The Carbon Cycle
1.
2.
3.
4.
The global carbon budget .......................................................................................... 1
The oceanic carbon cycle .......................................................................................... 7
The terrestrial carbon cycle and "missing sink" ....................................................... 12
Fossil fuel emissions of CO2 ................................................................................... 15
1. The Global Carbon Budget
Concerns about global climate change have arisen largely because of increasing concentrations
of atmospheric CO2, due fossil fuel burning and to deforestation, which releases CO2 from
oxidation of organic matter stored in trees and soils. In order to understand concerns over the
changing composition of the atmosphere, and to project future concentrations of greenhouse
gases, we will study the global cycles of carbon through the atmosphere, biosphere, soils, and
oceans.
Recent records of atmospheric CO2 concentrations date back only to the late 1950's. Before
1960, it was not even known that atmospheric CO2 concentrations were increasing with time.
Since that time much scientific work has been done to determine the history of atmospheric CO2.
An especially fruitful approach has been to analyze CO2 content of air trapped in polar ice cores.
Ice core data
The following figures show the records of CO2 retrieved from polar ice cores, using different
time intervals on the x-axis in a sequence leading up to the recent past. There have been peaks
and dips in CO2 concentrations over hundreds of thousands of years. This pattern repeats several
times back to the oldest ice ever recovered, about 450,000 years before the present. However, the
current peak, the one for which we are responsible, is the highest that has occurred on the planet
in 450,000 years. This graph was made in 1995, present concentrations are just under 370 ppm.
During the last ice age the atmospheric concentration of CO2, inferred from ice cores, was about
200 ppm. At the end of the ice age, mean surface temperatures increased by 10 C while the
atmospheric abundance of CO2 increased by 80 ppm to 280 ppm. (1 ppm (part per million)
corresponds to 2.1 billion tons (Gtons) of carbon in atmospheric CO2.) Atmospheric CO2 and
global temperature track each other over geological time. During warm periods ("inter-glacials"),
CO2 was typically about 280 ppm, and values were much lower (~180 ppm) during cold periods
of glacial advance. We cannot determine from the ice core record whether the CO2 increases
caused the temperature to increase at the end of the ice ages, or vice versa. However, we do
know that CO2 is a very efficient greenhouse gas, and most likely warming climate caused CO2
to increase and vice versa, i.e. there was positive feedback.
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In the following graph, you will notice the rise in CO2 concentrations over the past decades. You
should also notice that the rate at which the concentration is rising is beginning to slow. That is,
the graph appears to be leveling off a bit.
The graph on the following page shows the concentrations of CO2 at a number of stations
throughout the world. The latitudes of the different stations are included in the index. Can you
postulate as to why there is a "saw-toothed" appearance in the trend? Why are the "teeth" longer
for some stations than for others?
The saw tooth pattern superimposed on the long-term trend is due to the influence of
photosynthesis and respiration, especially by terrestrial (land) plants. In the spring and summer
when the surface vegetation is growing, photosynthesis uses up CO2 from the atmosphere to
make organic matter, and thus we see a decline in the concentration of atmospheric CO2,
reaching a minimum at the end of the summer. In fall and winter when leaves are decaying and
vegetation and soils respire, CO2 is released to the atmosphere and thus we see an increasing
concentration, reaching a maximum in late winter.
At sites very far removed from surface vegetation the amplitude of the saw tooth pattern is
significantly reduced. For example, similar measurements from Antarctica show the gradual
increase in the abundance of CO2, but the season cycle due to photosynthesis and respiration is
very small.
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One would also expect the seasonal signal to be smaller in the Southern Hemisphere since most
of the land mass on the Earth exists in the Northern Hemisphere. In fact, this is exactly what is
observed. The seasonal change is largest at the highest northern latitudes, where most of the
surface is covered by land and where the vegetation has the strongest seasonal growing pattern.
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These data show that the terrestrial biosphere plays an important role in the global carbon
budget. The seasonal cycle is much bigger than the year-to-year increase. Annually,
anthropogenic (human) activity releases about 7.6 G tons C/yr (1 G ton = 1 billion tons = 1.0 x
1012 kg of Carbon) to the atmosphere. However, we only see an increase of about 3 G tons
C/year (~1.5 ppm/yr) in the atmosphere (see Table 1 in Section 4, Emissions of CO2). Thus the
biosphere and the oceans must take up almost half of the CO2 released into the atmosphere by
combustion and deforestation yearly. To determine the fate of anthropogenic carbon in the future
we must understand how carbon is stored globally. Will the oceans and atmosphere continue
remove half of what we burn? There is no assurance that this might hold true for all time.
