Extreme Climate Change: The late Neoproterozoic

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Extreme Climate Change The late Neoproterozoic
Snowball Earth
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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
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Earth' Climate Through time(Including 30
increase in Solar Luminosity)Adapted from James
Lovelock, 1979)
Kirchvink, www.gps.caltech.edu/users/jkirschvink/
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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

Kirchvink, www.gps.caltech.edu/users/jkirschvink/
Hoffman and Schrag, 2002
Kirchvink, www.gps.caltech.edu/users/jkirschvink/
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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)

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Estimated Atmospheric CO2(Adapted from J.F.
Kasting, Science 259, p. 923, 1993)
Kirchvink, www.gps.caltech.edu/users/jkirschvink/
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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)

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

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Necessary FactorsReduced Solar Luminosity

• 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
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Necessary FactorsReduced Atmospheric CO2
Donnadieu et al., 2004

• 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
Baum and Crowley, 2001
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Necessary 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

Sturtian
Varanger
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Necessary FactorsContinental Configuration
Examples of plate configurations used in models.
Hyde et al., 2000
Donnadieu et al., 2004
Poulsen et al., 2002
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(Pierrehumbert, 2004)
Other 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

Winter hemisphere isothermal-radiative
equilibrium, Convection occurs in southern
hemisphere but low tropopause limits greenhouse
effects
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Other FactorsAtmospheric Dynamics
January

• Surface winds- important heat transfer

July
T
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Other 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 variations- unstable boundary layer
  during day and night stably stratfied- enhanced
  SH loss

Pierrehumbert, 2005
Pierrehumbert, 2005
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Other FactorsOcean 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
Poulesn et al., 2001
Ocean Surface Energy Budget
Poulsen and Jacob, 2004
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Other FactorsOcean Dynamics
January

• Strong cross equatorial heat flow
• Suggestion that a complete snowball may cause a
  stagnant, stratified ocean

July
Poulsen and Jacob, 2004
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Other FactorsSnow/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 0.01-0.1 Wm-2 at base (tropics)
  (Warrant et al., 2002)

Baum and Crowley, 2001
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Other FactorsSnow/Ice Dynamics

• Differences in albedo of sea-ice (.5, glacial
  ice (.2-.4), and snow cover (.6-.9) influence
  absorbed radiation
• Other ice factors- at lt -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),

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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