The Earth’s Carbon Cycle
12.340 Global Warming Science
March 22, 2012
David McGee
1
Main topics
• How negative feedbacks in the carbon cycle are thought
to lead to long-term (Myr-scale) climate stability
• Short-term (kyr or less) cycling of carbon
• Interactions between human activities and the carbon
cycle, including:
– Present uptake of anthropogenic CO2 by the ocean and
terrestrial biosphere
– Long-term fate of anthropogenic CO2
2
Table: Carbon budget for the Earth in the pre-industrial era
(Sarmiento and Gruber, 2002; Tyrell and Wright, 2001)
Reservoir
Mass of Carbon (1015g=Pg)
Atmosphere
600
Fossil Fuels
3,700
Ocean
38,000
Rocks (as calcium carbonate)
60,000,000
Rocks (organic)
14,000,000
Vegetation/Soils
2,000
Image by MIT OpenCourseWare.
3
Atmosphere
CO2
emissions
CO2 uptake
(silicate weathering)
CaCO3
Continent
Sediments
an
oe
Island
(mantle plume)
Vo
lc
Organic
carbon burial
Ocean
s
CO2 release
(organic C weathering)
Continent
Continent
Ocean Crust
CO2
Mantle
Su
bd
uc
tio
n
Mantle
Mantle
Image by MIT OpenCourseWare.
4
Long-term sinks for atmospheric CO2
• Burial of organic carbon produced by photosynthesis
Idealized as
CO2 + H2O + photons
CH2O + O2
Most organic carbon is quickly respired, but some is buried in
marine
sediments, peat bogs, swamps, etc.
• Silicate weathering and calcium carbonate burial
Idealized as
CaSiO3 + CO2 (in soils)
river transport of dissolved Ca, Si, CO2
CaCO3 + SiO2 (in the ocean)
5
Atmosphere
CO2
emissions
CO2 uptake
(silicate weathering)
CaCO3
Continent
Sediments
an
oe
Island
(mantle plume)
Vo
lc
Organic
carbon burial
Ocean
s
CO2 release
(organic C weathering)
Continent
Continent
Ocean Crust
CO2
Mantle
Su
bd
uc
tio
n
Mantle
Mantle
Image by MIT OpenCourseWare.
6
The global carbon cycle can be approximated as:
Atmosphere
CaCO3
Island
(mantle plume)
Sediments
Continent
oe
Ocean
an
Organic
carbon burial
s
CO2 release
(organic C weathering)
Vo
lc
dM CO2
= Fvolc Fsil Forg
dt
CO2
emissions
CO2 uptake
(silicate weathering)
Continent
Continent
Ocean Crust
CO2
Mantle
Where:
Su
bd
uc
tio
n
Mantle
Mantle
Image by MIT OpenCourseWare.
MCO2=Mass of CO2 in the atmosphere
Fvolc=Volcanic CO2 emissions (~0.2 GtCO2/yr)*
Fsil=Silicate weathering flux (~flux of Ca ~flux of CaCO3)
Forg=Organic C burial flux (can be positive or negative)
*(note: MCO2=MC*44/12)
7
The silicate weathering CO2 thermostat:
An explanation for long-term climate
stability
CO2
emissions
CO2 uptake
(silicate weathering)
Organic
carbon burial
Ocean
CaCO3
Continent
s
CO2 release
(organic C weathering)
Island
(mantle plume)
Sediments
Vo
lca
no
e
dM CO2
= Fvolc Fsil Forg
dt
Atmosphere
Continent
Continent
Ocean Crust
CO2
Mantle
Su
bd
uc
tio
n
Mantle
Mantle
Image by MIT OpenCourseWare.
IMPORTANT: Because the atmosphere contains so little carbon, these fluxes
cannot be out of balance on Myr timescales.
Forg is generally ~3x smaller than Fsil and is not thought to be strongly sensitive to
climate.
Fsil should increase with increasing atmospheric CO2 and temperature.
Thus, on Myr timescales we can write:
Fvolc = Fsil + Forg = ksil MCO2 + Forg
where ksil is the slope of the weathering-CO2 relationship.