Carbon Reservoirs
In the pre-industrial atmosphere the abundance of carbon was about 615 G tons (about 280 ppm
of CO2). Today it is about 776 G tons. As mentioned above, photosynthesis and respiration in the
biosphere play an important role in the carbon cycle. Annually, the biosphere and soils are
responsible for a flux of carbon out of and into the atmosphere of about 62 G tons. We assume
that the system is in steady-state (i.e. the fluxes out of and into each reservoir are equal). If, for
example, the flux into the biosphere were greater than the flux out of it, over the last few
thousand years, all the carbon in the atmosphere would have been accumulating in the biosphere.
Obviously this is not the case.
In the diagram on the next page we present a simplified picture of the various global reservoirs
of carbon. The budgets in the reservoirs are estimated for the pre-industrial atmosphere.
Photosynthesis and respiration by organisms in the surface of the oceans are also responsible for
a flux of about 60 G tons of carbon per year out of and into the atmosphere. The carbon absorbed
by the photosynthetic organisms is passed up the food chain in the oceans, and as organisms die,
the organic matter falls to the deep oceans. Along the way it may be consumed by other
organisms, or decay, releasing CO2. Eventually some of this organic matter may make it to the
ocean floor and deposited in the sediments. This flux of organic carbon and calcium carbonate
(skeletons of animals) out of the ocean reservoir, about 0.23 G ton/yr, is very small relative to the
fluxes into and out of other reservoirs. But as the diagram shows, the sediments are a significant
reservoir of carbon because the residence time in sediments is millions of years.
Sediments may either be transported to the surface through crustal up-lift or they may be
subducted down into the mantle. Once in the mantle the carbon can be returned to the surface as
CO2 either through volcanos or hot springs. On average, a carbon atom resides in the sediments
for 391 million years. In comparison, the residence time of a carbon atom in the ocean is only
about 614 years. Therefore, if plate tectonics were to cease, in a geologically short period of
time, all the carbon on earth would become locked up in the sediments. This includes the carbon
in our bodies - what would be the prospect for life on Earth?
The residence time for a given reservoir is the average time that an atom of carbon
resides in that reservoir. Mathematically, it is given by the total amount of carbon in the reservoir
divided by the flux out of the reservoir (or the flux into it, since we are assuming steady- state the
flux in must balance the flux out). We know that the system is no longer in balance since each
year human activity adds an extra 7.6 G tons of carbon to the atmosphere, extracted from the
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deep sediments. The question which we need to answer is how will the carbon from this new
source becomes partitioned among the reservoirs that we care about, especially the atmosphere.
If in time most of goes into the biosphere or the ocean then we have no reason to be concerned
about build-up of CO2 and possible associated global warming.
Currently we think about half of the CO2 released to the atmosphere by humans remains
in the atmosphere. But how sensitive are the biospheric and oceanic sinks to climatic
perturbations? What effect would global warming have on the ability of the oceans to continue
their current rate of carbon uptake? Would increased atmospheric CO2 cause the biosphere to
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increase or decrease its uptake of carbon? These are the questions we need to answer to address
the issue of global warming.
•
•
•
•
•
•
•
Major points of this section:
The atmospheric concentration of CO2 has risen over the past century from 280 ppm to
370 ppm. The current rate of increase is ~1.8 ppm per year, corresponding to a CO2
accumulation rate in the atmosphere of 4.0 G tons C/yr.
Note: 1 ppm CO2 = 2.1 Gtons Carbon
The rise in CO2 is due to fossil fuel combustion (presently about 6.0 G tons C/yr) and to
deforestation in the tropics (presently 1.6 G tons C/yr)). Thus the total rate of CO2
emission to the atmosphere is 7.6 G tons C/yr.
Comparison of the total rate of CO2 emission to the atmosphere (7.6 Gtons C/yr) to the
actual accumulation rate of CO2 in the atmosphere (4.0 Gtons C/yr) implies that 3.6
Gtons C/yr (or almost half of the CO2 emitted) is being removed from the atmosphere.
