Why Venus is hotter than Earth? Venus (460 o C) vs. Earth (15 o C)
Why Venus (460 o C) is hotter than the Earth (15 o C) ? Answer 1: Venus is closer to the Sun Distance to the sun: Venus/Earth=0.72 radiation reach Venus/Earth = (1 2 )/(0.72 2 )=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 atmosphereMost 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 + T g 4 - T 4 =0 Top = (1-a)S - T g 4 = 0 (or radiation balance for the glass layer: 2 T g 4 = T 4 ) T g = ((1-a)S/ ) 1/4 = T cc =255K, T cg =2 1/4 T g =288 o K=15 o C About right… T4T4 (1- )S glass Tg4Tg4 Major GHG on earth: H 2 O!
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 25 o C, 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 ==> T 4by /T 0by ~(S 4by /S 0by ) 1/4 ~(0.7) 1/4 ~0.915 T 4br =0.915*T 0br =0.915*288K=263K= -10 o C Would be frozen, But, incompatible with the evidence of premitive life found as far back as 3.5 Ba,
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: CO 2 +H 2 O in the atmosphere removes CO 2 from the atmosphere and is incorporated into ground water to form H 2 CO 3 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 : H 2 O (rain)+CO 2 (air) CaSiO 3 +H 2 CO 3 CaCO 3 +SiO 2 +H 2 O Silicate rock Carbonic acid shells of organism (Continent) soil
Carbon sink: Chemical Weathering II Dissolution: CO 2 +H 2 O in the atmosphere removes CO 2 from the atmosphere and forms H 2 CO 3 which attacks limestone caves, and the dissolved ions flow to the ocean in rivers. Dissolution : H 2 O (rain)+CO 2 (air) CaCO 3 +H 2 CO 3 CaCO 3 +H 2 O + CO 2 Limestone rock in soil shells of organism return to air Different from hydrolysis Dissolution much faster but leads to no net removal of CO 2 from the atmosphere So does not contribute to the lowering of CO 2 in the long run
Chemical weathering: earth’s thermostat through a higher temperature, rainfall and vegetation higher temperature increasing weather rate (10 o C double rate) higher precipitation raise ground water level in the soil increasing weather rate Increase vege photosynthesis removal CO 2 delivers into the soil where it combines with ground water to form H 2 CO 3, 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 CO 2 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 CO 2 down cooling
Life and CO 2 Organic carbon cycle, accounts for 20% of carbon fluxes
Root system effective removal of atmospheric CO 2 Primitive system ineffective in the removal of atmospheric CO 2 Evolution of Life and CO 2 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 CO 2 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 CO 2. 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…) (warmer more plants/plankton takes CO2 down cooling
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 ==> T 8Ma /T 0Ma ~(S 8Ma /S 0Ma ) 1/4 ~(0.94) 1/4 ~0.985 T 8Ma =0.985*T 0Ma =0.985*288K=283K= 10 o C 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
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