Long-term Evolution of Earth’s Atmosphere and Climate

James Kasting Department of Geosciences

Penn State University

Talk Outline

• Part 1: Precambrian climate evolution (in a nutshell)

• Part 2: Planetary climates revisited—the largely overlooked problem of Snowball Earth limit cycling

PhanerozoicTime

First shelly fossils

Age of fishesFirst vascular plants on landIce age

Ice age

First dinosaurs

Dinosaurs goextinct

Ice age (Pleistocene)

Geologic time

Rise of atmospheric O2 (Ice age)

First shelly fossils (Cambrian explosion)Snowball Earth ice ages

Warm (The ‘Boring Billion’)

Ice ages

Warm (?) Origin of life

‘Conventional’ interpretationof the Precambrian climaterecord

• 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

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

Greenhouse gases and CO2-climate feedbacks

• So, one needs more greenhouse gases, especially during the Archean

• CO2 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. AtmosphericCO2 increases when the climate cools because of slower rates of silicate weathering on land

What can we say empirically about CO2 levels in the distant past?

• Some controversial constraints on Archean CO2 can be derived from paleosols (ancient soils)

Precambrian pCO2 from paleosols

• First estimate for Archean pCO2 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 CO2 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)

• 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)

K&M paleosol analysis: ancient soils

Kanzaki & Murakami, GCA (2015)

• If the new paleosol analysis is correct, then CO2 could have been high enough to solve the faint young Sun problem by itself

Driese etal. (2011)

Som et al. (2012) – upper limit from raindrops

• That said, methane should also have been an important greenhouse gas during the Archean– Its lifetime is long in a low-O2 atmosphere– It’s a moderately good greenhouse gas (but

not nearly as good as CO2, contrary to popular opinion)

– The methanogens that produce it are thought to be evolutionarily ancient..

Anoxic ecosystem modeling

• Coupled photochemical-ecosystem modeling of an methanogen- or H2-based anoxygenic photosynthetic ecosystem predicts Archean CH4 concentrations of 200-2000 ppm

• This is enough to produce 10-15 degrees of greenhouse warming

• Higher warming by CH4 is precluded by the formation of organic haze at CH4/CO2 ratios greater than ~0.1

Kharecha et al., Geobiology (2005)

Archean CH4-CO2 greenhouse

• Diagram shows a hypothetical Archean atmosphere at 2.8 Ga

• The black curves show predicted surface temperatures with zero and 1000 ppm of CH4

• The loss of much of this CH4 at ~2.5 Ga could plausibly have triggered the Paleoproterozoic glaciations

2.8 GaS/So = 0.8

J.F. Kasting, Science (2013)

Driese et al. (2011)

Geologic time

Rise of atmospheric O2 (Ice age)

First shelly fossils (Cambrian explosion)Snowball Earth ice ages

Warm (The ‘Boring Billion’)

Ice ages

Warm (?) Origin of life

‘Conventional’ interpretationof the Precambrian climaterecord

• 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…

• 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

Menou’s new model

• One needs to simultaneously solve for surface temperature, Tsurf, as a function of pCO2 and for pCO2 as a function of Tsurf

• 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

Menou’s new model (cont.)

• The CO2 model balances removal by weathering, W, with production from volcanism, V

• The weathering rate parameterization is from Berner and Kothavala (2001)

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 CO2 buildup

• An Earth-like planet at 1.6 AU has no stable states but, rather, cycles between warm and cold (Snowball) climate states

IR coolingSolar heatingDifferent weathering rates

SnowballEarth

PresentEarth

Limit cycles

K. Menou, EPSL (2015)

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

Updated habitable zone(Kopparapu et al., 2013, 2014)

• The conservative HZ is the one predicted by climate models

Conservative HZ

Credit: Sonny Harman

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

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

Conclusions• Earth’s early climate was kept warm by a

combination of higher CO2 and CH4

– 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 CO2-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