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.

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

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

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

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

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

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

Carbon emissions rising faster than estimates

Global C emissions map Where emissions come from

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

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

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

Atmospheric CO2: Last 250 years

Atmospheric CO2: last 400,000 years!

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

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

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)

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)

Carbon ins and outs Source: Carbon Emissions: 7 Gt/year Deforestation: 2 Gt/year Sink: Obs. Atm increase: -4 Gt/year Ocean uptake: -2.5 Gt/year “missing sink”: -2.5 Gt/year 2002 data

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

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 (1984-1998)

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”

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.

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

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

Carbon ins and outs Source: Carbon Emissions: 7 Gt/year Deforestation: 2 Gt/year Sink: Obs. Atm increase: -4 Gt/year Ocean uptake: -2.5 Gt/year “missing sink”: -2.5 Gt/year 2002 data

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 ???

The Missing Sink (history)

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

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.

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 ?

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.

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

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)

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

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

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

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”