The two processes removing CO2 from the atmosphere are uptake by the biosphere
(photosynthesis) and dissolution in the oceans. We call these processes "sinks" for CO2.
Both are of comparable importance.
We presented a figure showing the global cycling of carbon between its geochemical
reservoirs for the natural ("pre-industrial") atmosphere. The carbon amounts in each
reservoir, and the fluxes of carbon between reservoirs, are estimated with variable
reliability. The total amount of CO2 in the atmosphere was 615 Gtons in 1800 (280 ppm,
the pre-industrial value). The rate of transfer of CO2 to and from the oceans is 60
Gtons/yr, to and from the terrestrial biosphere, 62 Gtons/yr.
The RESIDENCE TIME of carbon in any of its reservoirs is defined as the average time
that an atom of carbon resides in that reservoir. It is often estiamted from knowledge of
the components of the reservoir, e.g. the lifetime of a tree. Mathematically, the residence
time is calculated as the total amount of carbon in the reservoir divided by the flux out of
the reservoir. From the figure above you can see that the atmospheric residence time of
CO2 is 615/(60+62) = 5 years. CO2 is thus rapidly transferred to terrestrial vegetation and
to the oceans.and back again!
It is imprtant to distinguish between the size (Gtons of carbon) of a reservoir and the
flux (Gtons of carbon/yr) through the reservoir.
2. The role of the oceans in the budget of carbon dioxide
The largest fraction of all the carbon on Earth is stored in the oceans, apart from deep sediments
and rocks that don't enter into the carbon cycle on time scales of interest to us. Uptake of CO2
occurs at the surface of the ocean where there is contact between the atmosphere and the ocean.
CO2 taken up in surface waters can then be transported to other oceanic reservoirs, such the
intermediate ocean and the deep ocean. These large reservoirs, which are not in contact with the
atmosphere, account for the long residence time of carbon in the ocean.
The uptake of CO2 occurs through both biological and chemical processes. Organisms at the
surface of the ocean take up CO2 during photosynthesis. The organic carbon stored in these
organisms is then transferred up the food chain. CO2 can also be taken up by the ocean through
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the dissolution of atmospheric CO2 into the surface waters. The capacity for this chemical
process to take up CO2 is determined by the pH of the ocean, and by the transport rates of water.
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625 G tons
Atmosphere
55
6
56
17
796 Gton
warm surface water
30
1
1
5
46 Gton
cold surface water
22 <1
8
40
9744 Gtons
Intermediate Ocean
3
162
205
26280 Gtons
Deep Ocean
0.23
90,000,000 Gtons
Sediments
Carbon Cycle of the Ocean (after McElroy, 2000)
Reservoir sizes are in Gtons C, transfer fluxes in Gtons C/ yr. ----- Biological transport by sinking fecal
pellets in the ocean. This hypothetical schematic of the carbon cycle describes CO2 before the recent
increases in atmospheric concentrations (280 ppm in the atmosphere) and assumes that all reservoirs
are in steady state and undisturbed by man or by climate change.
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Ocean Circulation
The figure above shows a schematic picture of the ocean, with the CO2 amounts in each
reservoir. We have distinguished the warm and the cold surface ocean because only in cold
regions does water become sufficiently dense in winter to sink to the deep. Most of the deep
water in the ocean arrived there by cooling and sinking from the surface in polar regions. Thus
the ocean circulation looks something the atmospheric Hadley circulation in reverse: negative
buoyancy is created at the poles, cold dense water sinks, circulates to the tropics, and is
eventually forced up to the surface and warmed. Cold water can hold more CO2 than warm
water, and thus this circulation removes CO2 from the atmosphere and circulates it through the
deep ocean, a process that takes about 600 years. This circulation may be traced out on the
diagram.
Acidic and basic solutions
Water constantly dissociates into H+ and OH- ions. The ions in turn recombine rapidly to form
H2O again:
H+ + OH- <=> H2O.
Eq (I)
Reactions in both directions proceed rapidly, leading to a balance in aqueous (water) solutions
where
[H+] [OH-] = 10-14 (moles/liter)2
DEFINITION:An acidic solution has [H+] > [OH-]. (NOTE:[ ] denotes concentration.) A basic
solution has [H+] < [OH-]. From Eq. (I), [H+] > 10-7 moles/liter in an acid, and [H+] < 10-7
moles/liter is a base, and vice versa for [OH-].
