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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