1. Long-term Evolution of Earth’s Atmosphere and Climate James Kasting Department of Geosciences Penn State University

2. Talk Outline Part 1: Precambrian climate evolution (in a nutshell) Part 2: Planetary climates revisited—the largely overlooked problem of Snowball Earth limit cycling

3. Phanerozoic Time First shelly fossils Age of fishes First vascular plants on land Ice age First dinosaurs Dinosaurs go extinct Ice age (Pleistocene)

4. Geologic time Rise of atmospheric O 2 (Ice age) First shelly fossils (Cambrian explosion) Snowball Earth ice ages Warm (The ‘Boring Billion’) Ice ages Warm (?) Origin of life ‘Conventional’ interpretation of the Precambrian climate record

5. The fact that most of the Precambrian appears to have been warm is remarkable, because the Sun is thought (by essentially everyone) to have been less luminous early in Earth’s history 

6. 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

7. Greenhouse gases and CO 2 - climate feedbacks So, one needs more greenhouse gases, especially during the Archean CO 2 is a prime candidate because it is part of a negative feedback loop (see panel at right) We should be cautious about over-interpreting this model, though, because land area may have been much smaller during the Archean Diagram illustrating the (modern) carbonate-silicate cycle. Atmospheric CO 2 increases when the climate cools because of slower rates of silicate weathering on land

8. What can we say empirically about CO 2 levels in the distant past? Some controversial constraints on Archean CO 2 can be derived from paleosols (ancient soils)

9. Precambrian pCO 2 from paleosols First estimate for Archean pCO 2 was published by Rye et al. (1995) Criticized by Sheldon (2006) –Can’t use thermodynamic arguments when the entire suite of minerals is not present He presented an alternative analysis of paleosols based on mass balance arguments (efficiency of weathering) If Sheldon and Driese are right about Precambrian CO 2 levels, then other greenhouse gases would have been needed to keep the early Earth from freezing But, a new analysis method has recently been published.. N. Sheldon, Precambrian Res. (2006) Driese et al., 2011 (10-50 PAL)

10. Sheldon’s method –Mass balance on soil silicates (following Holland and Zbinden, 1988) –Involves assumptions about soil porosity, lifetime New method –Detailed chemical modeling of porewater composition, pH. Involves multiple assumptions about soil and groundwater parameters Geochimica et Cosmoschimica Acta 159, 190 (June, 2015)

11. K&M paleosol analysis: ancient soils Kanzaki & Murakami, GCA (2015) If the new paleosol analysis is correct, then CO 2 could have been high enough to solve the faint young Sun problem by itself Driese et al. (2011) Som et al. (2012) – upper limit from raindrops

12. That said, methane should also have been an important greenhouse gas during the Archean –Its lifetime is long in a low-O 2 atmosphere –It’s a moderately good greenhouse gas (but not nearly as good as CO 2, contrary to popular opinion) –The methanogens that produce it are thought to be evolutionarily ancient..

13. Anoxic ecosystem modeling Coupled photochemical- ecosystem modeling of an methanogen- or H 2 -based anoxygenic photosynthetic ecosystem predicts Archean CH 4 concentrations of 200- 2000 ppm This is enough to produce 10-15 degrees of greenhouse warming Higher warming by CH 4 is precluded by the formation of organic haze at CH 4 /CO 2 ratios greater than ~0.1 Kharecha et al., Geobiology (2005)

14. Archean CH 4 -CO 2 greenhouse Diagram shows a hypothetical Archean atmosphere at 2.8 Ga The black curves show predicted surface temperatures with zero and 1000 ppm of CH 4 The loss of much of this CH 4 at ~2.5 Ga could plausibly have triggered the Paleoproterozoic glaciations 2.8 Ga S/S o = 0.8 J.F. Kasting, Science (2013) Driese et al. (2011)

15. Geologic time Rise of atmospheric O 2 (Ice age) First shelly fossils (Cambrian explosion) Snowball Earth ice ages Warm (The ‘Boring Billion’) Ice ages Warm (?) Origin of life ‘Conventional’ interpretation of the Precambrian climate record

16. But, this analysis overlooks a phenomenon that could have been important on early Earth (although not necessarily) and that should be important on at least some Earth-like planets around other stars…

17. A new paper by Kristen Menou shows that planets near the outer edge of the habitable zone should not have stable, warm climates, despite the influence of the carbonate-silicate cycle See also Kadoya and Tajika (ApJ, 2014), along with earlier papers by Tajika, referenced therein

18. Menou’s new model One needs to simultaneously solve for surface temperature, T surf, as a function of pCO 2 and for pCO 2 as a function of T surf The radiation balance is done using a fit to Darren Williams’ 1997 EBM The EBM parameterization itself was created by fitting results from our own 1-D radiative-convective climate model

19. Menou’s new model (cont.) The CO 2 model balances removal by weathering, W, with production from volcanism, V The weathering rate parameterization is from Berner and Kothavala (2001)

20. Limit cycles on poorly lit planets An Earth-like planet at 1 AU from its parent star has a stable, warm climate state. Snowball climate states exist, but they go away because of volcanic CO 2 buildup An Earth-like planet at 1.6 AU has no stable states but, rather, cycles between warm and cold (Snowball) climate states IR cooling Solar heating Different weathering rates Snowball Earth Present Earth Limit cycles K. Menou, EPSL (2015)

21. Taking limit cycling into account may change our mental picture of the habitable zone around different stars 

22. Updated habitable zone (Kopparapu et al., 2013, 2014) The conservative HZ is the one predicted by climate models Conservative HZ Credit: Sonny Harman

23. Updated habitable zone (Kopparapu et al., 2013, 2014) Optimistic HZ Credit: Sonny Harman The optimistic HZ is one defined by early Mars and recent Venus

24. Updated habitable zone (Kopparapu et al., 2013, 2014) The outer (or occasionally) HZ is the region where limit cycling occurs Outer (or Occasionally) HZ Credit: Sonny Harman

25. Conclusions Earth’s early climate was kept warm by a combination of higher CO 2 and CH 4 –Life plays a role in climate regulation, but Earth should remain habitable even without it The carbonate-silicate cycle plays a key role in Earth’s climate stability, especially in countering the faint young Sun problem The CO 2 -climate feedback may work quite differently on planets that receive less starlight than Earth –The outer regions of the HZ around different stars may be less habitable than the inner regions because of limit cycling –Earth may actually be ideally situated within the HZ