Global Carbon Cycling
Where does it all go?
Main Concepts
Current CO2 levels: fluxes in and out
What are C reservoirs?
Natural CO2 sources and sinks:
The land breathes.
The ocean breathes.
The rocks breathe.
Carbon Residence time?
Timescales of carbon removal from the
atmosphere.
Atmospheric CO2
What are the major sources of C emissions?
How unique are modern CO2 levels?
Where does it all go?
How long will it stick around?
Fossil fuel CO2 emissions: Burning buried sunshine
Global C emissions map
Atmospheric CO2:Last 50 years
(1.5 ppm/year increase)
Atmospheric CO2: Last 250 years
Atmospheric CO2: last 400,000 years!
Atmospheric CO2:
Last 50 MILLION years
How unusual are
modern CO2 levels?
Atmospheric CO2:
Last 500 MILLION years
Estimating Ancient CO2:
Leaf Stomata
Living
Stomata density
related to CO2
high CO2 = low stomatal density
Fossil
Carbon fluxes (in Gt/yr), reservoirs (bold, Gt),
and residence times (years)
as of 1990
Note: 2006 emissions were 7.7 Gt / year,
How much is a gigaton (Gt)?
• One billion metric
tons (1015 g)
• It is about 2750
Empire State
Buildings.
• Global C emissions
are about 6 Gt.
Carbon cycle fluxes
These have significant errors and change year-to-year
Cycling
Ocean uptake
Flux rate (Gt/year)
-2.0
Photosynthesis
-120.0
Respiration
+120.0
Fossil Fuel
Combustion
Biomass burning
+6.0
+1.0
( “-” means removed from atmosphere; 1990 data)
The atmosphere only contains about
2/3 of the total C emissions - why?
Carbon “sinks”
Source:
Carbon Emissions: 6.0 Gt/year
Sink:
Obs. Atm increase: 3.2 Gt/year
Ocean uptake:
2.0 Gt/year
“missing sink”:
1.8 Gt/year
1990 estimates
Ocean-Atmosphere gas exchange
Air
Sea
CO2 + H2O  H+ + HCO3-
Vertical
Sections
through
the oceans
Air-Sea CO2 fluxes
Gases are more soluble in COLD water
Ocean uptake
Net:
-2 Gt/yr
Ocean release
Ocean uptake
Ocean uptake
Ocean release
Ocean and Atmoshere
C reservoirs
Ocean C: 39,000 Gt (as HCO3-, CO32-)
Atmosphere: 1580 Gt (as CO2)
Ocean has 50x more carbon than the
atmosphere.
Carbon Fluxes
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 CU undergrad
Reservoir: Size of Columbia’s UG Student
Body?
Input rate: Incoming 1st-year class size
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) = ?
Q1:
Carbon Cycle Budget
At steady state: Sources = Sinks
Sources: Respiration, FF, land use
Sinks: Photosynthesis, ocean uptake
But.. C is not at steady state… (CO2 is rising,
right? Equivalent to +3.2 x1015g/ year
So the following should be true:
+3.2 Gt/year = (Sources) + (Sinks)
The missing C sink (Gt/yr)
+3.2 = (Sources) + (Sinks) + (other)
+3.2 = (122+90+6+1) + (-120-92-2) + (other)
+3.2 = (219 - 214) + (other)
“other” = 5.0 - 3.2
“other” = missing sink of -1.8 Gt/year
This “missing sink” has been removing C from
atmosphere each year…(a good thing)
The Missing Sink (history)
Missing C sink: What is it?
CO2 fertilization of high-latitude forests
Plants grow faster/better at higher CO2
Plant C
uptake
Atm CO2 level
Current state of the Missing Sink
“Missing sink” was accelerated
growth of northern forests under
high CO2 (carbon uptake)
This sink is now completely offset
by tropical deforestation.
(additional carbon emission)
Bolivia (1984-1998)
The future of fossil fuel 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)
Fast C cycling: Air-Sea Exchange
Centuries
Moderate C Cycling
Neutralize deep ocean acidity by
Dissolving ocean CaCO3 sediments
104 years
Really Slow C cycling
Continental weathering (dissolves mountains!)
“Urey reaction” - millions of years
CaSiO3 + CO2 --> CaCO3 + SiO2
Carbon Reservoirs
(1Gt = 1015g)
Reservoir
Rocks
Size
65,000,000
Oceans
39,000
Soils
1,580
Land Plants
610
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”