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
Carbon emissions rising faster than estimates
Global C emissions map
Where emissions come from
Atmospheric CO2:Last 50 years
(2.0 ppm/year increase, or 0.5%)
391 ppm
It’s alive! Seasonal cycle
CO2 growth rates
http://www.esrl.noaa.gov/gmd/webdata/ccgg/trends/co2_data_mlo_anngr.png
CO2 growth rates
http://www.esrl.noaa.gov/gmd/webdata/ccgg/trends/co2_data_mlo_anngr.png
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.
Free air
Trapped air
Up to 800,000 year old ice…
with ancient trapped air
bubbles!
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?
Natural vs. human carbon fluxes
Cycling
Flux rate (Gt/year)
Photosynthesis
-122.0
Respiration
+120.0
Fossil Fuel emissions
+7.0
Deforestation
+2.0
Negative (positive) means removed from (added to) theatmosphere; 2002 data)
Carbon ins and outs
Source:
Carbon Emissions:
Deforestation:
7 Gt/year
2 Gt/year
Sink:
Obs. Atm increase:
Ocean uptake:
“missing sink”:
-4 Gt/year
-2.5 Gt/year
-2.5 Gt/year
2002 data
Human Carbon emissions
2012 emissions are ~9 Gt… were about 6 Gt when I started teaching this course !
Deforestation accounts for an
additional +2 Gt / year
Deforestation
- 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 one-third of our fossil fuel emissions
Air (CO2)
CO2 + H2O  H+ + HCO3Sea (CO2)
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
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:
Deforestation:
7 Gt/year
2 Gt/year
Sink:
Obs. Atm increase:
Ocean uptake:
“missing sink”:
-4 Gt/year
-2.5 Gt/year
-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: 1-2 Gt
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
Q: How quickly will the planet take up our CO2?
A: Not very quickly…
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”
Radiative Forcing
• Helps us quantify how global climate
responds to an imposed change
(“forcing”).
What is Radiative Forcing?
Radiative forcing: An imposed change in Earth’s radiative energy
balance. Measured in Watts per square meter (W/m2)
• “Radiative” because these factors change the balance
between incoming solar radiation and outgoing infrared radiation
within the Earth’s atmosphere. This radiative balance sets the
Earth’s surface temperature.
• “Forcing” indicates that Earth’s radiative balance is being
pushed away from its normal state.
Examples: Solar variability, volcanic emissions, greenhouse gases,
ozone, changes in ice cover (albedo), land use changes.
Our first climate model
Recall how to calculate Earth’s effective temperature
The Stefan-Bolzmann equation:
Blackbody radiation I (w/m2) = s T4
Earth incoming radiation (a = Earth albedo, or reflectivity)
I incoming = (1-a) Isolar = (1-a) s Tsun4
a Is ~0.3, or 30%
Our first climate model
Earth incoming radiation (a = Earth albedo, or reflectivity)
I incoming = ((1-a) Isolar ) / 4, or ((1-a) s Tsun4 )/ 4
Earth outgoing radiation
I outgoing = s Tearth4
Earth’s temperature with no greenhouse effect
Teffective = 254.8K (-18°C)
At equilibrium, I incoming = I outgoing
Set Sunlight = Earthlight
Solve for Tearth
Eqn. 3.1 in Archer Chapter 3
Volcanic eruption can
change albedo by 1%
a = ~30% on average
Teffective = 254.8K
Recalling I = (1-a) s T4
Increase a to 31%
New Teffective = 253.9K
or -1°C cooler due a volcanic eruption
Adding an atmosphere
Greenhouse gases are “selective absorbers”of
outgoing long wavelength radiation (Earthlight)
Spectrum of
IR light emitted
from earth
to space
Water Vapor Molecule (H2O)
Vibrational modes
H2O
bend
H2O
stretch
Carbon Dioxide Molecule (CO2)
Vibrational mode (~15µm)
CO2
bend
Natural CO2 radiative forcing
Makes Earth habitable
Pre-Industrial CO2 level of ~280 ppm
Increases surface temperature from -18°C
(effective temperature) to +15°C
(Water vapor is also important)
CO2 “Band Saturation”
More CO2 warms the Earth less and less
No CO2
10 ppm
100 ppm
1000 ppm
Notice the CO2
absorption band
CO2 and surface warming
just due to radiation changes - no feedbacks
Pre-Industrial = 280 ppm
Today = 390 ppm
Temp (K)
About 1°C per 100 ppm
So, w/o feedbacks: ~1.2°C
With feedbacks: ~3°C
(Feedbacks include water
vapor and sea ice changes)
CO2 ppm
Atmospheric CO2
CO2
(ppm)
• CO2 has increased by about +40%
• Long term average growth rate is +1.4% per year
• Last decade growth rate is +2.0% per year
All Radiative Forcing factors (1750-2005)
Sum = +1.6 W/m2
Radiative Forcing Contributions
•
•
•
•
•
GHGs warm (CO2, CH4, N2O)
H2O (vapor) warms
Tropospheric O3 warms, Strat O3 cools
Human and natural Aerosols cool
Solar irradiance warms
Net Effect: +1.6 W/m2