1. A snowball Earth versus a slushball Earth:
Results from Neoproterozoic climate modeling sensitivity experiments
So, this paper is similar to the snowball earth presentation we had last week. It is about whether we had a snowball earth or a slushball earth, but this paper talks about experiments that were conducted with an earth model.
by Arne Micheels & Michael Montenari
Alex Wang

2. INTRO The Neoproterozoic (1000-542Ma)
5 successive ice ages have been proposed for the entire Neoproterozoic
Only 2 major glaciations can be identified with confidence; the existence of a third is debated
Glacial conditions advanced into equatorial latitudes.
But was the Neoproterozoic Earth completely glaciated?
Lets start with a little background,
Neoproterozoic sedimentary deposits can be found across the world and they document an intense degree of glaciation for that time.
Based on lithostratigraphic and chemostratigraphic evidence from Australia, Canada, Congo, Namibia, and Spitzbergen five successive ages have been proposed during that time
The third, Possibly Marinoan-aged, major glaciation, evidenced by glacigenic deposits at the Varanger Peninsula in northern Norway
Glacial conditions advanced even into the equatorial latitudes.
Even though there is the widespread occurrence of glacial deposits in the Neoproterozoic, but was the Neoproterozoic Earth completely glaciated?

3. Experiments On a cool V.S. cold ocean
On a desert V.S. a glacier land surface
On a lower V.S. a higher carbon dioxide concentration
(All Neoproterozoic model experiments represent much colder conditions than today, and widespread glaciation)
Sensitivity experiments were performed with an Earth model of intermediate complexity for this period of dramatic global cooling.
The simulations focus on the climate response on

4. EMIC Planet Simulator
Earth system model of intermediate complexity(EMIC) Planet Simulator
Core module: the simple atmospheric general circulation model (AGCM) PUMA-2
PUMA-2 is based on the primitive equations representing
Used for the present day climate, proved its reliability.
Performed a present-day control experiment:
present-day sea ice cover (SIC)
present-day sea surface temperatures (SSTs)
modern day vegetation
atmospheric CO2 concentration of 280 ppm
They used the Earth system model of intermediate complexity Planet Simulator
Performed climate modeling sensitivity experiments for the Neoproterozoic.
The core module is the simple atmospheric general circulation model, PUMA-2, which is coupled to a slab ocean and thermodynamic sea ice model.
PUMA-2 is based on the primitive equations representing i.e. the conservation of mass and momentum, and the first principle of thermodynamics.
The planet simulator was used for the present day climate and proved its reliability.
With the simulator, they performed a present-day control experiment, and with the control experiment, they force the model with present-day…

5. Paleogeography and Paleo-orography
The land-sea distribution is based on the software “Plate Tracker”-Rodinia\
Assumed that the break up of the Rodinia initiated the transition into the Neoproterozoic glaciation
Assumed the supercontinent configuration with its positioning in lower latitudes is needed for an extreme degree of glaciation
Assumed the mountains at the continental plate margins were formely as much as 1000m high; paleoelevation of 50m
Lower solar luminosity by -6% as compared to today
The land-sea distribution is based on the software Plate Tracker and it represents almost one supercontinent known as Rodinia
They assume that the break up of the Rodinia initiated the transition in the Neoproterozoic glaciation
They assume the supercontintent configuration with its positioning in lower latitudes is needed for an extreme degree of glaciation
assume that mountains at the continental plate margins were formerly as much as 1000 m high. For interior parts of the landmasses, they used a paleoelevation of 50 m.
In their simulations, the paleo-orography is not necessarily fully realistic, but it is between previous estimates
Also, it is known that the orography was not the driving factor for the initiation of snowball conditions.

6. Land Surface Cover: Desert V.S. Glacier
Two scenarios: Desert & Glacier
Glacier simulations: fully ice-covered continents.
Desert simulation: completely ice-free situation with desert covering the landmasses; specified conditions of a higher albedo (α = 0.35)
A rather cold state in the model experiments.
For the land surface, they defined two scenarios: desert and glacier.
The glacier simulations (NEO-1 to NEO-4) include a set of surface parameters corresponding to fully ice-covered continents.
For the desert experiment (NEO-5), surface parameters refer to a completely ice-free situation with desert covering the landmasses.
For the desert scenario, they specified conditions of a sand desert because it has a higher albedo (α = 0.35) as compared to a normal desert (α = 0.20).
This setting should contribute to force a rather cold state in the model experiments.