DEFINITION: pH = - log10[H+] is convenient to describe [H+], which varies by many orders of
magnitude in solutions that we encounter every day. Examples: orange juice, vinegar, and Coca
Cola are very acidic (pH = 2 - 3), soap and seawater are basic (pH 8 - 9), and cleaning solution
for contact lenses is carefully kept neutral (pH = 7).
Chemical and biological transformations of CO2 in the oceans
I. Chemical transformations
The first step in dissolving CO2 in the oceans is the simple uptake of gaseous CO2:
CO2 (g) à CO2 (aq).
Eq (II)
Carbon dioxide (aqueous) reacts with liquid water in a reversible reaction (i.e., goes in both
directions):
Eq (III)
CO2 (aq) + H2O (liquid) ß à H2CO3 (aq)
(aqueous carbon dioxide + liquid water ßà carbonic acid)
Carbonic acid is a weak acid, which means that it that can liberate a hydrogen ion in a basic
solution. This process produces a bicarbonate ion HCO3-, and it also goes both ways:
H2CO3 (aq) ß à HCO3- (aq) + H+ (aq).
Eq (IV)
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In the present discussion we are considering the steps that take place when CO2 is added to the
atmosphere, which drives the reaction CO2 (aq) + H2O (liquid)àà HCO3- (aq) + H+ (aq). The
bicarbonate HCO3- can also can liberate a hydrogen ion if the solution is sufficiently basic,
producing a carbonate ion CO3-2:
HCO3- (aq) ß à CO3-2 (aq) + H+ (aq).
Eq (V)
Thus bicarbonate can act as either an acid, liberating H+, or a base (take up H+). The carbonate
ion is strictly a base, taking up H+ (reverse direction of Eq. V). The activity of CO3-2 as a base
may also be look at as a reaction of CO3-2 with water to release OH-, since H+ and OH- must
remain in balance (Eq. I),
CO3-2 + H2O à HCO3- + OH-.
Eq (V')
.
Seawater is basic, i.e. it has an excess of OH- ions over H+ ions, by a factor greater than 104.
Almost all dissolved CO2 is in the form of HCO3- with about 10% as CO3-2. Almost all of the H+
ions liberated in these reactions will be removed by the vast excess of OH- (the pH changes only
slightly) that are generated by Eq. V'. So the reactions of H2CO3 and HCO3- that release H+ are
followed immediately by
H+ + OH- à H2O
We can summarize this series of reactions as
CO2 + OH- à HCO3Adding CO2 to seawater neutralizes an OH- ion, and the process that re-supplies OH- is the
reaction of water with carbonate ion,
CO3-2 + H2O à HCO3- + OHThe net change in concentrations of "inorganic carbon" (CO2, HCO3-, CO3-2) due to adding CO2
to seawater is therefore
CO2 + CO32- à 2 HCO3- .
Dissolution of CO2 in seawater: how much CO2 is removed by the oceans today?
If we take equal volumes of air and seawater at an average temperature, say 18 C, add 15
molecules of CO2 to the air, and wait for the process of dissolution to go to completion,
approximately 14 will end up in the seawater and only one will remain in the air.
This is one of the main processes that removes anthropogenic CO2 from the atmosphere. It is
reversible: the amount of CO2 removed is limited by the quantity of seawater that comes in
contact with the air, and changes in the ocean temperature, which makes CO2 less soluble in
seawater, can cause CO2 to be emitted from the ocean.
The effective depth of the atmosphere is the scale height, H (7 km), and of the oceans, 4 km. If
we wait a very long time, all of the ocean water eventually comes into contact with the
atmosphere (about 500 years for one "stirring"), so after a very long time (1000-1500 years) only
7- 10% of fossil fuel CO2 will remain in the atmosphere. This is the ultimate limit on the amount
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of CO2 that can be removed from the atmosphere by dissolution in the oceans on time scales of
hundreds of years. Slower processes (104 - 106 years) involving sediments can eventually take
up more CO2, but these can be of little practical interest.
Current estimates suggest that 2 Gtons C/yr of fossil fuel CO2 dissolves in the oceans. Why is
this so much smaller than the 90% that can be taken up by reaction of CO2 with CO3-2?
