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?