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=0.72
radiation 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???
Answer 2: The CO2 GHG effect
Venus
vs.
Earth
The same amount of total carbon
All carbon in the atmosphere
Most carbon in rocks
Carbon reservoirs on the earth
0.001%
0.025%
0.057%
99.9%
Greenhouse Effect
(1-)S
Tg4
(1-)S
glass
T4
Heat fluxes:
surface = (1-a)S +  Tg4 -  T4 =0
Top
= (1-a)S -  Tg4 = 0
(or radiation balance for the glass layer: 2 Tg4 =  T4 )
Tg= ((1-a)S/ )1/4 = Tcc=255K ,  Tcg =21/4Tg=288oK=15oC
About right…
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
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,
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?
Carbon exchange
Carbon Source: Volcanic Eruption
Renewal /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 I
Hydrolysis: 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
Silicate rock Carbonic acid
(Continent)
soil

CaCO3 +SiO2 +H2O
shells of organism
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
Limestone rock in soil
 CaCO3 +H2O + CO2
shells of organism
return to air
Different from hydrolysis
Dissolution much faster
but leads to no net removal of CO2 from the atmosphere
So 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 water
to 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…)
warmermore plants/planktontakes CO2 downcooling
Life and CO2
Organic carbon cycle, accounts for 20% of carbon fluxes
Primitive system ineffective in the
removal of atmospheric CO2
Root system effective removal
of atmospheric CO2
Evolution of Life and CO2 removal efficiency
The Debate on Gaia Hypothesis
life is involved in the weathering process (vegetation, plankton shell…)
(warmermore plants/planktontakes CO2 downcooling
•
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.
Thermostat Malfunction: A Snowball Earth?
2-4 times glacial deposits, at least once in the tropics
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
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