Extreme Climate Change: The late Neoproterozoic “Snowball Earth”What is the snowball Earth?Earth' Climate Through time (Including 30% increase in Solar Luminosity) Adapted from James Lovelock, 1979)Slide 4Slide 5Slide 6Estimated Atmospheric CO2 (Adapted from J.F. Kasting, Science 259, p. 923, 1993)Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Extreme 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 radiationChandler and Sohl, 2000Earth' Climate Through time(Including 30% increase in Solar Luminosity)Adapted from James Lovelock, 1979)Runaway Greenhouse -- Possible Upper LimitIce Catastrophy -- Possible Lower Limit (?)SteamIceActualEarth's ClimateWater150100504-50Average Global Temperature, CTime Before Present (Billion Years)03 210?Kirchvink, www.gps.caltech.edu/users/jkirschvink/Evidence for a snowball EarthGeologic Evidence• Low-latitude glaciogenic deposits: diamictites, dropstones, glacially striated bedrock , varves• Banded Iron Formations (BIF)• Globally extensive “cap” carbonates Hoffman and Schrag, 2002Kirchvink, www.gps.caltech.edu/users/jkirschvink/Kirchvink, www.gps.caltech.edu/users/jkirschvink/Evidence for a snowball EarthGeochemical• Carbon isotope excursions, pre-glacial /postglacial positive/negativeClimatological• 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 CO2(Adapted from J.F. Kasting, Science 259, p. 923, 1993)Ocean-Covered, Fast Young EarthHuronian Snowball Earth(-50 C)Late ProterozoicSnowball Earth (-50 C)30% Solar FluxReduction (0 C) C3 Photo-synthesisoooTime Before Present (Billion Years)3.52.51.50.54.5-1-2-3-410101010101CO Partial Pressure (bar)2CO2 limit fromLack of sideriteIn paleosols (Rye et al., 1995)Kirchvink, www.gps.caltech.edu/users/jkirschvink/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 Cycle1. low-latitude continental break-up2. CO2 reduction, glaciation3. Runaway ice/albedo feedback4. Ocean/Atmosphere isolation CO2 build up5. Extreme Greenhouse7. low-latitude continental break-upClimatic Forcing Factors to get a Snowball EarthNecessary 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 FactorsReduced Solar Luminosity94% 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 considered94% of current, ~ 6% lessNecessary FactorsReduced 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 snowballHoffman and Schrag, 2002Donnadieu et al., 2004Baum and Crowley, 2001Necessary FactorsContinental 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 SturtianVarangerNecessary FactorsContinental ConfigurationPoulsen et al., 2002Examples of plate configurations used inmodels.Hyde et al., 2000Donnadieu et al., 2004Other FactorsAtmospheric 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 effectsOther FactorsAtmospheric DynamicsJanuaryJulyT• Surface winds- important heat transferOther FactorsAtmospheric 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