Ocean Carbon Cycle Figure credit US-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 SIO CO2 program Polar ice core data Instrumental record
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 k M G T P kilo mega giga tera peta
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 Oceans Biosphere 15
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?