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Ocean Carbon Cycle
Figure creditUS-OCB Program
Why study ocean carbon cycle?
• CO2 is an important greenhouse gas
• Ocean is a major sink of fossil fuel CO2
– Impacts on ocean chemistry
Rising atmospheric CO2
Polar ice core data Instrumentalrecord
SIO CO2 program
Global average ~ 1.2 TC/year/person
Units for atmospheric CO2
• What do we mean by 390 ppmv?• About 0.039% of all gas molecules in the
atmosphere is CO2
• CO2 is a minor component of the atmospheric gases
Current atmospheric CO2 is about 390 ppmv, which is equivalent of 780 GTC.
Simple rule of thumb
Unit matters
• 1 molC = 12 gC = 44 gCO2
• 1 gC = 3.7 gCO2
• molC and gC are most frequently used in scientific literature
• gCO2 is also used in media, etc.– Watch out for different units!
Global carbon inventory estimates
• Atmosphere : 780 GTC– CO2 gas
• Land biosphere : 2,000 GTC– Organic matter in forest and soils
• Oceans : 38,000 GTC– Dissolved Inorganic Carbon (DIC) = CO2 gas dissolved in the
seawater• Geological reservoirs– Seafloor sediment : 150 GTC– Oil reservoir : 500 GTC– Coal reservoir : 5,000 GTC
Order of magnitude
• 1 metric ton of C (TC) = 106 gC– In 2009, US per capita carbon emission is 4.6 TC– Global CO2 emission in 2008 is about 9 GTC
• 1 giga ton of C (GTC)– 1GTC = 109TC = 1015gC = 1 PgC– 1PgC = 1015gC x (1molC/12gC) = 8.3 x 1013molC
100 103 106 109 1012 1015
kilo mega giga tera peta
k M G T P
Concept test
• Company A sells a product that reduces carbon emission at a cost of $10/TCO2
• Company B sells similar product at a cost of $20/TC
• Which company is offering more cost-effective product?
Concept test
• If we consume entire oil reservoir, 500 GTC, how many ppmv increase do we expect in the atmosphere?– (case 1) No absorption into the ocean and/or land– (case 2) 50% absorption– (case 3) 75% absorption
Emission due to human activities
• DOE: Carbon Dioxide Information Analysis Center
CO2 changes in the last 50 yr
OceansBiosphere
The Carbon sinks
• CO2 emission due to human activity– About 9 GTC/year
• Time rate of increase in atmospheric CO2
– Late 2000s– Observed increase is about 4 GTC/year
• Only about 45% of CO2 emitted by the human activity is remaining in the atmosphere
• Where has the rest of CO2 gone?
Land and ocean carbon sinks
• Land biosphere– Carbon molecule is stored in trees, plants and soils
• Ocean CO2 uptake– Absorption of CO2 into the seawater
• The carbon cycle has a stabilizing mechanism, mediating the growth of atmospheric CO2 by absorption into the land and oceans
Photosynthesis and respiration
• Solar energy + CO2 + H2O Sugar + O2
• Most of energy stored in sugar is released by respiration when the biomass is consumed by animals, bacteria and people
• If there is a net surplus in biomass production, biosphere can become a carbon sink
Carbon uptake by the land biosphere
• (Land uptake) = (Photosynthesis) –
(Respiration)• Photosynthesis– Water, nutrients, sunlight
• Respiration– Decomposition of organic matter– Organic matter CO2
– Episodic events: ex. forest fires
Air-sea gas transfer• Driven by turbulence at the air-sea interface– What controls the turbulent motion?
Emerson and Hedges (2010)
Gas transfer coefficient: G [ms-1]• Measures the strength of the air-sea gas transfer
Modeling air-sea gas transfer
• Sea-to-air gas flux [mol m-2 s-1] – Measures exchange of gas molecules per unit area
and per unit time– Proportional to the degree of saturation
Henry’s law coefficient
Chemical reactions with the seawater
• A series of reactions
CO2 + H2O H2CO3
(carbonic acid)H2CO3 H+ + HCO3
-
(bicarbonate ion)HCO3
- H+ + CO32-
(carbonate ion)
Textbook chapter 6, page 180-183
Carbonate chemistry
• DIC = (Dissolved Inorganic Carbon)= [CO2] + [H2CO3] + [HCO3
-] + [CO32-]
• DIC is a conserved quantity with respect to the carbonate chemistry
Buffer (Revelle) factor
• Fractional changes in pCO2 is related to that of DIC with a constant factor, B.
• Buffer (Revelle) factor is about 10 for the modern oceans
• 10% increase in pCO2 leads to 1% increase in DIC
Concept test
• Since industrial revolution, atmospheric pCO2 has increased from 280 to 390 ppmv.
• How accurate do we need to measure DIC in order to detect the influence of rising CO2 on the surface ocean DIC concentration?
A little more challenging test
• Let’s assume that we consume entire oil reservoir, 500 GTC, and half of coal reservoir, 2500 GTC.
• If this carbon remains in the atmosphere, what would be the resulting pCO2?
• What happens if we allow the entire ocean carbon reservior to respond to the rising CO2?
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