1. Climate Modeling of the Early Earth: What Do We Know and How Well Do We Know It? James Kasting Department of Geosciences Penn State University

2. Phanerozoic Time First shelly fossils Age of fish First vascular plants on land Ice age First dinosaurs Dinosaurs go extinct Ice age (Late Cenozoic) Warm

3. Geologic time Warm (?) Rise of atmospheric O 2 First shelly fossils (Cambrian explosion) Snowball Earth ice ages Warm Ice age Origin of life (Ice age)

4. G.M. Young et al., J. Geol. (1998) Evidence for 2.9-Ga glaciation 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

5. 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)

6. 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 (SiO 2 ), appear to indicate that the Achean Earth was hot, 70  15 o C, 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 Warm Cold (SMOW) P. Knauth, Paleo 3 219, 53 (2005)

7. Surface temperature vs. pCO 2 and fCH 4 Kasting and Howard, Phil. Trans. Roy. Soc. B (2006) 3.3 Ga S/S o = 0.77 Getting surface temperatures of ~70 o C at 3.3 Ga would require of the order of 3 bars of CO 2 (10,000 times present), possibly more, as the greenhouse effect of CH 4 was overestimated in these calculations pH of rainwater would drop from 5.7 to ~3.7 Would this be consistent with evidence of paleoweathering?

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

9. 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)

10. 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)…

11. The faint young Sun problem Kasting et al., Scientific American (1988) T e = effective radiating temperature = [S(1-A)/4  ] 1/4 T S = average surface temperature

12. Feedbacks are important in the climate system One way to describe these is with systems diagrams 

13. Systems Notation = system component = positive coupling = negative coupling

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

15. The faint young Sun problem The best solution to this problem is higher concentrations of greenhouse gases in the distant past (but not H 2 O, which only makes the problem worse) Less H 2 O More H 2 O

16. 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 CO 2, H 2 O, and CH 4 –H 2 O, 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 CO 2, higher CH 4, or both. Let’s consider CO 2 first 

17. The carbonate-silicate cycle (metamorphism) Silicate weathering slows down as the Earth cools  atmospheric CO 2 should build up This is probably at least part of the solution to the faint young Sun problem

18. Negative feedback loops (stabilizing) The carbonate-silicate cycle feedback (−)(−) Surface temperature Rainfall Silicate weathering rate Atmospheric CO 2 Greenhouse effect

19. CO 2 vs. time if no other greenhouse gases (besides H 2 O) J. F. Kasting, Science (1993) In the simplest story, atmospheric CO 2 levels should have declined monotonically with time as solar luminosity increased Geochemists, however, insist on making the story more complicated…

20. Sheldon’s pCO 2 estimate from paleosols First published estimate of Archean CO 2 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 pCO 2 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 pCO 2 at 2.7 Ga (Driese et al., 2011): 10-50 PAL N. Sheldon, Precambrian Res. (2006) Driese et al., 2011

21. Rosing et al.: CO 2 from BIFs J. F. Kasting Nature (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 CO 2 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.. (pCO 2  3 PAL)

22. Sagan and Mullen, Science (1972) Sagan and Mullen liked ammonia (NH 3 ) and methane (CH 4 ) as Archean greenhouse gases As a result of Preston Cloud’s work in the late 1960’s, they were aware that atmospheric O 2 was low on the early Earth

23. But Sagan and Mullen hadn’t thought about the photochemistry of ammonia 

24. Problems with Sagan and Mullen’s hypothesis Ammonia is photochemically unstable with respect to conversion to N 2 and H 2 (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)

25. CH 4 as an Archean greenhouse gas CH 4 is a better candidate for providing additional greenhouse warming Substrates for methanogenesis should have been widely available, e.g.: CO 2 + 4 H 2  CH 4 + 2 H 2 O Methanogens (organisms that produce methane) are evolutionarily ancient –We can tell this by looking at their DNA

26. 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 CH 4 and further warming the climate

27. CH 4 -climate positive feedback loop Surface temperature CH 4 production rate Greenhouse effect (+) Methanogens grow faster at high temperatures

28. But, If CH 4 becomes more abundant than about 1/10 th of the CO 2 concentration, it begins to polymerize 

29. Titan’s organic haze layer The haze is formed from UV photolysis of CH 4 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

30. Possible Archean climate control loop Surface temperature CH 4 production Haze production Atmospheric CH 4 /CO 2 ratio CO 2 loss (weathering) (–)

31. CH 4 /CO 2 /C 2 H 6 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 pCO 2 from Driese et al. (2011) Possible additional warming from higher pN 2 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)

32. In any case, the climate crashed when pO 2 rose at 2.4 Ga, wiping out much of the methane greenhouse and triggering glaciations 

33. Huronian Supergroup (2.2-2.45 Ga) Redbeds Detrital uraninite and pyrite Glaciations S. Roscoe, 1969 Low O 2 High O 2

34. Conclusions The Sun really was ~30% dimmer during its early history, at least after the first 200 m.y. CO 2, CH 4, and C 2 H 6 may all have contributed to the greenhouse effect back when atmospheric O 2 levels were low –Higher pN 2 and lower planetary albedo could also have helped warm the climate, but these mechanisms (in my view) are more speculative High atmospheric CH 4 /CO 2 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 O 2 and loss of the methane component of the atmospheric greenhouse