climate modeling of the early earth: what do we know and how well do we know it? james kasting...
TRANSCRIPT
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Climate Modeling of the Early Earth: What Do We Know and
How Well Do We Know It?
James KastingDepartment of Geosciences
Penn State University
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PhanerozoicTime
First shelly fossils
Age of fishFirst vascular plants on land
Ice age
Ice age
First dinosaurs
Dinosaurs go extinct
Ice age (Late Cenozoic)
Warm
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Geologic time
Warm (?)
Rise of atmospheric O2
First shelly fossils (Cambrian explosion)Snowball Earth ice ages
Warm
Ice age
Origin of life
(Ice age)
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G.M. Young et al., J. Geol. (1998)
Evidence for 2.9-Gaglaciation
• One finds diamictites and dropstones in the 2.9-Ga Mozaan Group in South Africa, within the Pongola Basin
• Diamictites are called tillites once their glacial origin is established
• Diamictites are also found in the Belingue Greenstone Belt in Zimbabwe (E. M. Nisbet et al. Geology, 1993), although they were not identified as glacial by the authors
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2.9-Ga dropstone
• Dropstones are fist-sized (or larger) chunks of rocks found in laminated marine sediments that are thought to have been deposited by melting icebergs
• This one was found in the muddy upper layers of the diamictites shown in the previous slide
G.M. Young et al., J. Geol. (1998)
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Oxygen isotope evidence for a hot early Earth?
• The geologic data suggest that the Archean was cool (but not frozen), at least some of the time
• By contrast, oxygen isotope data from cherts (SiO2), appear to indicate that the Achean Earth was hot, 7015oC, at 3.3 Ga
• Indeed, data from carbonates (not shown) suggest that the Earth remained warm up until ~400 Ma
• These isotopic data have multiple interpretations, some of which will come up during Paul Knauth’s talk
WarmCold
(SM
OW
)P. Knauth, Paleo3 219, 53 (2005)
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Surface temperature vs. pCO2 and fCH4
Kasting and Howard, Phil. Trans. Roy. Soc. B (2006)
3.3 GaS/So = 0.77
• Getting surface temperatures of ~70oC at 3.3 Ga would require of the order of 3 bars of CO2 (10,000 times present), possibly more, as the greenhouse effect of CH4 was overestimated in these calculations• pH of rainwater would drop from 5.7 to ~3.7• Would this be consistent with evidence of paleoweathering?
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• From a theoretical standpoint, it is curious that the early Earth was warm, because the Sun is thought to have been less bright
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Why the Sun gets brighter with time
• H fuses to form He in the core
• Core becomes denser• Core contracts and
heats up• Fusion reactions
proceed faster• More energy is produced
more energy needs to be emitted
Figure redrawn from D.O. Gough,Solar Phys. (1981)
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• Various astronomers have come up with proposals for altering the standard solar evolution model, and we will hear about that at this meeting
• For now, I will assume that the young Sun was indeed faint
• This has big implications for planetary climates, as first pointed out by Sagan and Mullen (1972)…
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The faint young Sun problem
Kasting et al., Scientific American (1988)
Te = effective radiating temperature = [S(1-A)/4]1/4
TS = average surface temperature
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• Feedbacks are important in the climate system
• One way to describe these is with systems diagrams
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Systems Notation
= system component
= positive coupling
= negative coupling
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Positive Feedback Loops(Destabilizing)
Surfacetemperature
AtmosphericH2O
Greenhouseeffect
Water vapor feedback
(+)
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The faint young Sun problem
• The best solution to this problem is higher concentrations of greenhouse gases in the distant past (but not H2O, which only makes the problem worse)
LessH2O
MoreH2O
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Greenhouse gases
• Greenhouse gases are gases that let most of the incoming visible solar radiation in, but absorb and re-radiate much of the outgoing infrared radiation
• Important greenhouse gases on Earth are CO2, H2O, and CH4
– H2O, though, is always near its condensation temperature; hence, it acts as a feedback on climate rather than as a forcing mechanism
• The decrease in solar luminosity in the distant past could have been offset either by higher CO2, higher CH4, or both. Let’s consider CO2 first
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The carbonate-silicate cycle
(metamorphism)
• Silicate weathering slows down as the Earth cools atmospheric CO2 should build up• This is probably at least part of the solution to the faint young Sun problem
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Negative feedback loops(stabilizing)
The carbonate-silicate cycle feedback
(−)
Surfacetemperature
Rainfall
Silicateweathering
rate
AtmosphericCO2
Greenhouseeffect
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CO2 vs. time if no other greenhouse gases (besides H2O)
J. F. Kasting, Science (1993)
• In the simplest story, atmospheric CO2 levels should have declined monotonically with time as solar luminosity increased• Geochemists, however, insist on making the story more complicated…
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Sheldon’s pCO2 estimate from paleosols
• First published estimate of Archean CO2 from paleosols came from Rye et al. (1995) (probably incorrect)
• Sheldon presented an alternative analysis of paleosols based on mass balance arguments (efficiency of weathering)
• Sheldon’s calculated pCO2 at 2.2 Ga is also about 0.008 bar, or 28 PAL, with a quoted uncertainty of a factor of 3
• New estimate for pCO2 at 2.7 Ga (Driese et al., 2011): 10-50 PAL
N. Sheldon, Precambrian Res. (2006)
Driese et al.,2011
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Rosing et al.: CO2 from BIFs
J. F. KastingNature (2010)
Rye et al.(old)
Ohmoto
Sheldon
von Paris et al.