Unfortunately, only a very small portion of the ocean is in intimate contact with the atmosphere
in a year (the "mixed layer")--about 150 m (3%), so only about 0.15*15/7 = .30 (30%) of added
CO2 is removed annually from the atmosphere by this process. Therefore, of the 5-7 Gtons of
carbon humans add to the atmosphere each year, about 30% dissolves in the ocean.
There is another factor that reduces oceanic uptake of CO2. The capacity of the ocean to take up
CO2 will diminish slowly with time, as the pH of the ocean declines due to uptake of CO2. The
ocean becomes (slightly) acidified and the dissolution reactions slow down. This effect is
especially important in the surface waters that are exposed to the highest CO2 concentrations.
Uptake of CO2 by chemical dissolution is limited by the rate for exchange between deep
ocean water and surface water, and by acidification of seawater as CO2 is added.
II. Biological processes
The "biological pump"
Biological processes can help to transfer carbon from the surface waters of the ocean to the deep
ocean, helping to remove fossil fuel CO2 from the atmosphere. CO2 is removed by growing
plants via photosynthesis:
CO2 + H2O + sunlight + nutrients à "CH2O"
Most of the growing plants in the ocean are small organisms, plankton that float near the surface
so that they can remain in waters where sunlight is available for photosynthesis. When animals
(small ones like Daphnia, or large ones like whales) eat and excrete these organisms, they are
packaged into fecal pellets large enough to sink into the deep. This provides a "rain of organic
matter" that extracts organic carbon from the near-surface environment, where exchange with the
atmosphere is rapid, to the deep ocean, where carbon is stored for centuries. Eventually almost
all of it is oxidized to CO2, but it can't return to the atmosphere until the deep water exchanges
with surface water, a process that takes hundreds of years.
A small fraction is not oxidized, but is incorporated into organic-rich sediments. A *very* small
portion of that may be converted into the oil deposits used as fossil fuel.
Nutrient limitation on the "biological pump"
The rate at which the biological pump can operate is limited by the supply of nutrients (N, P, Fe,
essentially fertilizer) in the surface waters. These nutrients have to come from the deep waters,
and when you bring them up, you bring up the CO2 that descended in past centuries! Deep waters
always contain more CO2 than would be there in equilibrium with the atmosphere, and can serve
to release additional CO2 to the atmosphere.
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In this figure, we represent organic matter as [C:N:P] to emphasize that marine organic material
has (approximately) a mean composition with molar ratios C:N:P=106:16:1. Organic matter that
descends by gravitational settling (sedimentation) is mostly oxidized in the deep water, releasing
CO2, NO3- (nitrate) and PO43- (phosphate) in the same molar ratios. Growth of marine plants,
followed by grazing and sedimentation, effectively strips the upper ocean of nutrients.
Nutrients are supplied by upwelling of deep water to the surface, and also from land as dust or
run-off in rivers. When deep water is brought to the surface, CO2 comes along with the nutrients.
This process keeps high concentrations of CO2 in the deep, but there is little effect on removing
anthropogenic CO2 from the atmosphere. However when nutrients are added from the land, the
effect can be a net removal of CO2 from the atmosphere.
Recently attention has focussed on iron as a limiting nutrient for the "biological pump" of
CO2. Iron is a very common element, and it is essential for plants to make chlorophyll, but iron
is very insoluble in seawater, and thus it is in short supply in most of the ocean. Experiments in
which iron has been added to the sea have shown increased growth of phytoplankton, and a shift
in the type of organisms present to favor larger plankton. Some scientists have argued that we
should consider adding iron to seawater to stimulate growth of phytoplankton and remove CO2
from the atmosphere. Others think this is a "risky scheme" and voice concerns about the impact
of shifts in the species of plankton, or the possibility that carbon removed in this way may return
to the atmosphere in a brief time by upwelling from deeper waters.
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Main points of Ocean Processes
The oceans have a great capacity to take up CO2, due to both chemical and biological
processes:
• Chemical processes involve reaction of atmospheric CO2 (a weak acid) with basic
ions (OH-) in seawater. The ocean becomes slightly acidified, and the over-all
reaction may be represented by
CO2 + CO3-2 à 2 HCO3-.
• The rate of removal of fossil fuel CO2 is limited by the rate for exchange of cold
deep ocean water with surface water.
