Climate Change

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From first lecture:

Components of the climate system e.g. atmosphere, cryosphere, biosphere, hydrosphere and lithosphere.

Greenhouse Effect and the Carbon Cycle

Forcing (internal, timescale dependent. External - the sun, volcanic forcing) and Feedbacks (positive - enhancing, negative)

Proxy archives: short timescale - tree rings, corals, historical documents. Long timescale - marine sediments, geological record

Today:

Deep time 4.6Gya - 540 Mya

Why do we care?

Early earth was rather exotic. An excellent period to study feedbacks and climate stabilisation. Need stability for life to evolve - you can have extreme environments but it must have some sort of stability.

Hadean: 4.6 to 3.9Gya

Archean: 3.9 to 2.5Gya - early life - methanogens

Proterozoic: 2.5Gya to 540Mya - early life - cyanobacteria

Phanerozoic: 540Mya to present - fossils of larger lifeforms - plants and animals

Formation - 4.6Gya. 50-70Mya later Mars sized object collides with Earth to form the Moon/

Hadean - first 600My of Earth's history.

Sun was 30% fainter than at present

Formed with no primary atmosphere but outgassing resulted in an atmosphere which was likely water vapour, co2, hydrogen sulphide, sulphur dioxide

Zircons show oceans and continental material had formed by 4.4Gya (0.1Gya after formation)

Impacts occurred from formation until the LHB (3.9Gya)

STERILISING impacts probably occurred 6-12 times during Hadean. If any life were to evolve the whole process would reset

No real idea of what the temperature was

No rocks from this period - they have been recycled

Archean - 3.9 to 2.5Gya

Oldest rocks on Earth from this period

Evidence for lack of oxygen - Witwatersrand gold ore (3Gya), detrital pyrite was not oxidised during weathering, banded iron formations. Alternating layers. Iron rich sediments- no oxidation on deposition

Evolution of life 3.7 - 3.5Gya - methanogens (prokaryotes) cannot survive with free oxygen. Co2+4H2 ---> CH4 + 2H2O

Methanes retention time in the atmosphere is much less than co2 - only 30 decades so if methane is present it means it must be being generated

Archean ends with rise in global oxygen levels - cyanobacteria (eukaryotes) 2.8Gya

Tectonics different - higher head flow

Nothing like today. Lots of co2, ammonia, methane, water vapour, sulphur dioxide.

Temperature likely warmer than today - oxygen isotopes in archean rocks suggest oceans twice as warm as todays tropical oceans (-50C). This is contentious

The Faint Sun Paradox

No atmosphere? Earth should have been frozen. For bulk of geological history, if the sun was the only driving force then the planet would have been frozen. 3.8Gya - suns luminosity 75% of present value. Yet, during the Archean, liquid water was prevalent on the surface. In fact, the geological and palaeoclimatic record strongly suggests Earth has maintained a moderate climate through history - BUT WITH WOBBLES

......

The Greenhouse Effect!

CO2 supplied for volcanoes, methane from volcanoes but also requires life - methanogens.

Co2 has varied but has not simply increased year on year for the last 4.6Gya, No runaway greenhouse effect like Venus

Does this mean volcanic activity has decreased?

No! Rates of tectonism decreased slowly through

Something is removing co2 from the system - slow chemical weathering of continental rocks balances input of co2 into atmosphere

Chemical weathering reactions important. CO2 gets locked up in calcium carbonate

Hydrolysis.

Reaction of silicate minerals with carbonic acid to form clay minerals and dissolved ions

This process accounts for 80% of co2 removal

Slow silicate rock weathering balances long term build up of atmospheric co2

Chemical weathering influenced by temperature. Rates double per 10C rise in temp

Precipitation - h20 required for hydrolysis. h20 increases as temperature increases

Vegetation (not relevant for archean) - respiration in soil increases co2. co2 in soils 100-1000x higher than in atmosphere

Warming leads to an increase in hydrological cycle. Air holding more water than cold air. Landmass vs latitude.

Chemical weathering is a NEGATIVE FEEDBACK

Volcanic outgassing of co2 - counteracted the lower radiant energy from sun and warmed planet

As earth warmed it became wetter, increased rates of weathering

Methane is a more potent greenhouse gas. Methane from volcanoes. Methanogens become more productive as it gets warmer. Feedback.

If methane becomes more abundant than co2 an organic haze begins to form. Haze from UV photolysis (decomposition) of methane. Anti greenhouse effect. Haze absorbs sunlight in the stratosphere and radiates back into space.

High co2, no methane, Then long period of them being similar. Hazy. Oxygen flat all the way through.

Archean/Proterozoic transition

A time of significant change

Cyanobacteria

Stromatolites

Evolution of abundant photosynthetic organisms (cyanobacteria) was one of the most critical periods in history

2.4Gya - oxygen increase- The Great Oxygenation Event

increasing cyanobacteria led to increased weather rates....oxidation!!!

Led to decrease in CO2 shifts the balance

CO2 reduced than allowed global temperature to dip below 0

Methanogens outcompeted by cyanobacteria - decrease in methane

Prolonged period where planet was relatively warm and stable

2,2Gya to 750Mya

Was it stable?

Why?

Tectonically style changed - small to large plates. First supercontinent

Rodinia: 1Gya to 750Mya

Related Grenville Orogeny

when this broke up things started going crazy. Stable because of supercontinent

Neoproterozoic Snowball Earth - multiple periods of severe global glaciation. Lots of geologic evidence of low latitude glaciation

Carbonistotopes

Sun 6% less luminous than today. Planet still wanted to be a little bit cooler. We know radical changes in atm composition. Breakup of Rodinia. Continent positions high vs low latitude weathering differences Obliquity changes in earths orbit

Ice accumulated at the poles and movies southwards. Runaway ice albedo

Evidence for at least 2 glacial events

Its the greenhouse effect.

Ice - intense albedo. But tectonics still happening. Volcanoes. CO2 slowly produced. Zero weathering, hydrological cycle negligible. CO2 steadily builds up. At some point it will overcome the albedo effect. Unstable situation. Once ice started melting albedo would have quickly reduced and due to high co2 atmospheric concentrations warming would really kick in

Catastrophic ice melting global sea levels rise, it gets hot, really hot, really quick;y

After snowball - hot house. Carbonate silicate kicks in to high gear. Emergence of multicellular life. Snowball seawater was laden with nutrients due to hydrothermalactivity. The rise in free oxygen is likely the cause of the cambrian explosion of life