• More recently, Rosing et al. (Nature, 2010) have tried to place even more stringent constraints on past CO2 using banded iron-formations (BIFs)• I have argued with these estimates, but I’ll let the authors present them first, and then we can argue..
(pCO2 3 PAL)
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Sagan and Mullen, Science (1972)
• Sagan and Mullen liked ammonia (NH3) and methane (CH4) as Archean greenhouse gases• As a result of Preston Cloud’s work in the late 1960’s, they were aware that atmospheric O2 was low on the early Earth
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• But Sagan and Mullen hadn’t thought about the photochemistry of ammonia
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Problems with Sagan and Mullen’s hypothesis
• Ammonia is photochemically unstable with respect to conversion to N2 and H2 (Kuhn and Atreya, 1979)
• There may be ways to salvage this hypothesis, or part of it, by shielding ammonia with fractal organic haze (see talk by Feng Tian)
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CH4 as an Archean greenhouse gas
• CH4 is a better candidate for providing additional greenhouse warming
• Substrates for methanogenesis should have been widely available, e.g.:
CO2 + 4 H2 CH4 + 2 H2O• Methanogens (organisms that produce
methane) are evolutionarily ancient– We can tell this by looking at their DNA
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Feedbacks in the methane cycle
• Furthermore, there are strong feedbacks in the methane cycle that would have helped methane become abundant
• 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
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CH4-climate positive feedback loop
Surfacetemperature
CH4
productionrate
Greenhouseeffect
(+)
• Methanogens grow faster at high temperatures
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But,
• If CH4 becomes more abundant than about 1/10th of the CO2 concentration, it begins to polymerize
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Titan’s organic haze layer
• The haze is formed from UV photolysis of CH4
• It creates an anti-greenhouse effect by absorbing sunlight up in the stratosphere and re-radiating the energy back to space
• This cools Titan’s surface Image from Voyager 2
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Possible Archean climate control loop
Surfacetemperature
CH4
production
Hazeproduction
AtmosphericCH4/CO2
ratio
CO2 loss(weathering)
(–)
(–)
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CH4/CO2/C2H6 greenhouse with haze
• The Archean climate system would arguably have stabilized in the regime where a thin organic haze was present• We don’t quite make it to 288 K, using pCO2 from Driese et al. (2011) • Possible additional warming from higher pN2 or from albedo feedbacks?
Late Archean Earth?
Water freezes
J. Haqq-Misra et al., Astrobiology (2008)
(Old—Rye et al.)
Driese et al. (2011)(10-50 PAL, 2.7 Ga)
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• In any case, the climate crashed when pO2 rose at 2.4 Ga, wiping out much of the methane greenhouse and triggering glaciations
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Huronian Supergroup (2.2-2.45 Ga)
Redbeds
Detrital uraniniteand pyrite
Glaciations
S. Roscoe, 1969
Low O2
High O2
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Conclusions• The Sun really was ~30% dimmer during its early
history, at least after the first 200 m.y.• CO2, CH4, and C2H6 may all have contributed to the
greenhouse effect back when atmospheric O2 levels were low– Higher pN2 and lower planetary albedo could also have
helped warm the climate, but these mechanisms (in my view) are more speculative
• High atmospheric CH4/CO2 ratios can trigger the formation of organic haze. This has a cooling effect.– Stability arguments suggest that the Archean climate may
have stabilized when a thin organic haze was present• The Paleoproterozoic glaciation at ~2.4 Ga may
have been triggered by the rise of O2 and loss of the methane component of the atmospheric greenhouse