Slushball Earth
Neoproterozoic ‘snowball Earth’ simulations with a coupled climate/ice-sheet model
(Hyde et al. 2000)
Nick Cowan
February 2006
Outline
• Paleomagnetic and geological evidence points towards snowball Earth events in the late Proterozoic.
• Run simulations to verify how easy (or difficult) it is to bump terrestrial climate into a SBE state.
• Comment on any unexpected simulation results.
Ice Sheet Model and Boundary Conditions
• Ice Flow
• Mass Balance
• Temperature (diffusive, 2-D EBM)
• Bedrock Sinking = 4000 yrs
Simulation Inputs
• Paleogeography (Dalziel’s reconstruction)
• Atmospheric CO2 (strong dependence)
• Precipitation (0.6 mm/day)
• Milankovitch Forcing (orbital effects)
• Solar Luminosity (6% below present)
Paleogeography
(Longitude may be BS… latitude, too)
Atmospheric CO2
• Sharp discontinuity at an IR cooling of ~5 W/m2
• Below this, we get normal Earth.
• Above, we get Snowball Earth.
• 5 W/m2 corresponds to 130 ppm of CO2
Precipitation
• Less precipitation yields thinner ice.
• With no precipitation, calving of icebergs eventually reduces ice volume to zero.
• Even after 10 Myrs, the ice volume is twice that of the Pleistocene max.
Ice-Sheet Model
• Ice is hard to melt.• Ice sheets can
expand into areas which are originaly too hot to freeze.
• Their thermal inertia allows these more temperate regions to freeze, too.
Climate Simulations
• Plenty of ice, just not at the equator.
• The ice sheets are cold (below feezing)
• The rain falls mainly back on the water belt, where it doesn’t freeze.
Simulation Results (part I)• With present-day CO2 ice sheets reach 40o
along the coast and 50o in the interior of the supercontinent.
• For CO2 levels below 130 ppm (5 W/m2), the entire Earth is covered in ice.
• The transition to/from SBE happens in a couple thousand years.
• Greater continental freeboard results in better cooling.
• The exact configuration of continents is largely unimportant.
Simulation Results (part II)
• The interactive ice-sheet is important.• In some simulations, a band of open
equitorial water survived the so-called SBE state: colder, dryer, lower albedo.
• The humidity stays above the water belt.• This conflicts with 13C measurements, unless
metazoans evolved around then.• The huge amounts of ice make for salty
oceans, but this isn’t a problem.• On a colder Earth, there is less precipitation,
and calving of icebergs decreases ice-volume.
Conclusions & Discussion
• Hoffman et al. was right!
• It isn’t very hard to bump an ancient Earth into a SBE state: a slightly dimmer Sun and a bit less CO2 do the trick.
• Open waters may have existed near the equator (good for metazoans!).
Hoffman Strikes Back
• Observations require that the oceans were briefly anoxic, hence the glaciations must have been complete.
• Volcanism? Weathering?
• Eukaryotic life is tougher than you think. SBE wouldn’t have killed it, it would have built character.
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Return of the Hyde• Raising CO2 by
degassing takes too long to be important.
• CO2 affects glaciation in a non-linear way.
• Metazoans are wimps.• “We believe that the
open-water solution is much more favorable to the survival of metazoans, allowing their remote progeny to continue this discussion.”
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Nick’s Musings
• Hoffman and Hyde should agree… why do they fight?• The idea that there is a 1:1 correspondence between
atmospheric CO2 levels and the Earth’s cooling constant seems awfully naïve.
• Having constant precipitation seems too simple.• What about volcanism and tectonism? And biology?
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