the faint young sun problem. systems notation = system component = positive coupling = negative...
TRANSCRIPT
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)
The carbonate-silicate cycle feedback
(-)
Surfacetemperature
Rainfall
Silicateweathering
rate
AtmosphericCO2
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...
Archean Climate Control Loop
Surfacetemperature
CH4
production
Hazeproduction
AtmosphericCH4/CO2
ratio
CO2 loss(weathering)
(–)
(–)
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)
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
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
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
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