2. ARNE MICHEELS, MICHAEL MONTENARI
A SNOWBALL EARTH VERSUS A SLUSHBALL EARTH: RESULTS FROM
NEOPROTEROZOIC CLIMATE MODELING SENSITIVITY EXPERIMENTS
3. WHAT IS THE
NEOPROTEROZOIC
SNOWBALL EART H
HYPOTHESIS?
4. SNOWBALL EARTH HYPOTHESIS
• An intense degree of global glaciation between 800-600 million years ago
• At least two major glaciations can be identified, with a possible third (though
some posit as many at five)
• Glacial conditions advanced into the equatorial latitudes via a runaway
feedback state (ice-albedo feedback)
• Oceans froze to a depth of ~1 km, forming a dense non-light-transmissive layer
• Possible causes include a reduction in greenhouse gases, break-up of Rodinia,
reduced insolation
5. PROBLEMS WITH A
SNOWBALL EARTH
• Evidence of an actively working
hydrological cycle during the
Neoproterozoic
• Existence of a widespread, light-dependent,
complex microbial
ecosystem
• No evidence for a prominent
extinction phase during that time
6. AN ALTERNAT I V E :
SLUSHBALL EARTH
• Less severe glaciation
• Ice-free ocean areas at the equator,
allowing photosynthesis to take place
7. EXPERIMENTAL DESIGN
• Earth system model of intermediate complexity (EMIC) Planet Simulator
• Reliably used for present-day and Miocene climate modeling
• Neoproterozoic Boundary Conditions for 8 sensitivity experiments
• adapted paleogeography and paleo-orography
• lower solar luminosity by -6%
• present-day orbital parameters
8. EXPERIMENTAL DESIGN
• Control Experiment (CTRL)
• Present day geography, orography, vegetation, sea surface temperature
(SST) and sea ice cover (SIC)
• Atmospheric CO2 set to preindustrial concentration of 280 ppm
• Two different ocean settings: a cool and a cold situation
• Two different global land surface cover settings: desert versus glaciated
• Variations of atmospheric CO2: higher versus lower concentrations (510 vs 280)
10. FIGURE 1
THE PALEOGEOGRAPHY AND PALEO-OROGRAPHY (IN METERS) AS
USED FOR ALL NEOPROTERO- ZOIC SENSITIVITY EXPERIMENTS.
11. DESERT VS GLACIER
LAND SURFACE COVER
• Glacier simulations (NEO-2 to
NEO-4): fully ice-covered continents
• Desert simulations (NEO-1 and
NEO-5): completely ice-free with
sand desert
• Sand desert used for its higher
albedo (α = 0.35) than a normal
desert (α = 0.20)
12. HIGHER VS LOWER
CO2 CONCENTRATION
• 510 ppm vs 280 ppm
• Atmospheric CO2 concentration of
510 ppm in a first set of sensitivity
experiments (NEO-1 to NEO-5)
• Atmospheric CO2 concentration of
280 ppm in a second set of sensitivity
experiments (NEO-3-280 to
NEO-5-280)
13. COOL VS COLD
OCEAN CONDITIONS
• Cold ocean conditions: NEO-1 and NEO-2
• Global constant SSTs of 271 K
• Global ice cover of a depth of 1 m
• Cool ocean conditions: NEO-3 to NEO-5
• At the equator, initial SSTs set to 280 K
and decline to 265 K at the poles.
• Ice cover where SSTs are below the
freezing point and set ice depth to 1 m
14. RESULTS
• Performed eight Neoproterozoic sensitivity experiments
• NEO-1 and NEO-2 run in the Planet Simulator for 4 k.y.
• Result in a snowball Earth
• NEO-3 to NEO-5-280 run in the Planet Simulator for 1 k.y.
