The sixth element

Atmospheric CO2

Where do emissions come from? How unique are modern CO2 levels? Where does it all go? How long will it stick around?

Main Concepts

Forms of C

Current CO2 levels and fluxes

What are C reservoirs?

What are C transfer rates? Why important?

Residence time?

Timescales of C cycling

How much is a gigaton (Gt)?

• One billion metric tons.

• It is about 2750 Empire State Buildings.

• 142 million African elephants.

• Global C emissions are about 6 Gt.

Storage Reservoir type Author(s)

750 Gt (1.) In atmosphere, CO2, CH4 (1990) IPCC (1990) (s.)

11,500 Gt (2.) Methane clathrates Macdonald (1992)

38,725 Gt (3.) Dissolved or in suspension in oceans IPCC (1990) (s.)

700 Gt (4.) Dissolved organic C; intermediate & surface waters IPCC. (1990) (s.)

4,000 Gt (5.) Fossil fuel Johns. & Ker. (s.)

Calculating Residence time

Residence time is a “replacement time”: time required to affect a reservoir given a certain flux.

(years) = reservoir / input rate

Example: Residence time of a Stony Brook undergrad

Reservoir: Size of Student Body?Input rate: Incoming 1st-year class size

Carbon cycle fluxes

Cycling Flux rate (Gt/year)

Ocean uptake -2.0

Photosynthesis -120.0

Respiration +120.0

Fossil Fuel Combustion

+6.0

Biomass burning +1.0

( “-” means removed from atmosphere)

Atmospheric CO2:Last 50 years

Atmospheric CO2:Last 250 years

Atmospheric CO2: last 400,000 years!

Calculating residence time of Carbon due to air-sea exchange

Ocean uptake rate: -2.0 Gt / year

Total Ocean C reservoir : 39,000 Gt

Surface Ocean C reservoir : 600 Gt

C residence time (whole ocean) = ?

C residence time (surface only) = ?

The fate of fossil CO2

How quickly would the planet take up our CO2?

Fast: “solubility pump” Air-Sea CO2 exchange (centuries)

Moderate: “Deep ocean acid neutralization” (tens of thousands of years)

Really slow: “Weathering of continental rocks” (millions of years)

Moderate C Cycling

Neutralize deep ocean acidity by Dissolving ocean CaCO3 sediments

104 years

75% in 300 years

25% “forever”

Bottom Line

Human C Emissions are large

Nature can’t keep up

Natural C sinks are diminishing

Lifetime of CO2 from your tailpipe:

“300 years, plus 25% that lasts forever”