Earth’s Early Climate Jim Kasting Meteo 470

Solar luminosity versus time See The Earth System, ed. 2, Fig The fundamental problem of long-term climate evolution: the Sun was ~30% dimmer when it formed and has brightened more or less linearly since that time

Stellar nucleosynthesis Most of the Sun’s energy is produced by the proton-proton chain (at right) Overall reaction is 4 1 H + 2 e --> 4 He + 2 neutrinos + 6 photons Wikipedia: wiki/Image:FusionintheSun.png

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

Now, consider the implications for Earth’s climate This problem was first pointed out in 1972 by Carl Sagan and George Mullen

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

Question: How did Earth remain habitable during its early history when the Sun was less bright? Was it Gaia, or was it something else?

First presented in the 1970s by James Lovelock The Gaia hypothesis

According to this hypothesis, organisms (plants and algae pulled CO 2 out of the atmosphere at precisely the right rate to offset increasing solar luminosity CO 2 + H 2 O  CH 2 O + O 2 Is this idea teleological?

Before trying to solve the faint young Sun problem, let’s look briefly at Earth’s long- term climate history 

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

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

Now, let’s return to the FYS problem…

Possible solutions to the FYS problem Think back to the planetary energy balance equation  T e 4 = S (1 – A) 4 Albedo changes? –A possible feedback involving biogenic sulfur gases and cloud condensation nuclei has been suggested (Rosing et al., Nature, 2010), but it is probably not strong enough to solve the problem Geothermal heat? –Too small (0.09 W/m 2 vs. ~240 W/m 2 from absorbed sunlight

Possible solutions to the FYS problem Increasing the greenhouse effect also works T s = T e +  T g Possible greenhouse gases –NH 3 : Doesn’t work very well (photolyzes rapidly) –CO 2 : Works! (supplied by volcanoes) –CH 4 : Also works (probably requires life) – H 2 : This is a new (and surprising!) idea Sagan and Mullen (1972) liked methane (CH 4 ) and ammonia (NH 3 ) because they were aware that O 2 levels were low on the early Earth. We’ll return to that thought later.

To get a handle on climate evolution, it’s important to understand climate feedbacks For this, it is useful to define some systems notation…

Systems Notation = system component = positive coupling = negative coupling

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

The faint young Sun problem Kasting et al., Scientific American (1988) Less H 2 O More H 2 O

Positive feedback loops (destabilizing) Surface temperature Snow and ice cover Planetary albedo Snow/ice albedo feedback (+) If this feedback were included in models of early climate, the FYS problem would be even worse

So, we need one or more negative feedback loops to stabilize Earth’s climate What could these be?

Negative feedback loops (stabilizing) IR flux feedback Surface temperature (-) Outgoing IR flux This feedback loop is so straightforward that it is often overlooked  But it can break down when the atmosphere heats up and becomes H 2 O-rich

There must also be negative feedbacks that operate on long time scales –Need this to counter the change in solar luminosity What could these be?

The carbonate-silicate cycle Atmospheric CO 2 builds up as the climate cools, so higher atmospheric CO 2 is a natural solution to the FYS problem

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

CO 2 vs. time if no other greenhouse gases (besides H 2 O) J. F. Kasting, Science (1993) Snowball Earth events

Is CO 2 the solution to the FYS problem? Thus, high CO 2 levels could, in principle, have solved the FYS problem Unfortunately, geochemists have made this problem more difficult by attempting to measure paleo-CO 2 concentrations using data from paleosols (ancient soils) –An early effort was made by R. Rye et al. (Nature, 1995), but their analysis is now considered to be incorrect, so we shall skip it J. F. Kasting, Science (1993)

CO 2 from paleosols Catling & Kasting, in prep. Sheldon (and Driese et al.) both derived estimates for pCO 2 that are well below those needed to keep the climate warm However, the latest estimates (Kanzaki & Murakami, 2015) are consistent with the climate calculations Who is right? This remains to be determined...

Let’s talk a little more about Snowball Earth episodes…

Low Latitude Glaciations Paleomagnetic data indicate low-latitude glaciation at 2.3 Ga, 0.75 Ga, and 0.6 Ga Paleoproterozoic glaciations (~2.3 Ga) may be triggered by the rise of O 2 and the corresponding loss of CH 4 Late Precambrian glaciations studied by Hoffman et al., Science 281, 1342 (1998)

The Great Infra-Cambrian Ice Age W. Brian Harland & M.J.S. Rudwick, Scientific American 211 (2), 28-36, 1964 Courtesy of Joe Kirschvink

Using Magnetic Data to Determine Paleolatitudes Courtesy of Adam Maloof Polar Equatorial

Periglacial Outwash Varves From the Elatina Formation, South Australia Courtesy of Joe Kirschvink

Late Precambrian Geography * (according to Scotese) Hyde et al., Nature, 2000* glacial deposits

Energy Balance Models (EBMS) EBMs are sometimes referred to as 1.5-D climate models They treat latitudinal heat transport like diffusion – The diffusion coefficient is adjusted to get the right equator-to-pole T gradient – The surface heat capacity is adjusted to get the right seasonal cycle – Radiative transfer is done by parameterizing results from 1-D radiative-convective models Caldeira & Kasting, Nature (1992) These models can be used to calculate ice-lines, and they exhibit both stable and unstable equilibrium solutions

Ghaub Glaciation (Namibia) Glacial Tillite Courtesy of Joe Kirshvink Maieberg “cap”

Snowball Earth results The evidence for Snowball Earth episodes in the Neoproterozoic ( Ga) is very strong –Whether these were of the ‘hard Snowball’, ‘thin- ice’, or ‘Jormungand’ variety remains to be determined –The fact that photosynthetic life survived these events argues in favor of one of the less extreme varieties

Overall conclusions for Earth’s early climate The faint young Sun problem is real, i.e., the Sun was indeed 30% fainter in the past Some combination of CO 2 and CH 4 was probably the solution to the problem Climate limit cycling is possible on the early Earth, but details remain sketchy because of the scarcity of geologic evidence, especially during the Archean/Hadean