The Faint Young Sun Problem

Systems Notation

= system component

= positive coupling

= negative coupling

Positive Feedback Loops(Destabilizing)

Surfacetemperature

AtmosphericH2O

Greenhouseeffect

Water vapor feedback

(+)

Positive Feedback Loops(Destabilizing)

Surfacetemperature

Snow and icecover

Planetaryalbedo

Snow/ice albedo feedback

(+)

Negative Feedback Loops(Stabilizing)

Surfacetemperature

IR flux feedback

(-) OutgoingIR flux

Runaway Greenhouse: FIR and FS

J. F. Kasting, Icarus (1988)

The Carbonate-Silicate Cycle

Negative Feedback Loops(Stabilizing)

The carbonate-silicate cycle feedback

(-)

Surfacetemperature

Rainfall

Silicateweathering

rate

AtmosphericCO2

Greenhouseeffect

Model pCO2 vs. Time

J. F. Kasting, Science (1993)

pCO2 from Paleosols (2.8 Ga)

Rye et al., Nature (1995)

Geological O2 Indicators

H. D. Holland, 1994

The Universal Tree of Life

Kasting and Brown (1998)

Pavlov et al., JGR (2000)

CH4-Climate Feedback Loop

Surfacetemperature

CH4

productionrate

Greenhouseeffect

(+)

CH4-Climate Feedback Loop

• Doubling times for thermophilic methan-ogens are shorter than for mesophiles

• Thermophiles will therefore tend to outcompete mesophiles, producing more CH4, and further warming the climate But

• If CH4 becomes more abundant than CO2, organic haze begins to form...

Titan’s Organic Haze Layer

The Anti-greenhouse Effect

Archean Climate Control Loop

Surfacetemperature

CH4

production

Hazeproduction

AtmosphericCH4/CO2

ratio

CO2 loss(weathering)

(–)

(–)

Huronian Supergroup (2.2-2.45 Ga)

Redbeds

Detrital uraniniteand pyrite

Glaciations

Snowball Earth Glaciations

• Paleomagnetic data indicate low-latitude glaciation at 2.3 Ga, 0.75 Ga, and 0.6 Ga

• Huronian glaciation (2.3 Ga) may be triggered by the rise of O2 and the corresponding loss of CH4

• Late Precambrian glaciations studied by Hoffman et al., Science 281, 1342 (1998)

Model pCO2 vs. Time

J. F. Kasting, Science (1993)

Late Precambrian Geography

Hyde et al., Nature, 2000* glacial deposits

Triggering a Snowball Earth episode

• Hoffman et al.: Continental rifting created new shelf area, thereby promoting burial of organic carbon

• Marshall et al. (JGR, 1988): Clustering of continents at low latitudes allows silicate weathering to proceed even as the global climate gets cold

Caldeira and Kasting, Nature, 1992

Recovering from a Snowball Earth episode

• Volcanic CO2 builds up to ~0.1 bar

• Ice melts catastrophically (within a few thousand years)

• Surface temperatures climb briefly to 50-60oC

• CO2 is rapidly removed by silicate weathering, forming cap carbonates

Hoffman et al.,Science, 1998

‘Cap’ carbonate(400 m thickness)

How did the biota survive the Snowball Earth?

• Refugia such as Iceland?

• Hyde et al. (Nature, 2000): Tropical oceans were ice free

• C. McKay (GRL, 2000): Tropical sea ice may have been thin

Snowball EarthIce Thickness

Fg

Ts

Toc 0oC

Let k = thermal conductivity of ice z = ice thickness T = Toc – Ts

Fg = geothermal heat flux

z

Ice Thickness (cont.)

The diffusive heat flux is: Fg = kT / z Solving for z gives:

z = kT / Fg

2.5 W/m/K(27 K)/ 6010-3 W/m2

= 1100 m

Heat Flow Through Semi-transparent, Ablating Ice

Ref: C. P. McKay, GRL 27, 2153 (2000)

k dT/dz = S(z) + L + Fg

where k = thermal conductivity of ice S(z) = solar flux at depth z in the ice L= latent heat flux (balancing ablation) Fg = geothermal heat flux

Comparative Heat Fluxes

Geothermal heat flux: Fg = 6010-3 W/m2

Solar heat flux (surface average):Fs = 1370 W/m2(1 – 0.3)/4 240 W/m2

Equatorial heat flux:Feq 1.2 Fs 300 W/m2

Ratio of equatorial heat flux (from Sun) vs. geothermal heat flux: Feq/Fg 300/0.006 = 5000

Ice Transmissivity

C. McKay, GRL (2000)

Heat Fluxes (cont.)

Now, lettR = ice transmissivity

Then, scaling ice thickness inversely withtransmitted heat flux yields:

tR z10-3 ~200 m10-2 ~20 m10-1 ~2 m

CONCLUSIONS

• Earth’s climate is stabilized on long time-scales by the carbonate-silicate cycle

• Higher atmospheric CO2 levels are a good way of compensating for the faint young Sun

• CH4 probably made a significant contribution to the greenhouse effect during the Archean when O2 levels were low

CONCLUSIONS (cont.)

• Earth’s climate is theoretically susceptible to episodes of global glaciation. It can recover from these by buildup of volcanic CO2

• The first such “Snowball Earth” episode at ~2.4 Ga may have been triggered by the rise of O2 and loss of the methane component of the atmospheric greenhouse

CONCLUSIONS (cont.)

• The true “Snowball Earth” model (complete glacial ice cover) best explains the geological evidence, particularly the presence of cap carbonates