Lecture 17: Long-Term Climate Record and some mysteries

Main Glaciations throughout Earth’s history:

GLACIATION MYA

Huronian 2500
Glaciations

Late 850-550
Proterozoic
Glaciations


Late Ordovician ca. 440
Glaciations

Permo- ca. 310-270
Carboniferous
Glaciations

Pleistocene 1.8 -

Causes of Long-Term Climate Change
- Positioning of the Continents
Part of the answer. Big ice sheets most likely occur when continents are near one of the poles (assuming that Earth’s obliquity is low and poles are cold)

- Variations in Atmospheric CO2
CO2 in the atmosphere is critical GH gas. Variations in the amount have large impacts on climate.

Mystery of Earth’s long-term habitability.
Faint Young Sun paradox:
Sun was ca. 30% fainter 4.5 bya. But Earth had running water throughout its history, and life dates back 3.5 Bya, proving that Earth was habitable.
Higher CO2 in the atmosphere
- During this early period, rates of volcanic input of CO2 were probably higher. But that doesn’t explain how it is that the content of CO2 in the atmosphere has kept Earth habitable over all this time.
- How has it managed to decrease CO2 over time to match increasing solar luminosity?

What mechanism acting as a Thermostat allowing the Earth to regulate amounts of CO2 in the atmosphere ?
Volcanic eruptions are a result of internal action deep in the Earth and therefore isolated from surface conditions and climate

Chemical Weathering:
- To avoid long-term build up of CO2 levels over time, the input to atmosphere by volcanoes must be countered by CO2 removal
- The chemical reactions involved in Chemical Weathering act to remove CO2 from the atmosphere (not to be confused with the organic carbon that can be released during weathering, on much smaller scale). In particular, the process of Hydrolysis, which involves minerals from continental rocks, water from rain and CO2 from atmosphere. Very basically, rain and CO2 combine to produce carbonic acid, that then combines with silicate rock from continents and forms carbonates that are carried to the ocean, form shells of organisms and subsequently are buried in the ocean floor.
- Rates of Chemical weathering, and of volcanic production must have matched fairly nearly to avoid the earth system getting out of balance. The rates of each (i.e., input by volcanoes and removal by weathering) have varied during earth’s history because of natural changes in tectonic processes, but the long-term habitability of the planet has remained.
- Climatic factors that control Weathering
o Temperature – as T. increases, weathering increases
o Precipitation – as ppt increases, weathering increases
o Vegetation – as ppt and T increase, so does vegetation productivity and that also increases weathering
o Chemical weathering as thermostat: negative feedback moderating long term climate change. So that: initial change -> warmer climate -> increased T, ppt, veg -> increased chem. Weathering -> increased CO2 removal -> reduction of initial warming
And similarly with cooling change
This doesn’t mean that no climate change occurs at all. Processes succeed in initially in warming the Earth, but by a smaller amount than it would have.
Mystery of Fait Young Sun paradox:
Earth early history had more volcanic eruptions and the surface may even have been molten for a few hundred my after 4.55 bya. Also earth was probably being bombarded
Chemical weathering probably slower because earth probably cooler due to faint young sun. Thus the rate of CO2 removal was slower. As more CO2 was in atm, then Earth didn’t freeze. As earth warmed, weathering increased and so removed more atmospheric CO2.
Other mysteries:
Why was the Earth glaciated during late Proterozoic?
Position of continents implies that Earth would have been warmer.
During the late Proterozoic, a period which lasted 330 mys, continents around the Equator would have been warmer -> increased weathering, which would have removed CO2 from atmosphere, causing cooling of the entire planet.