1. Ocean Carbon Cycle
Figure credit
US-OCB Program

2. Why study ocean carbon cycle?
CO2 is an important greenhouse gas
Ocean is a major sink of fossil fuel CO2
Impacts on ocean chemistry

3. Rising atmospheric CO2 SIO CO2 program Polar ice core data
Instrumental
record

6. Global average ~ 1.2 TC/year/person

7. 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

8. Current atmospheric CO2 is about 390 ppmv, which is equivalent of 780 GTC.
Simple rule of thumb

9. 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!

10. 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

11. 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

12. 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?

13. 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

14. Emission due to human activities
DOE: Carbon Dioxide Information Analysis Center

15. CO2
changes in the last 50 yr
Oceans
Biosphere 15

16. 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?

17. 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

18. 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

19. 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

20. Air-sea gas transfer Driven by turbulence at the air-sea interface
What controls the turbulent motion?
Emerson and Hedges (2010)

21. Gas transfer coefficient: G [ms-1]
Measures the strength of the air-sea gas transfer

22. 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

23. Chemical reactions with the seawater
A series of reactions
CO2 + H2O H2CO3
(carbonic acid)
H2CO3 H+ + HCO3-
(bicarbonate ion)
HCO H+ + CO32-
(carbonate ion)
Textbook chapter 6, page

24. Carbonate chemistry DIC = (Dissolved Inorganic Carbon)
= [CO2] + [H2CO3] + [HCO3-] + [CO32-]
DIC is a conserved quantity with respect to the carbonate chemistry

25. 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

26. 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?

27. 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?