deglaciating snowball earth - university of chicago
TRANSCRIPT
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Princeton and Chicago seminars, Dec. 2004
Deglaciating Snowball Earth
Raymond T. Pierrehumbert
References: Nature June 2004. JGR-Atmospheres, in the press.
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“Certainty of Death,Small chance of
success,What’r we waitin’
for?
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An Introduction to the Snowball Earth Problem
• ”Soft Snowball” = low latitude glaciation with open tropical water
• ”Hard Snowball” = globally frozen ocean
• Two Neoproterozoic episodes (600-700 Million Years Ago)
• Sun was 6% fainter than it is now
• Duration of Snowball 1-20 Million Years
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• No multicellular life, no land plants
• Immediately precedes ”Cambrian Explosion”
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The Evidence: Low Latitude Glaciation
• Striations, dropstones, diamictites
• Paleolatitude estimates based on paleomagnetism are now consid-ered quite reliable
• Low latitude glaciation does not demand a hard snowball
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The Evidence: Cap Carbonates”
• Thick, unusual, rapidly deposited marine carbonate layers;widespread in Neoproterozoic times.
• If global, cap carbonates require (suggest?) a ”hard snowball”
• Scenario: CO2 build up during Snowball, followed by deglaciation,followed by precipitation of accumulated carbon as carbonate.
• .2 bar atmospheric CO2 = 10m layer of marine CaCO3, with muchmore coming from dissolved carbonate, and carbonate weathering af-ter deglaciation.
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The Evidence: 13C
• Cap Carbonates show a large negative excursion of δ13C
• Indicative of a long period with little organic carbon burial
• Complications:
– Methane
– Dissolution of marine carbonate reservoir during snowball
– Dissolution of marine and land carbonates after deglaciation
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– Fractionation in silicate, carbonate and bicarbonate reactions
• Reduction of δ13C starts before glaciation
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Mantle, -6‰
HCO3 , 0‰ (Today)
Marine Carbonate+2‰ (today)
Organic C, -20‰
Lan
d ca
rbon
ate
Silic
ate
(Wea
ther
ing)
C in Marine Carbonates13
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But can we deglaciate with .2 bars of CO2?
Energy balance models say yes, easily (Principally: Caldeira and Kasting,Tajika, Ikeda and Tajika). Estimates from Baum and Crowley, Hyde et alhard to evaluate.)
Note: Most people quote .12 bars for N.P. deglaciation, but C&K actuallyimplies .3 bars if you read it carefully. Tajika: .2 bars, under different as-sumptions.
EBM’s say, should be close to deglaciation by .2 bars.
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Problems with energy balance model estimate:
• Cloud radiative forcing. generally held constant
• Dynamic heat transport and meridional temperature gradient
• Seasonal cycle. Ignored (unnecessarily) in many EBM estimates
• Lapse rate of temperature. Implicitly held constant at adiabat, in green-house calculation.
• Strong diurnal cycle (leads to cold mean ice-surface temperature)
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• Surface albedo too low (.6, corresponding to sea ice) because of ne-glect of snow cover.
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GCM Experiments
• FOAM with mixed layer ocean (Basically CCM3)
• Solar constant 94% of present value
• Idealized rectangular Equatorial supercontinent with coastal moun-tains
• First run model to global glaciation, at 100ppm CO2
• Sequence of 20-year runs at 100ppm,400ppm,1600ppm, .1bar,2bar
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• Data shown is for last 10 years of each 20-year run
• All analysis done using Python.
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Low thermal inertia, extreme seasonal cycle
(Anticipated by Walker)
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160
180
200
220
240
260
-90 -60 -30 0 30 60 90
Zonal Mean Air Temperature over Ice at 100ppm
JanuaryJulyAprilOctoberT
empe
ratu
re (
Deg
rees
K)
latitude
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The Greenhouse Effect
G = OLR− σT4s
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-20
-10
0
10
20
30
-90 -60 -30 0 30 60 90
January clear-sky Greenhouse Effect, 100ppm
G(jan)
OL
R r
educ
tion,
W/m
2
latitude
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The Lapse Rate
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100 150 200 250
100
1000
January temperature profile (100ppm CO2)
Dry adiabatT(46N)Dry AdiabatT(46S)
Temperature (Kelvin)
Pres
sure
(m
b)
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The Hadley Circulation
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-80 -60 -40 -20 0 20 40 60 80
100
200
300
400
500
600
700
800
900
1000
-80 -60 -40 -20 0 20 40 60 80
100
200
300
400
500
600
700
800
900
1000
Pre
ssur
e
-200
-150
-100
-50
0
50
100
150
200
-80 -60 -40 -20 0 20 40 60 80
100
200
300
400
500
600
700
800
900
1000
Latitude
January April
-200
-150
-100
-50
0
50
100
150
200
-80 -60 -40 -20 0 20 40 60 80
100
200
300
400
500
600
700
800
900
1000
October
Latitude
Pre
ssur
e
July
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Test of IR radiation code at high CO2
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-1
0
1
2
3
4
1 2 3 4 5 6 7 8 9 10 11 12
OLR(CCM) - OLR(Kasting) at 46 N
.2 bar
.1 bar
OL
R d
iffe
renc
e (W
/m2 )
month
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Do we deglaciate?
