Global Carbon CyclingWhere does it all go?
Main Concepts
Pre-anthropogenic CO2 fluxes in and out
Current CO2 fluxesWhat are C reservoirs?Carbon Residence time?Timescales of carbon removal from the
atmosphere.
IPCC AR5 (2013)
Carbon: Ins and Outs
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%)
400 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
What do we know aboutgreenhouse 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)
Note: 2010 emissions were 9 Gt / year
1990s data
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 2012.
How much does global population weigh?
7 x 109 people x 102 kg/person7 x 1011 kg = 0.7 Gt
AR5 Observed carbon fluxes
Reservoir Pre-Ind Fluxes (Gt/year)
Current Flux (Gt/year)
Photosynthesis -108.9 -123.0
Respiration +107.2 +118.7
Ocean +0.7 -2.3
Fossil fuels emissions +7.8
Land Use changes +1.1
“Other” (volcanoes, lakes, rivers)
+1.0 +0.3
Atmosphere CO2 increase
- 0 - +4
Negative (positive) means removed from (added to) the atmosphere; IPCC AR5 data)
Carbon ins and outs
Source:
Carbon Emissions 7.8 Gt/year
Deforestation 1.1 Gt/year
Sink:
Obs. Atm increase -4.0 Gt/year
Ocean uptake -2.3 Gt/year
“missing sink” -2.6 Gt/year
IPCC AR5 data
Human Carbon emissions
2012 emissions are ~9 Gt… were about 6 Gt when I started teaching this course !
Deforestation accounts for an additional +1.1 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 -2.3 Gt / year• Roughly one-third of 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
Ocean uptake
Ocean release
Gases are more soluble in COLD water
Ocean uptake
Ocean uptake Ocean release
Net:-2 Gt/yr
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 7.8 Gt/year
Deforestation 1.1 Gt/year
Sink:
Obs. Atm increase -4.0 Gt/year
Ocean uptake -2.3 Gt/year
“missing sink” -2.6 Gt/year
IPCC AR5 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: -2.6 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)
Atm CO2 level
Plant Cuptake
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 undergradReservoir: 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 yearsCaSiO3 + CO2 --> CaCO3 + SiO2
75% in 300 years
25% “forever”
Time of removal
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) = T4
Earth incoming radiation ( = Earth albedo, or reflectivity)
I incoming = (1-) Isolar = (1-) Tsun4
Is ~0.3, or 30%
Our first climate model
Earth incoming radiation ( = Earth albedo, or reflectivity)
I incoming = ((1-) Isolar ) / 4, or ((1-) Tsun4 )/ 4
Earth outgoing radiation
I outgoing = 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%
= ~30% on average
Teffective = 254.8K
Increase to 31%
New Teffective = 253.9K
or -1°C cooler due a volcanic eruption
Recalling I = (1-) T4
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
H2Ostretch
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
10 ppm
1000 ppm100 ppm
No CO2
Notice the CO2 absorption band
CO2 and surface warming just due to radiation changes - no feedbacks
About 1°C per 100 ppm
Pre-Industrial = 280 ppm
Today = 390 ppm
So, w/o feedbacks: ~1.2°C
With feedbacks: ~3°C
(Feedbacks include water vapor and sea ice changes)
CO2 ppm
Tem
p (K
)
Atmospheric CO2
• 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
CO2 (ppm)
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