• Result in a slushball Earth
18. CALCULATING THE GLOBAL
AVERAGE TEMPERATURE
• T (K) is the global average temperature
• S↓ and S↑ (W/m2) are the solar and
terrestrial radiation flux
• S0 (W/m2) is the solar constant
• α (fractional) is the planetary albedo,
• σ = 5.67·10−8 W m–2 K–4 is the Stefan-
Boltzmann constant
• RE = 6378 km, the radius of the Earth
19. CALCULATING THE GLOBAL
AVERAGE TEMPERATURE
• For the present-day planetary albedo of
α = 0.3, this simple energy budget
results in a global average T = –18.2 °C
• CTRL simulation is warmer than the
theoretical value (ΔT = +32.4 °C) due
to neglecting the greenhouse effect
• With a planetary albedo corresponding
to ice (α = 0.7), the resulting T = –70.5 °C
• NEO-2, global average T = –68.2 °C;
(ΔT = +2.3 °C)
20. RESULTS
THE GLOBAL AVERAGE TEMPERATURES AND THE GLOBAL AVERAGE SEA ICE
COVER OF THE PRESENT DAY CONTROL RUN AND THE NEOPROTEROZOIC
EXPERIMENTS
21. GLOBAL TEMPERATURE AND SEA ICE COVER
• The snowball experiments NEO-1 and NEO-2 demonstrate much colder
global temperatures as compared to other modeling studies
• The global average temperatures of NEO-3 to NEO-5-280 are closer to
other Neoproterozoic model studies
• Snowball Earth is obtained (NEO-1 and NEO-2) only if the setup strongly
pushes the model into this situation
22. EFFECTS OF CO2
• Reduction of atmospheric CO2 from 510
ppm to 280 ppm triggers the climate toward
cooler conditions
• The stronger the degree of the Earth’s
glaciation in the simulations, the less
sensitive is the climate system reaction to
variations of greenhouse gas concentrations.
• An escape from an extreme glaciation
should require a strongly enhanced CO2
concentration, eventually resulting in a
super-greenhouse environment (freeze-fry)
23. EFFECTS OF LAND
SURFACE COVER
• Due to lower albedo, desert
simulations represent globally
warmer conditions and less sea ice
than the runs with continental
glaciers
• The formation of continental glaciers
via the positive ice-albedo feedback
might have contributed significantly
to a widespread freezing of the
Neoproterozoic Earth
24. RESULTS
THE MEAN ANNUAL TEMPERATURES (C) AND SEA ICE
MARGIN OF THE NEOPROTEROZOIC EXPERIMENTS
25. RESULTS
THE MEAN ANNUAL TEMPERATURES (C) AND SEA ICE
MARGIN OF THE NEOPROTEROZOIC EXPERIMENTS
26. RESULTS
THE MEAN ANNUAL TEMPERATURES (C) AND SEA ICE
MARGIN OF THE NEOPROTEROZOIC EXPERIMENTS
27.
28. HYDROLOGICAL CYCLE AND THE FOSSIL RECORD
• Evidence that contradicts the snowball hypothesis agrees with the ice-free
ocean belt in the slushball scenarios.
• NEO-3 to NEO-5-280 supports that there may have been ice-free
regions; Thick layers from some formations of the Neoproterozoic glacial
phase support that there was an actively working hydrological cycle.
• The scenario of a snowball Earth would show a massive extinction,
especially affecting the light-dependent organisms, such as
photoautotrophic prokaryotes and eukaryotes; There is no reliable
evidence for a global extinction event.
29. LOW TEMPERATURES AND CO2
• Minimum temps. in NEO-2 fall below the sublimation/deposition point of CO2
• If the Neoproterozoic was so cold (-110 °C in some models), is it possible and
reasonable that CO22 could have changed from gas to solid phase in winter?
• The greenhouse effect of increasing concentrations of CO2 could be nullified
• The possibility of the occurrence of carbon dioxide ice increases
• The escape out of a snowball Earth becomes difficult if atmospheric carbon
dioxide and, therefore, the greenhouse effect are reduced due to phase
changes of CO2
30. FIGURE 5
THE ZONAL AVERAGES OF THE MEAN TEMPERATURES
OF THE NEOPROTEROZOIC EXPERIMENTS
31. WEAK POINTS OF THE MODEL RUNS
• Used 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
• Uncertainties with respect to the paleogeography and paleo-orography
• Further model experiments could focus on the sensitivity with respect to
the paleogeography and paleo-orography
32. SUMMARY
• Earth system model of intermediate
complexity, Planet Simulator
• Cool versus a cold ocean
• Desert versus a glacier land surface
• Higher versus a lower atmospheric
concentration of carbon dioxide
33. SUMMARY
• NEO-1 and NEO-2 support the snowball earth
hypothesis with extremely cold conditions
• NEO-3 to NEO-5 support a less severe
slushball earth with moderately cold conditions
• Relatively ice-free equatorial belt and lower
latitudes
• A strongly enhanced CO2 concentration is
required to escape the frozen situation, which
would result in an extreme greenhouse world.
• Future model studies should address how
much CO2 is needed to melt a snowball
earth
35. REFERENCE
• Micheels, A., and M. Montenari (2008), A snowball Earth versus a slushball
Earth: Results from Neoproterozoic climate modeling sensitivity
experiments, Geosphere, v. 4, no. 2, p. 401-410, doi: 10.1130/GES00098.1
