S N O W B A L L E A R T H

E R I N W I L L I A M S O N | E S S 4 3 3 S E M I N A R

A S N O W B A L L E A R T H V E R S U S A S L U S H B A L L E A R T H : R E S U LT S F R O M N E O P R O T E R O Z O I C C L I M AT E M O D E L I N G S E N S I T I V I T Y E X P E R I M E N T S

A R N E M I C H E E L S , M I C H A E L M O N T E N A R I

N E O P R O T E R O Z O I C S N O W B A L L E A R T H H Y P O T H E S I S ?

W H A T I S T H E

S N O W B A L L E A R T H H Y P O T H E S I S

• 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

P R O B L E M S W I T H A S N O W B A L L E A R T H

• 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

A N A LT E R N AT I V E : S L U S H B A L L E A R T H

• Less severe glaciation

• Ice-free ocean areas at the equator, allowing photosynthesis to take place

E X P E R I M E N TA L D E S I G N

• 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

E X P E R I M E N TA L D E S I G N

• 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)

C L I M AT E M O D E L I N G S E N S I T I V I T Y E X P E R I M E N T S

T H E PA L E O G E O G R A P H Y A N D PA L E O - O R O G R A P H Y ( I N M E T E R S ) A S U S E D F O R A L L N E O P R O T E R O - Z O I C S E N S I T I V I T Y E X P E R I M E N T S .

F I G U R E 1

D E S E R T V S G L A C I E R L A N D S U R FA C E C O V E R

• 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)

H I G H E R V S L O W E R C O 2 C O N C E N T R AT I O N

• 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)

C O O L V S C O L D O C E A N C O N D I T I O N S

• 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

R E S U LT S

• 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

T H E T I M E S E R I E S O F T H E G L O B A L AV E R A G E T E M P E R AT U R E

R E S U LT S

T H E G L O B A L S E A I C E D E P T H O F T H E N E O P R O T E R O Z O I C E X P E R I M E N T S

R E S U LT S

C A L C U L AT I N G T H E G L O B A L AV E R A G E T E M P E R AT U R E

• 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

C A L C U L AT I N G T H E G L O B A L AV E R A G E T E M P E R AT U R E

• 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)

T H E G L O B A L AV E R A G E T E M P E R AT U R E S A N D T H E G L O B A L AV E R A G E S E A I C E C O V E R O F T H E P R E S E N T D AY C O N T R O L R U N A N D T H E N E O P R O T E R O Z O I C E X P E R I M E N T S

R E S U LT S

G L O B A L T E M P E R AT U R E A N D S E A I C E C O V E R

• 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

E F F E C T S O F C O 2

• 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)

E F F E C T S O F L A N D S U R FA C E C O V E R

• 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

T H E M E A N A N N U A L T E M P E R AT U R E S ( C ) A N D S E A I C E M A R G I N O F T H E N E O P R O T E R O Z O I C E X P E R I M E N T S

R E S U LT S

T H E M E A N A N N U A L T E M P E R AT U R E S ( C ) A N D S E A I C E M A R G I N O F T H E N E O P R O T E R O Z O I C E X P E R I M E N T S

R E S U LT S

T H E M E A N A N N U A L T E M P E R AT U R E S ( C ) A N D S E A I C E M A R G I N O F T H E N E O P R O T E R O Z O I C E X P E R I M E N T S

R E S U LT S

H Y D R O L O G I C A L C Y C L E A N D T H E F O S S I L R E C O R D

• 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.

L O W T E M P E R AT U R E S A N D C O 2

• 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

T H E Z O N A L AV E R A G E S O F T H E M E A N T E M P E R AT U R E S O F T H E N E O P R O T E R O Z O I C E X P E R I M E N T S

F I G U R E 5

W E A K P O I N T S O F T H E M O D E L R U N S

• 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

S U M M A R Y

• 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

S U M M A R Y

• 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

N E O P R O T E R O Z O I C S L U S H B A L LS E N S I T I V I T Y E X P E R I M E N T S W I T H T H E P L A N E T S I M U L A T O R S U P P O R T A

M O R E T H A N A S N O W B A L L

R E F E R E N C E

• 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

Q U E S T I O N S ?