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160
180
200
220
240
260
-90 -60 -30 0 30 60 90
January Ice-masked air Temperature
100ppm400ppm1600ppm12800ppm.1bar.2bar
Tem
pera
ture
(D
egre
es K
)
latitude
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Clear sky Greenhouse
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-20
0
20
40
60
80
-90 -60 -30 0 30 60 90
January clear-sky Greenhouse Effect
100ppm400ppm1600ppm12800ppm.1bar.2bar
OL
R r
educ
tion,
W/m
2
latitude
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Cloud Radiative Forcing
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-2
0
2
4
6
8
10
12
-90 -60 -30 0 30 60 90
January Cloud Longwave Forcing
100ppm400ppm1600ppm12800ppm.1bar.2bar
OL
R r
educ
tion,
W/m
2
latitude
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Snow cover
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-1.5
-1
-0.5
0
0.5
1
-90 -60 -30 0 30 60 90
Ann. P-E, 100ppmAnn. P-E, 12800ppmAnn. P-E, .2bar
Net
acc
umul
atio
n (c
m. p
er y
ear
wat
er e
quiv
)
latitude
Annual Mean P-E
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Distribution of Snow Cover
30
0
60
90
-30
-60
-90
Lat
itud
e
Longitude-180 0 180
10cm - 1m
1cm-10cm
1mm-1cm
Bare ice
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Synoptic eddy heat flux
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-0.5
0
0.5
1
1.5
2
2.5
3
-90 -60 -30 0 30 60 90
January Transient Eddy Dry Static Energy FluxE
nerg
y Fl
ux, P
etaw
atts
latitude
100ppm
400ppm
1600ppm12800ppm
.1 bar
.2 bar
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• Snow is not in equilbrium, but enough accumulates to provide fullalbedo effect
• After about 100K years, when snow gets thick, ”sea glaciers” wouldflow (Goodman and Pierrehumbert, JGR)
• Therefore, tropical ice would be glacier ice, not bare sea ice.
• Bare sea-ice albedo in model is .5, vs. .6 for blue glacier ice (Warrenet al).
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Sea Glaciers, Salt and Tropical albedo
EQ NPSP
Accumulation Accumulation
Ablation
?????
Glacier ice =.6Clear sea ice = .5Marine ice = .3Subeutectic ice = .8
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A huge diurnal cycle in temperature and precipitation
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-50
-40
-30
-20
-10
0
10
20
30
40
50
-90 -60 -30 0 30 60 90
January PrecipJanuary EvapJanuary P-E
Net
acc
umul
atio
n (c
m/y
ear
wat
er e
quiv
)
latitude
January water balance, .2 bars
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Insulating effect of snow leads to strong nocturnal boundary layer
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-20-15-10
-505
-90 -60 -30 0 30 60 90
January T(surf) - T(air), .2 bars
Tem
p. d
iff.
, K
-80
-60
-40
-20
0
20
40
60
80
100
120
140
-90 -60 -30 0 30 60 90
Solar(Jan)LW(Jan)Sens(Jan)Latent(Jan)Net
Ene
rgy
flux
, W/m
2
latitude
January Surface Energy Balance, .2 bars
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In the quest for deglaciation, I tried:
• Dusty snow
• Increasing cloud water content all the way to modern value
• Dark marine sea ice
All these produced substantial summer warming, but only weak meanequatorial warming. Effect of cold winter hemisphere is just too strong.
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Conclusions Prior estimates have been over-optimistic about how easyit is to deglaciate Snowball Earth in Neoproterozoic conditions:
• Weak lapse rate in Winter hemisphere, and low tropopause, inhibitsGreenhouse Effect
• Synoptic eddies draw heat into the cold Winter hemisphere
• Widespread snow cover increases albedo
• Cloud effects are weak (little water, even less high up where it’sneeded to make clouds which trap OLR, little summer storm activity)
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Logarithmic extrapolation indicates even 2 bars would not be enough todeglaciate, and at those levels CO2 condensation becomes important.
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Is there no hope for the snowball?
Don’t give up yet. I can believe ten impossible things before Breakfast, ifthe data says they really happened.
Possibilities from hydrothermal plumes, ice fracture.
Suppression of convection in Winter hemisphere is sensitive to stableboundary layer physics. There may still be surprises from clouds and dustysnow.
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“It is a strange fate that so much fear and doubt should be causedby so small a thing ... such a little thing”
Boromir, son of Denethor
Dust Particles in Snow
Bubbles in ice
Salt Particles in Brine
Crystal structure (creep rate) of cold ice