8
Long-term climate change can thus be
interpreted in terms of
a) Changes in volcanic outgassing (e.g.,
due to changes in oceanic crust
production at mid-ocean ridges)
This image has been removed due to copyright restrictions. Please see:
Figure 1. Beerling, D. J., D. L. Royer, et al. "Convergent Cenozoic CO2
history". Nature Geoscience 4, no. 7 (2011): 418–20. doi:10.1038/
ngeo1186.
b) Changes in ksil through changes in
the “weatherability” of continents
(e.g., due to mountain building,
concentration of land masses near
the equator, or volcanic activity
producing large, highly-weatherable
areas of Earth’s surface)
9
Silicate weathering CO2 thermostat:
examples
Increased volcanic emissions
Rising atmospheric pCO2
Increasing silicate weathering
Stabilized pCO2
Increasing C burial
as CaCO3
Atmospheric pCO2 rises until weathering is sufficient to balance increased
volcanic inputs, stabilizing atmospheric pCO2.
10
Walker feedback: examples
Increased weatherability
Stabilized pCO2
11
Walker feedback: examples
Increased weatherability
Increased burial of CaCO3
Declining pCO2
Stabilized pCO2
Less weathering
Atmospheric pCO2 falls until weathering falls back into balance with volcanic
inputs, stabilizing atmospheric pCO2.
12
The Paleocene-Eocene Thermal Maximum
(PETM), 55 Myr ago
Addition of carbon to the
atmosphere and ocean
over ~10kyrs
Removal chiefly by
silicate weathering
feedback over ~200 kyrs
This image has been removed due to copyright restrictions. Please see the image on page
http://www.springerimages.com/Images/RSS/1-10.1007_978-1-4020-4411-3_168-1.
13
Short-term carbon fluxes
This image has been removed due to copyright restrictions. The image is from Sarmiento, Jorge L.
and Nicolas Gruber. "Sinks for Anthropogenic Carbon." Physics Today. August 2002. (c) American
Institute of Physics, S-0031-9228-0208-010-9. Please see the image on page http://www.atmos.
ucla.edu/~gruberpublication/images_publ/pt_02/carboncycle_rev3d.pdf
Sarmiento and Gruber, 2002
14
Image courtesy of NOAA.
15
Image courtesy of NOAA.
16
Carbon emissions from fossil fuel use
This image has been removed due to copyright restrictions. Please see the image
on page http://en.wikipedia.org/wiki/File:Global_Carbon_Emissions.svg.
17
<60% of trend expected
from fossil fuel emissions
Image courtesy of NOAA.
Where is the rest of the CO2?
18
Important short-term CO2 sinks
Terrestrial biosphere: Increase due to afforestation, CO2 fertilization?
Ocean: Increase in CO2 content of ocean waters?
One way of checking: measure changes in the O2 content of the atmosphere
O2/N2 RATIO (per meg)
LA JOLLA, CALIFORNIA
-100
-200
-300
-400
1989
1991
1993
1995
1997
1999
2001
2003
2005
2007
CO2 (ppm)
Year
390
370
Burning of fossil fuels
leads to declining O2,
while increase in size
of terrestrial
biosphere causes
rising O2, and ocean
uptake of CO2 doesn’t
affect O2.
350
1989
1991
1993
1995
1997
1999
2001
2003
2005
2007
Year
Image by MIT OpenCourseWare.
Suggests net terrestrial sink is smaller than the marine sink
19
These images have been removed due to copyright restrictions. Please see the images
on page http://www.globalcarbonproject.org/carbonbudget/12/images.htm.
Note large uncertainties in terrestrial source and sink!
Global Carbon Project 2010; Updated from Le Quéré et al. 2009, Nature Geoscience; Canadell et al. 2007, PNAS
20
Why is so much CO2 going into the ocean?
Argon (similar mass to CO2):
~40x more Ar in the atmosphere than in the ocean
CO2:
>60x more CO2 in the ocean than in the atmosphere!
21
Why is so much CO2 going into the ocean?