7. CO2: Higher V.S. Lower Present-day Control: 280 ppm
Lack of knowledge for the concentration of CO2 for the Neoproterozoic
Breakup of the supercontinent Rodinia lead to a decrease of atmospheric CO2, resulting in values of ~510 ppm (or even lower) because of increased continental weathering rates
Based on the study, set the CO2 concentration at 510 ppm for NEO-1 to NEO-5
Because of the uncertainty, 280 ppm for NEO to NEO-5-280
For the CO2 concentration,
The present-day control was set with a carbon dioxide concentration of 280ppm.
The experiment also require a concentration for the Neoproterozoic, but the they didn't’t have enough information.
So, they followed the idea that the breaking up of the supercontinent Rodinia led to a decrease of atmospheric CO2, which is at about 510 ppm, because of increased continental weathering rates.
Based on the study, they set the CO2 concentration at 510 ppm for the first set of experiments and because of the uncertainty, they set the second set of experiments with 280ppm(preindustrial CO2 level)

8. Initial Ocean Conditions: Cool Ocean V.S. Cold Ocean
Adjusted deep sea temperature(DST), the sea surface temperatures(SSTs), and the depth of the mixed-layer (MLD)
Considers a global constant temperature close to the freezing point (DSTglobal = 273 K); depth of the mixed-layer (MLDglobal = 50 m)
NEO-1 and NEO-2=Cold Ocean; SSTs of 271K; global ice cover of 1 m
NEO-3 to NEO-5=Cool Ocean; SSTs varys as a function of the cosine of the latitude [SST(φ) = f(cos φ)]; allow ice cover where SSTs are below the freezing point and set ice depth to 1 m
At the equator, initial SSTs are set to 280 K and decline to 265 K at the poles.
For the ocean setup, they adjusted the deep- sea temperature (DST), the SST, and the depth of the mixed-layer (MLD).
The initiation of the deep ocean considers a global constant temperature close to the freezing point.
The depth of the mixed-layer is also initialized with a globally constant value (MLDglobal = 50 m).
They defined cold (NEO-1 and NEO-2) and cool (NEO-3 to NEO-5) ocean scenarios. The cold ocean scenario uses global constant SSTs of 271 K, which correspond to the freezing point of seawater (Tfreeze = K) in the Planet Simulator.
Contrarily, SSTs of the cool scenario vary as a function of the cosine of the latitude [SST(φ) = f(cos φ)].
At the equator, initial SSTs are set to 280 K and decline to 265 K at the poles.

9. Performed 8 Neoproterozoic sensitivity experiments
With the set of boundary conditions, they performed eight Neoproterozoic sensitivity experiments, and here are the experiments specifications.

10. Results NEO-1 and NEO-2 represent a Neoproterozoic snowball Earth.
NEO-3 to NEO-5 achieve moderately cold slushball Earth condition.
Climate response to CO2 level is minor
Overall: the sensitivity experiments supports Slushball Earth
NEO-1 and NEO-2 represent a Neoproterozoic snowball Earth with extremely cold conditions. This is not surprising because the initial glaciation of both runs provides a strong model forcing via the ice-albedo feedback mechanism.
The simulations NEO-3 to NEO-5 achieve moderately cold slushball conditions with a more or less distinct ice-free ocean belt in lower latitudes.

12. Weak Points of the Model Runs
Use no explicit flux correction.
Do not consider that ocean currents east of Rodinia should transport warmer water masses toward middle and latitudes; western ones should bring cooler water into low latitudes.
Uncertainties with respect of paleogeography and paleo-orography.
They use no explicit flux correction and do not consider that ocean currents east of Rodinia should transport warmer water masses toward middle and high latitudes, while western ones should bring cooler water into low latitudes. This might modify but should not fundamentally change the climate response in their simulations.
Also, their sensitivity studies might include some uncertainties with respect to the paleogeography and paleo-orography