The Faint Young Sun Problem

Systems Notation = system component = positive coupling = negative coupling

Positive Feedback Loops (Destabilizing) Surface temperature Atmospheric H 2 O Greenhouse effect Water vapor feedback (+)

Positive Feedback Loops (Destabilizing) Surface temperature Snow and ice cover Planetary albedo Snow/ice albedo feedback (+)

Negative Feedback Loops (Stabilizing) Surface temperature IR flux feedback (-) Outgoing IR flux

Runaway Greenhouse: F IR and F S J. F. Kasting, Icarus (1988)

The Carbonate-Silicate Cycle

Negative Feedback Loops (Stabilizing) The carbonate-silicate cycle feedback (-) Surface temperature Rainfall Silicate weathering rate Atmospheric CO 2 Greenhouse effect

Model pCO 2 vs. Time J. F. Kasting, Science (1993)

pCO 2 from Paleosols (2.8 Ga) Rye et al., Nature (1995)

Geological O 2 Indicators H. D. Holland, 1994

The Universal Tree of Life

Kasting and Brown (1998)

Pavlov et al., JGR (2000)

CH 4 -Climate Feedback Loop Surface temperature CH 4 production rate Greenhouse effect (+)

CH 4 -Climate Feedback Loop Doubling times for thermophilic methan- ogens are shorter than for mesophiles Thermophiles will therefore tend to outcompete mesophiles, producing more CH 4, and further warming the climate But If CH 4 becomes more abundant than CO 2, organic haze begins to form...

Titan’s Organic Haze Layer

The Anti-greenhouse Effect

Archean Climate Control Loop Surface temperature CH 4 production Haze production Atmospheric CH 4 /CO 2 ratio CO 2 loss (weathering) (–)

Huronian Supergroup ( Ga) Redbeds Detrital uraninite and 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 O 2 and the corresponding loss of CH 4 Late Precambrian glaciations studied by Hoffman et al., Science 281, 1342 (1998)

Model pCO 2 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 CO 2 builds up to ~0.1 bar Ice melts catastrophically (within a few thousand years) Surface temperatures climb briefly to o C CO 2 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 Earth Ice Thickness FgFg TsTs T oc  0 o C Let k = thermal conductivity of ice  z = ice thickness  T = T oc – T s F g = geothermal heat flux zz

Ice Thickness (cont.) The diffusive heat flux is: F g = k  T /  z Solving for  z gives:  z = k  T / F g  2.5 W/m/K(27 K)/ 60  W/m 2 = 1100 m

Heat Flow Through Semi-transparent, Ablating Ice Ref: C. P. McKay, GRL 27, 2153 (2000) k dT/dz = S(z) + L + F g where k = thermal conductivity of ice S(z) = solar flux at depth z in the ice L= latent heat flux (balancing ablation) F g = geothermal heat flux

Comparative Heat Fluxes Geothermal heat flux: F g = 60  W/m 2 Solar heat flux (surface average): F s = 1370 W/m 2 (1 – 0.3)/4  240 W/m 2 Equatorial heat flux: F eq  1.2 F s  300 W/m 2 Ratio of equatorial heat flux (from Sun) vs. geothermal heat flux: F eq /F g  300/0.006 = 5000

Ice Transmissivity C. McKay, GRL (2000)

Heat Fluxes (cont.) Now, let t R = ice transmissivity Then, scaling ice thickness inversely with transmitted heat flux yields: t R  z ~200 m ~20 m ~2 m

CONCLUSIONS Earth’s climate is stabilized on long time- scales by the carbonate-silicate cycle Higher atmospheric CO 2 levels are a good way of compensating for the faint young Sun CH 4 probably made a significant contribution to the greenhouse effect during the Archean when O 2 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 CO 2 The first such “Snowball Earth” episode at ~2.4 Ga may have been triggered by the rise of O 2 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