• Biological processes involving sinking of organic matter from the surface. The
rate for removal of fossil fuel CO2 is limited by the rate of supply of nutrients to
surface waters. Some people think that anthropogenic nutrients (N, P in sewage,
N and S in combustion exhaust gases, Fe in dust) could be stimulating uptake of
CO2 by the oceans today.
The oceans currently take up about 1/3 of the CO2 entering the atmosphere each year due
to human use of fossil fuels and to clearance of forests for agriculture.
The capacity of the oceans to take up CO2 each year by the chemical process may be
expected to decline over time as the ocean becomes more acidic (pH declines).
The ultimate capacity of the ocean to absorb anthropogenic CO2 is very great, leaving <
15% in the atmosphere, but the time scale for this process is many hundreds years.
Oceanic uptake is reversible! Just as the oceans "buffer" atmospheric levels of CO2 by
storing > 90% of what is added, they will tend to keep the higher levels in place so that
the perturbation persists for a very long time.
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3. The carbon cycle and the fate of anthropogenic carbon dioxide.
SOURCES
SINKS
Dissolution in
the ocean
atmosphere
Fossil fuel
Forests
(deforestation
Forests
(reforestation
React organic (reduced)
carbon with O2
Inorganic C, oxidation state = 0
Organic C, oxidation state = -4
The fate of anthropogenic carbon dioxide
Anthropogenic carbon dioxide is emitted to the
atmosphere from burning of fossil fuel and
from clearing of forests, with minor sources
from production of cement and from use of
wood as fuel. The amount of carbon emitted
per unit of usable energy is not the same for
different fuels--highest for coal (by far the most
abundant fuel), lowest for natural gas (the least
abundant, according to present ideas). The
more hydrogen in a fuel, the lower the
oxidation state of carbon, the more energy is
derived from oxidizing -H to water, as
summarized in the following table.
oxidation state
0
-1 to -2.5
-2.5 to -3
-4
stoichiomtery
(CH2O)n
(CH2±)
C2H6 ; C4H10
CH4
fuel
wood, peat
coal, oil
LPG
natural gas
The budget of anthropogenic carbon presents a mystery
The sources of carbon dioxide from human
CO2 budget for 1980-1990 (IPCC, 1995)
activity in the last decade were:
sources
Average 1980-1990
• Burning of fossil fuel 5.3 Gtons of C
per year, averaged over the last decade
fossil fuel
5.3 Gt C/yr
(from U.N. estimates, +/- 5% (?))
tropical deforestation
1-2
• Tropical deforestation 1 - 2 Gtons of C
total input
6.3 – 7.3
per year (from U.N. estimates, +/- 50100% (?))
sinks and
Average 1980-1990
It
takes
2.12 Gtons of carbon to raise the
accumulation
concentration in the atmosphere by 1 ppm, so
the
amount released is sufficient to raise the
atmospheric increase
3.2.
atmospheric concentration by 3.5 - 4 ppm / yr.
ocean uptake
2.1
The
observed rise is 1 - 2 ppm/yr, averaging
"Missing Sink"
1–2
about
1.5 ppm/yr over the last decade.
Note: 2.12 Gtons C = 1 ppm atmospheric CO2
Where we think the carbon is going:
• Stored in the atmosphere: 3.2 Gtons/yr (atmospheric data, +/-15%(?))
• Stored in the oceans: 2.1 Gtons/yr (from ocean partial pressure of CO2, from 14 CO2, and
from from 13 CO2; +/-20%(?))
The "missing sink" is 1 – 2 Gtons/yr. This CO2 is "missing" from our budget analysis. If the
missing carbon is going into the deep ocean, that's good news: it is gone for a very long time (see
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next lecture). If it's going into forests, the uptake will stop as forests mature, or may even reverse
if more forests are degraded or converted to agriculture. We need to identify and understand the
"missing sink" in order to be able to predict the future concentrations of CO2: will there be 400
ppm or 700 50 years from now?
Analysis of atmospheric data to infer the nature of the "missing sink"
A classic analysis of the carbon cycle takes observed variations of the CO2 concentration in the
atmosphere and tries to work backwards to infer the sources. For example, Tans, Conway and
Takahashi (1989) analyzed the observed concentrations of CO2 and examined the latitude
distribution of sources and CO2 concentrations. The CO2emitted to the atmosphere enters mostly
in the northern hemisphere and in the tropics, very little in the southern hemisphere. On average,
this is expected to give rise to a mean excess of CO2 in the northern hemisphere. Some of the
CO2 is removed by dissolution in the oceans and by growth of plants; this removal is seasonally-
varying, peaking in late spring and early fall, and it is partially reversed (net input from
vegetation) in the other seasons. The net effect is a rather complex pattern of CO2 concentrations
over time and latitude, superimposed on a long-term increase (recall the figure earlier in the
chapter with the different CO2 monitoring stations and their characteristic 'sawtooth' patterns).
