Princeton and Chicago seminars, Dec. 2004

Deglaciating Snowball Earth

Raymond T. Pierrehumbert

References: Nature June 2004. JGR-Atmospheres, in the press.

1

“Certainty of Death,Small chance of

success,What’r we waitin’

for?

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

2

• No multicellular life, no land plants

• Immediately precedes ”Cambrian Explosion”

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

3

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.

4

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

5

– Fractionation in silicate, carbonate and bicarbonate reactions

• Reduction of δ13C starts before glaciation

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

6

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.

7

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)

• Surface albedo too low (.6, corresponding to sea ice) because of ne-glect of snow cover.

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

8

• Data shown is for last 10 years of each 20-year run

• All analysis done using Python.

Low thermal inertia, extreme seasonal cycle

(Anticipated by Walker)

9

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

The Greenhouse Effect

G = OLR− σT4s

10

-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

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)

The Hadley Circulation

12

-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

Test of IR radiation code at high CO2

13

-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

Do we deglaciate?

14

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

Clear sky Greenhouse

15

-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

Cloud Radiative Forcing

16

-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

Snow cover

17

-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

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

19

-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

• 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).

20

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

22

-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

Insulating effect of snow leads to strong nocturnal boundary layer

23

-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

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.

24

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.

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.

26

“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