Anthropogenic CO2 invasion

I. Anthropogenic CO2 uptake

Tacoma, WA (1891)

A. “Perturbed” carbon cycle (1990s)

Pre-industrialAnthropogenic

Anthro Sources:Fossil fuels: 244 GtCDeforestation: ~140 GtC

Anthro Sinks:Ocean: 118 19 GtC (~30%)Reforestation: ~100 GtC (~25%)Atmosphere: 165 GtC (~45%)

Pre-anthropogenic ocean:atmosphere DIC:CO2 >60:1 (98%)Anthropogenic ocean:atmosphere uptake ~0.7:1 (40%)

What limits ocean uptake of CO2?

How do we measure ocean uptake?What is the equilibrium capacity for uptake?What are the kinetic constraints?

B. Measuring ocean uptake1. Direct measurement• measure increase of DIC over time• get expected rate of change from Revelle Factor• measureable but small compared to spatio-temporal variability

2. Isolating anthro component of DIC inventory• if pre-industrial preformed DIC were known, then it could be subtracted from observed preformed DIC• pre-industrial preformed DIC estimated by additionally considering ventilation age (complicated)

Spatial pattern: Revelle Factor• higher values mean smaller DIC rise for a given pCO2 rise• higher DIC waters have higher Revelle Factor (high lats)• at constant Alk, adding DIC shifts equilibrium to left, towards CO2(aq)• keeps equilibrium pCO2 high and limits uptake of CO2

Sabine et al. (2004) Science

• low latitude surface has more uptake due to lower Revelle Factor (lower DIC)

• greater penetration into subtropical gyres and NADW

Sabine et al. (2004) Science

Sabine et al. (2004) Science

Water column inventory of anthropogenic CO2

C. Ocean’s equilibrium uptake capacity

1. DIC buffer

• ocean’s uptake capacity is large due to reaction with DIC• calculate equilibrium uptake using Revelle Factor• for top 75 m in equilibrium: 8% in ocean (ignoring land)• for entire ocean volume: 81% in ocean

• timescale depends on mixing (e-folding ~300 y)

• since Revelle Factor rises as DIC rises, capacity will decrease in future

2. CaCO3 buffer• dissolution of seafloor CaCO3 raises Alk:DIC 2:1• this drives DIC away from CO2(aq) and allows more uptake• total ocean uptake now ~90%• timescale depends on pore water diffusion and dissolution kinetics (e-folding ~4000 y)

3. Weathering buffer• reaction with all CaCO3 on land still leaves 8% in atmosphere• silicate weathering ultimately removes rest of perturbation

Remaining in atmosphere~50% after 400 y~20% after 2000 y~8% after 40,000 y

IPCC4

II. Ocean acidification• CO2 addition shifts DIC away from CO3

2- and OH- (lower pH)• surface pH has already dropped by 0.1• at constant Alk, 2X pCO2 (560 ppm): CO3

2- ↓30%, pH ↓0.3

• aragonite lysocline has shoaled by o(100 m) in Indo-Pacific• slight buffering due to reduced calcification (alkalinity gain)• greater buffering possible via sediment dissolution (longer)

Feely et al. (2004) Science