climate modeling of the early earth: what do we know and how well do we know it? james kasting...

Climate Modeling of the Early Earth: What Do We Know and

How Well Do We Know It?

James KastingDepartment of Geosciences

Penn State University

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

Geologic time

Warm (?)

Rise of atmospheric O2

First shelly fossils (Cambrian explosion)Snowball Earth ice ages

Warm

Ice age

Origin of life

(Ice age)

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

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)

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)

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?

• From a theoretical standpoint, it is curious that the early Earth was warm, because the Sun is thought to have been less bright

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)

• 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)…

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

• Feedbacks are important in the climate system

• One way to describe these is with systems diagrams

Systems Notation

= system component

= positive coupling

= negative coupling

Positive Feedback Loops(Destabilizing)

Surfacetemperature

AtmosphericH2O

Greenhouseeffect

Water vapor feedback

(+)

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

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

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

Negative feedback loops(stabilizing)

The carbonate-silicate cycle feedback

(−)

Surfacetemperature

Rainfall

Silicateweathering

rate

AtmosphericCO2

Greenhouseeffect

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…

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

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)

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

• But Sagan and Mullen hadn’t thought about the photochemistry of ammonia

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)

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

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

CH4-climate positive feedback loop

Surfacetemperature

CH4

productionrate

Greenhouseeffect

(+)

• Methanogens grow faster at high temperatures

But,

• If CH4 becomes more abundant than about 1/10th of the CO2 concentration, it begins to polymerize

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

Possible Archean climate control loop

Surfacetemperature

CH4

production

Hazeproduction

AtmosphericCH4/CO2

ratio

CO2 loss(weathering)

(–)

(–)

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)

• In any case, the climate crashed when pO2 rose at 2.4 Ga, wiping out much of the methane greenhouse and triggering glaciations

Huronian Supergroup (2.2-2.45 Ga)

Redbeds

Detrital uraniniteand pyrite

Glaciations

S. Roscoe, 1969

Low O2

High O2

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