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 CO 2 (strong dependence) Precipitation (0.6 mm/day) Milankovitch Forcing (orbital effects) Solar Luminosity (6% below present)

Paleogeography (Longitude may be BS… latitude, too)

Atmospheric CO 2 Sharp discontinuity at an IR cooling of ~5 W/m 2 Below this, we get normal Earth. Above, we get Snowball Earth. 5 W/m 2 corresponds to 130 ppm of CO 2

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 CO 2 ice sheets reach 40 o along the coast and 50 o in the interior of the supercontinent. For CO 2 levels below 130 ppm (5 W/m 2 ), 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  13 C 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 CO 2 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.

Return of the Hyde Raising CO 2 by degassing takes too long to be important. CO 2 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.”

Nick’s Musings Hoffman and Hyde should agree… why do they fight? The idea that there is a 1:1 correspondence between atmospheric CO 2 levels and the Earth’s cooling constant seems awfully naïve. Having constant precipitation seems too simple. What about volcanism and tectonism? And biology?