Since most fossil fuel is burned in the northern hemisphere, most models predict a large excess
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of CO2 in the north.
The figures show that north/south gradient for CO2 varies greatly over the course of the year,
and changes from year to year also. On average the northern hemisphere has 2-3 ppm more CO2
than the southern hemisphere, and this is about half of what most computer models predict if
there is no additional sink in the northern hemisphere. The conclusion from this study was that
the "missing sink" lies in the northern hemisphere, probably in the re-growing forests of North
America and Scandinavia
This is an elegant analysis, but possibly incorrect for several reasons:
• We don't know how much CO2 the ocean might transfer from one hemisphere to the other
(recall our graph of CO2 in the ocean, showing the gas moving from cold to warm
waters—it could also move from north to south. . Some scientists studying ocean
circulation believe that the ocean waters cool in the North Atlantic, circulate through the
cold interior of the ocean, and warm up in the southern hemisphere, releasing CO2. These
authors claim that, in the absence of fossil fuel inputs, there would be less CO2 in the
northern hemisphere than the southern hemisphere. Anthropogenic CO2 just fills in that
gradient, in their view. Thus they don't accept the interpretation of the atmospheric data.
• A relatively small change in the gradient for CO2 inferred from measurements could have
a large effect on the conclusions. Due to the large seasonal variation, and the fact that
measurements of atmospheric CO2 are all made at the surface, there is some additional
doubt about the interpretation of the model results. For example, the stations near the
surface may be more strongly affected by seasonal uptake of CO2 but the models may not
correctly simulate this.
What can we do to find out the identity of the "missing sink"?
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•
Main points of this section
The turnover time for atmospheric CO2, due to inputs and outputs of CO2 from
vegetation, is short, less than 10 years. However, net fluxes to/from vegetation must be
small; inputs and outputs must balance almost exactly, since uptake occurs mostly to
short-lived materials (leaves) that decay within a year.
Burning of fossil fuels leads to release of CO2 from large, very old stores of organic
material. This leads to long-term increases in concentrations of CO2 in the atmosphere
and oceans; some may be converted to organic material and stored in forests and in the
deep ocean.
Scientists are presently not able to account fully for the fate of CO2 in the atmosphere.
There is evidence to support the idea that a significant fraction of CO2 released from
burning of fossil fuels is presently being taken up and stored by terrestrial vegetation.
Modest net growth (or depletion) of organic carbon reservoirs, in soils or forests, might
occur over the long term, affecting atmospheric levels of CO2 by up to 100 ppm +/-.
It is important to understand the role of terrestrial vegetation: is there is net growth, and
if so, why? There is concern that, if uptake by vegetation is large, it may stop, or even
reverse, over periods of decades (perhaps uptake might be sustained a little longer if
forests are deliberately grown), leading to an acceleration of CO2 increases in the
atmosphere.
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4. Emission rates of CO2 from fossil fuels and other sources
What is the history of CO2 emissions? How do these emission rates compare to natural fluxes
of CO2?
In the following section we examine data pncarbon sources and sinks in nature, and try to
understand the issues raised by attempts to restrain the growth rates of atmospheric CO2.
Historical rates of CO2 emissions
Global emissions of CO2 are increasing rapidly; the rate of increase for atmospheric CO2 is
actually slowing. It is important to understand this paradox, and to determine if rapid
acceleration of CO2 growth rates in the atmosphere is likely in the future.
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US emissions, total and per capita
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German emissions, total and per capita
Japanese emissions, total and per capita
Chinese emissions, total and per capita
Former USSR Emissions, total and per
capita
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The contributions of various countries have changed radically in the past 50 years
After World War II, the US emrged as the dominant economy in the world, accounting for nearly
45% of global fossil fuel use. The fraction is about 20% today and declining rapidly as China,
Japan and other far-eastern countries grow rapidly. Brazil, India and other large countries are
also increasing their shares.
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