002), Other Factors Snow/Ice Dynamics

Summary Reduced solar luminosity, low- pCO2, and a favorable continental configuration needed to simulate snowball More advanced Ocean/Atmosphere coupled GCMs produce variable results, in particular the nature of the ocean heat transfer Jury still out if a Snowball was possible climatically Many aspects of the Neoproterozoic climate are inadequately modeled, particularly cloud effects, wind-driven ocean circulation and surface albedos