1. Global Carbon Cycling Where does it all go?

2. 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.

3. Quiz Q1: The surface ocean currents are characterized by what form of circulation?
Thermohaline overturning circulation
Wind-driven circulation
Hadley circulation
Walker circulation
Upwelling circulation

4. Quiz Q2: By which process are surface ocean waters transported by winds ?
The surface wind friction coupling
Ocean waves
The Ekman spiral
Surface drag
Walker circulation

5. Quiz Q3: Why does the Gulf Stream flow clockwise?
Surface trade winds and westerlies pile up ocean waters in subtropical “hills”.
Geostrophic flow.
The Gulf Stream is pushed by the winds directly.
Answers A, B and C
Answers A and B

6. Quiz Q4: Why are ocean temperatures cool off coastal California and West Africa (20°N) ?
Atmospheric heat transport
Ocean heat transport
Coastal upwelling due to surface winds
Equatorial upwelling due to trade winds
Surface evaporation

7. Quiz Q5: Wind-driven ocean circulation extends to what water depth?
Upper 10m
Upper 50m
Upper 1000m
Upper 3000m
It extends all the way to the seafloor

8. 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?

9. Fossil fuel CO2 emissions: Burning buried sunshine

10. Carbon emissions rising faster than estimates

12. Global C emissions map
Where emissions come from

13. Atmospheric CO2:Last 50 years (2.0 ppm/year increase)

14. What do we know about greenhouse gases and past climate change?

15. Glacial ice “traps” ancient air
Snow accumulates…
Snow becomes ice
Pore spaces are sealed and they trap ambient air.
Up to 800,000 year old ice… with ancient trapped air bubbles!
Free air
Trapped air

16. Atmospheric CO2: Last 250 years

17. Atmospheric CO2: last 400,000 years!

18. Atmospheric CO2: Last 50 MILLION years
How unusual are
modern CO2 levels?

19. Carbon fluxes (in Gt/yr), reservoirs (bold, Gt), and residence times (years)
1990s data
Note: 2010 emissions were 9 Gt / year

20. How much is a gigaton (Gt)?
One billion metric tons (1012 kg)
It is about 2750 Empire State Buildings.
Global C emissions are about 9 Gt as of 2010.
How much does global population weigh?
Global population = 6 x10^9 people * 100 kg/person = 600 x10^9 kg, or 0.6 x 10^12 kg (~0.5 Gt)

21. Natural vs. human carbon fluxes
Cycling
Flux rate (Gt/year)
Photosynthesis
-122.0
Respiration
+120.0
Fossil Fuel emissions
+7.0
Deforestation
+2.0
Natural carbon cycling by plants is a lot larger, but there’s little net difference between photosynthesis and respiration
( “-” means removed from atmosphere; 2002 data)

22. Carbon ins and outs Source: Carbon Emissions: 7 Gt/year
Deforestation: 2 Gt/year
Sink:
Obs. Atm increase: -4 Gt/year
Ocean uptake: Gt/year
“missing sink”: Gt/year
2002 data

23. Human Carbon emissions
… were about 6 Gt when I started teaching this course !

24. Deforestation accounts for an additional +2 Gt / year
- Mainly tropical rainforests
- Cutting down forests to make agricultural land is a net source of carbon to the atmosphere.
CH2O + O2  CO2 + H2O
Bolivia ( )

25. Where do our carbon emissions go?
Ocean takes up about -2.5 Gt / year
Roughly 1/3 or our fossil fuel emissions
Air (CO2)
Sea (CO2)
CO2 + H2O  H+ + HCO3-
Oceanic “Buffer reaction”

26. Why does the ocean take up CO2?
CO2 gas is soluble in the ocean
Gas solubility is highest in colder water
CO2 enters the oceans at the poles
CO2 is converted to HCO3- by “buffer reaction”
The ocean acidifies as a direct result
Ocean “buffer chemistry” can take up only a finite amount of CO2.

27. Air-Sea CO2 fluxes Net: -2 Gt/yr Gases are more soluble in COLD water
Ocean uptake
Net:
-2 Gt/yr
Ocean release
Ocean uptake
Ocean uptake
Ocean release

28. Where is our carbon in the oceans ?
Vertical
Sections
through
the oceans
Total ocean uptake is about -2.5 Gt / year

29. Carbon ins and outs Source: Carbon Emissions: 7 Gt/year
Deforestation: 2 Gt/year
Sink:
Obs. Atm increase: -4 Gt/year
Ocean uptake: Gt/year
“missing sink”: Gt/year
2002 data

30. What is the “missing sink”
The “missing sink” is the amount of carbon required to balance sources and sinks.
It is a big number: -2.5 Gt Carbon / year !
What is it ???

31. The Missing Sink (history)

32. Missing C sink: CO2 fertilization
“CO2 fertilization” of high-latitude forests
Plants grow faster/better at higher CO2
But … the effect is assymptotic (not linear)
Plant C
uptake
Atm CO2 level

33. Other things we need to know
Not only Fluxes of carbon in/out (Gt / year)
Sizes of the carbon reservoirs
Residence Time of carbon in each reservoir
These additional factors determine who the biggest players are and how quickly they will act.

34. Why these things matter
What would happen to CO2 levels if we stopped all emissions today?
What if the ocean warms up a lot?
What if deep ocean circulation were to change ?
Does Arbor day matter ?

35. Ocean and Atmoshere C reservoirs
Atmosphere: 1580 Gt (as CO2)
Ocean C: 39,000 Gt (as HCO3-, CO32-)
Ocean has 50x more carbon than the atmosphere.

36. 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

37. 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 (surface only) = ?
C residence time (whole ocean) = ?

38. The fate 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)

39. Fastest response (decades to centuries): The CO2 solubility pump
Air-Sea gas
exchange

40. Medium response time (104 years): Neutralize ocean acidity
Neutralize deep ocean acidity by
Dissolving ocean CaCO3 sediments
CaCO3  Ca2+ + CO32-

41. Really Slow response time (106 years)
Continental weathering (dissolves mountains!)
“Urey reaction” - millions of years
CaSiO3 + CO2 --> CaCO3 + SiO2

42. 75% in 300 years
25% “forever”

43. 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”