For both gases, air-sea fluxes are proportional to the difference in partial
pressures between the atmosphere and surface ocean:
pCO2air
pCO2sea
pArair
pArsea
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Why is so much CO2 going into the ocean?
But while Ar stops there, CO2 undergoes a number of transformations:
pCO2air
pArair
pCO2sea
H2CO3*
pArsea
Carbonic acid + aqueous CO2
HCO3-
Bicarbonate ion
CO32-
Carbonate ion
23
(note: full equations not shown for clarity )
The full equations:
Dissolved inorganic carbon = DIC = TCO2
=H2CO3* + HCO3- + CO32Where H2CO3* (sometimes shown simply as CO2) denotes the combination of
H2CO3 and CO2(aq), which are hard to distinguish analytically.
24
Note that adding CO2 to the ocean acts to
increase H+ concentrations (i.e., reduce pH)
25
100
2CO3
CO2 (=H2CO3)
90
HCO3
2-
70
60
-
CO2, HCO3, CO3 (%)
80
50
40
30
20
10
0
4
5
6
7
8
9
10
11
pH
Image by MIT OpenCourseWare.
At the pH of seawater (~8.2), DIC is ~0.5% H2CO3*, 88.5% HCO3-, and 11% CO32-
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100
2CO3
CO2 (=H2CO3)
90
HCO3
2-
70
60
-
CO2, HCO3, CO3 (%)
80
50
40
30
20
10
0
4
5
6
7
8
9
10
11
pH
Image by MIT OpenCourseWare.
As pH falls, carbonate ion concentrations go down and H2CO3* concentrations
go up, reducing the ocean’s ability to take up more CO2.
27
“Pumps” and vertical DIC gradients
28
Image courtesy of Michael Follows. Used with permission.
Image courtesy of Michael Follows. Used with permission.
29
“Pumps” and vertical DIC gradients
Image courtesy of Michael Follows. Used with permission.
30
“Pumps” and vertical DIC gradients
Image courtesy of Michael Follows. Used with permission.
31
Image courtesy of Michael Follows. Used with permission.
32
Image courtesy of Michael Follows. Used with permission.
33
Data from Takahashi et al (2002)
This image has been removed due to copyright restrictions. Please see the image on
page http://www.nature.com/nature/journal/v407/n6805/fig_tab/407685a0_F1.html.
34
0
Depth (m)
500
1000
1500
2000
0
1.0
2.0
3.0
mol C m-2y-1
Image by MIT OpenCourseWare.
35
Image courtesy of Michael Follows. Used with permission.
36
Image courtesy of Michael Follows. Used with permission.
This image has been removed due to copyright restrictions. Please see the image on
page http://earthguide.ucsd.edu/earthguide/imagelibrary/emilianiahuxleyi.html.
37
Image courtesy of Michael Follows. Used with permission.
38
Alkalinity
The ocean’s acid-buffering capability.
Equivalent to the charge imbalance between strong (i.e., unchanging) positive
and strong negative ions. Silicate weathering increases alkalinity by
preferentially adding ions like Ca2+, Mg2+, etc.
The charge imbalance between strong cations and strong anions is largely
balanced by carbonate and bicarbonate ions, meaning that the higher the
imbalance and the lower the DIC, the more DIC must exist as carbonate ion (-2
charge).
Rule of thumb: Alk-DIC ~ [CO32-].
39
Alkalinity
(negative charge deficit)
SO42-
40
Right now anthropogenic CO2 is causing DIC in the oceans to increase while
alkalinity stays roughly constant. What do you predict is happening to carbonate
ion concentrations?
41
The “long tail” of anthropogenic CO2
Uptake of 1000 Pg
C pulse
This image has been removed due to copyright restrictions. Please see Figure 1 on page
http://www.annualreviews.org/doi/pdf/10.1146/annurev.earth.031208.100206.
Different experiments reflect model runs with varying feedbacks included; the dotted
lines include the most feedbacks (climate, sediments, weathering, vegetation)
42
The “long tail” of anthropogenic CO2
Archer et al., Annual Reviews of Earth and Planetary Science 2009 (a nice paper)
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12.340 Global Warming Science
Spring 2012
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