Lecture 4: CO2 and Long Term Climate Change

(Ch. 3)

Atmospheric CO2 Evolution

Why Venus is hotter than Earth?

Venus (460oC) vs. Earth (15oC)

Why Venus (460oC) is hotter than the Earth (15oC) ?

Answer 1: Venus is closer to the Sun

Distance to the sun: Venus/Earth=0.72radiation reach Venus/Earth

= (12)/(0.722)=1.93

But, the albedo is 80% on Venus and 26% on Earth

So the solar radiation received Venus/Earth

= 1.93x (0.20/0.74)=0.52

So, Venus should be colder than the earth???

Venus vs. Earth

All carbon in the atmosphere Most carbon in rocks

The same amount of total carbon

Answer 2: The CO2 GHG effect

Carbon reservoirs on the earth

99.9%

0.057%

0.025%

0.001%

Greenhouse Effect

Heat fluxes: surface = (1-a)S + Tg

4 - T4 =0 Top = (1-a)S - Tg

4 = 0 (or radiation balance for the glass layer: 2 Tg

4 = T4 )

Tg= ((1-a)S/ )1/4 = Tcc=255K , Tcg =21/4Tg=288oK=15oC About right…

T4

(1-)S

(1-)S

glassTg4

Major GHG on earth: H2O!

Atmospheric CO2 Evolution

Last 4.5 Byr:

Why the earth is not that cold?

---The faint young Sun Paradox

The faint young Sun paradox

In spite of the much weaker Sun (30%) in the early stage of the earth, the earth has remained inhabitable, instead of largely frozen (a snowball earth).

Something keeps the earth warmer!

But, this factor must not be functioning today, which otherwise would heat the present earth inhabitable? (above 25oC, at least)

a thermostat (temperature regulator) is functioning!

Was the Earth more like the Venus in the past, with more carbon in rocks?

Assuming the same climate sensitivity:

T~(S)1/4==> T4by/T0by~(S4by/S0by)1/4~(0.7)1/4~0.915 T4br=0.915*T0br=0.915*288K=263K= -10oC

Would be frozen,But, incompatible with the evidence of premitive life found as far back as 3.5 Ba,

Carbon exchange

Carbon Source: Volcanic EruptionRenewal /depletion time:

Atmosphere: 600/0.15=4000 yr

Combined surface reservoir: 3700/0.15=24,700yr

Including deep ocean reservoir: 41,700/0.15=278,000yr, short compared with the history of the earth

Volcanic flux is sufficient to provide carbon for the atmosphere (actually the entire surface earth system: atmos+soil+ocean) at long term

0.15 GT/yr

But, volcanic eruption of CO2 has no direct feedback and therefore alone can’t form

the thermostat mechanism! Some feedback that feels the climate is needed.

Carbon sink: Chemical Weathering IHydrolysis: CO2+H2O in the atmosphere removes CO2 from the atmosphere and is incorporated into ground water to form H2CO3 in soil, which attaches rocks and dissolve ions, and transported into the ocean in river, and store in the shells of marine plankton which eventually is deposited into the ocean bottom

Hydrolysis : H2O (rain)+CO2 (air)

CaSiO3 +H2CO3 CaCO3 +SiO2 +H2OSilicate rock Carbonic acid shells of organism(Continent) soil

Carbon sink: Chemical Weathering II

Dissolution: CO2+H2O in the atmosphere removes CO2 from the atmosphere and forms H2CO3 which attacks limestone caves, and the dissolved ions flow to the ocean in rivers.

Dissolution : H2O (rain)+CO2 (air)

CaCO3 +H2CO3 CaCO3 +H2O + CO2 Limestone rock in soil shells of organism return to air

Different from hydrolysisDissolution much fasterbut leads to no net removal of CO2 from the atmosphereSo does not contribute to the lowering of CO2 in the long run

Chemical weathering: earth’s thermostat

through a higher temperature, rainfall and vegetation

higher temperature increasing weather rate

(10oC double rate)

higher precipitation raise ground water level in the soil

increasing weather rate

Increase vege photosynthesis removal CO2 delivers

into the soil where it combines with ground waterto form H2CO3, increasing weather rate

Chemical weathering forms the earth’s thermostat through T, P, V

Chemical weathering is an excellent candidate for Earth’s thermostat

A negative feedback mechanism for the fainted young Sun paradox:

Weaker Sun => cooler/less P/less vege => less chemical weathering

=> More CO2 left in the atmosphere => stronger greenhouse effect =>compensates the weaker Sun.

Chemical weathering is an excellent candidate for Earth’s thermostat (James Walker, Paul Hays and James Kastings)

In contrast to chemical weathering, water vapor feedback is a positive feedback

The Gaia Hypothesis

The ultimate control of climate: Life

Life itself has been responsible for regulating earth’s climate (J. Lovelock and L. Margulis, 1980)

life is involved in the weathering process (vegetation, plankton shell…)

warmermore plants/planktontakes CO2 downcooling

Life and CO2Organic carbon cycle, accounts for 20% of carbon fluxes

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Evolution of Life and CO2 removal efficiency

The Debate on Gaia Hypothesis

• Critics:

early life too primitive to play an significant role in weathering,

modern plants (root system) developed last 540 Ma

marine shells develop after 540Ma (before chemical precip in shallow tropical seas…),

• Support:

bacteria in early time can help reduce CO2 too

life evolution matches the earth’s need for progressively greater chemical weather through time. Later, more complex life leads to stronger weathering, reducing more CO2.

• Critics:

early life too primitive to play an significant role in weathering,

marine shells develop after 540Ma (before chemical precip in shallow tropical seas…),

life is involved in the weathering process (vegetation, plankton shell…) (warmermore plants/planktontakes CO2 downcooling

Thermostat Malfunction: A Snowball Earth?

Chemical weathering not working: a 6% reduction of insolation, not cold enough

Assuming the same climate sensitivity:

T~(S)1/4==> T8Ma/T0Ma~(S8Ma/S0Ma)1/4~(0.94)1/4~0.985 T8Ma=0.985*T0Ma=0.985*288K=283K= 10oC

So, a lower CO2 is needed (according to climate models). But, with chemical weathering thermostat, cooling reduced weathering higher CO2

2-4 times glacial deposits, at least once in the tropics

Reading Material for L4

• Hoffman P. and D. Schrag, 2002: The snowball Earth hypothesis: testing the limits of global change. Terra Nova, 14, 129-155

• Schrag, D. Berner, R. , P. Hoffman and G. Halverson, 2002: On the initiation of a snowball Earth. Geocheistry, Geophysics, Geosystems, 3, 10.1029/2001GC000219

The End