Order Code RL34059
The Carbon Cycle: Implications for
Climate Change and Congress
Updated March 13, 2008
Peter Folger
Specialist in Energy and Natural Resources Policy
Resources, Science, and Industry Division

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The Carbon Cycle:
Implications for Climate Change and Congress
Summary
Huge quantities of carbon are actively exchanged between the atmosphere and
other storage pools, including the oceans, vegetation, and soils on the land surface.
The exchange, or flux, of carbon among the atmosphere, oceans, and land surface is
called the global carbon cycle. Comparatively, human activities contribute a
relatively small amount of carbon, primarily as carbon dioxide (CO ), to the global
2
carbon cycle. Despite the addition of a relatively small amount of carbon to the
atmosphere, compared to natural fluxes from the oceans and land surface, the human
perturbation to the carbon cycle is increasingly recognized as a main factor driving
climate change over the past 50 years.
If humans add only a small amount of CO to the atmosphere each year, why is
2
that contribution important to global climate change? The answer is that the oceans,
vegetation, and soils do not take up carbon released from human activities quickly
enough to prevent CO concentrations in the atmosphere from increasing. Humans
2
tap the huge pool of fossil carbon for energy, and affect the global carbon cycle by
transferring fossil carbon — which took millions of years to accumulate underground
— into the atmosphere over a relatively short time span. As a result, the atmosphere
contains approximately 35% more CO today than prior to the beginning of the
2
industrial revolution (380 ppm vs 280 ppm). As the CO concentration grows it
2
increases the degree to which the atmosphere traps incoming radiation from the sun
(radiative forcing), warming the planet.
The increase in atmospheric CO concentration is mitigated to some extent by
2
two huge reservoirs for carbon — the global oceans and the land surface — which
currently take up more carbon than they release. They are net sinks for carbon. If the
oceans, vegetation, and soils did not act as sinks, then the concentration of CO in the
2
atmosphere would increase even more rapidly. A key issue to consider is whether
these two sinks will continue to store carbon at the same rate over the next few
decades, or whether their behavior will change. Currently, most of the total global
carbon sink is referred to as the unmanaged, or background, carbon cycle. Very little
carbon is removed from the atmosphere and stored, or sequestered, by deliberate
action.
Congress is considering legislative strategies to reduce U.S. emissions of CO2
and/or increase the uptake of CO from the atmosphere. Congress may also opt to
2
consider how land management practices, such as afforestation, conservation tillage,
and other techniques, might increase the net flux of carbon from the atmosphere to
the land surface. How the ocean sink could be managed to store more carbon is
unclear. Iron fertilization and deep ocean injection of CO are in an experimental
2
stage, and their promise for long-term enhancement of carbon uptake by the oceans
is not well understood. Congress may consider incorporating what is known about
the carbon cycle into its legislative strategies, and may also evaluate whether the
global carbon cycle is sufficiently well understood so that the consequences of long-
term policies aimed at mitigating global climate change are fully appreciated.

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Contents
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Carbon Storage, Sources, and Sinks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Carbon Flux, or Exchange, with the Atmosphere . . . . . . . . . . . . . . . . . . . . . 5
Land Surface-Atmosphere Flux . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Ocean-Atmosphere Flux . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Policy Implications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
List of Figures
Figure 1. (a) Storage or Pools (GtC); and (b) Annual Flux or Exchange
of Carbon (GtC per year) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
List of Tables
Table 1. Carbon Stocks in the Atmosphere, Ocean, and Land Surface,
and Annual Carbon Fluxes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

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The Carbon Cycle: Implications for
Climate Change and Congress
Introduction
Congress is considering several legislative strategies that would reduce U.S.
emissions of greenhouse gases — primarily carbon dioxide (CO ) — and/or increase
2
uptake and storage of CO from the atmosphere. Both approaches are viewed by
2
many observers as critical to forestalling global climate change caused, in part, by the
buildup of greenhouse gases in the atmosphere from human activities. Others point
out that the human contribution of carbon to the atmosphere is a small fraction of the
total quantity of carbon that cycles naturally back and forth each year between the
atmosphere and two huge carbon reservoirs: the global oceans and the planet’s land
surface. A question raised is whether the human fraction of the global carbon cycle
— the exchange, or flux, of carbon between the atmosphere, oceans, and land surface
— is large enough to induce climate change and global warming.
Despite the addition of a relatively small amount of carbon to the atmosphere,
compared to natural fluxes from the oceans and land surface, the human perturbation
to the carbon cycle is increasingly recognized as a main factor driving climate change
over the past 50 years. For most of human history, the global carbon cycle has been
roughly in balance, and the concentration of CO in the atmosphere has been fairly
2
constant at approximately 280 parts per million (ppm). Human activities, namely the
burning of fossil fuels, deforestation, and other land use activities, have significantly
altered the carbon cycle. As a result, atmospheric concentrations of CO have risen
2
by over 35% since the industrial revolution, and are now greater than 380 parts per
million (ppm).1
An understanding of the global carbon cycle has shifted from being of mainly
academic interest to being also of policy interest. Policy makers are grappling with,
for example, how to design a cap-and-trade system that accurately accounts for
carbon sequestration by components of the land surface sink, such as forests. Yet
how much CO forests are capable of taking up in the future is largely a scientific
2
question. More broadly, a cap-and-trade system that limits emissions, and is
designed to keep atmospheric CO below a specific concentration, would depend
2
inherently on continued uptake of carbon by the oceans and land surface. How those
two carbon reservoirs will behave in the future — how much CO they will take up
2
or release and at what rate — are also topics of active scientific inquiry.2
1 World Data Centre for Greenhouse Gases (WDCGG), WMO Greenhouse Gas Bulletin:
The State of Greenhouse Gases in the Atmosphere Using Global Observations through 2005
(Geneva, Switzerland: 2006); at [http://gaw.kishou.go.jp/wdcgg.html].
2 In addition to its climate warming effect, the buildup of CO in the atmosphere is also
2
changing the chemistry of the ocean’s surface waters, a phenomenon known as ocean
acidification, which could harm aquatic life.

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Thus the scientific understanding of the carbon cycle is integral to many aspects
of the current congressional debate over how to mitigate climate change. This report
puts the human contribution of carbon to the atmosphere into the larger context of
the global carbon cycle. The report focuses almost entirely on CO , which alone is
2
responsible for over half of the change in Earth’s radiation balance.3 Moreover,
according to the Intergovernmental Panel on Climate Change (IPCC), CO is the
2
most important greenhouse gas released to the atmosphere from human activities.4
Methane, black carbon, and organic carbon pollution are also part of the carbon cycle
and have roles in human-induced climate change (e.g., methane accounts for about
an additional 20% of the change in the Earth’s radiation balance).
Carbon Storage, Sources, and Sinks
The atmosphere, oceans, vegetation, and soils on the land surface all store
carbon. (See Figure 1a.) Geological reservoirs also store carbon in the form of
fossil fuels; for example, oil, gas, and coal.5 Of these reservoirs (or pools), dissolved
inorganic carbon in the ocean is the largest, followed in size by fossil carbon in
geological reservoirs, and by the total amount of carbon contained in soils. (See
Figure 1a and Table 1.) The atmosphere itself contains nearly 800 billion metric
tons of carbon (800 GtC),6 which is more carbon than all of the Earth’s living
vegetation contains.7 Carbon contained in the oceans, vegetation, and soils on the
land surface is linked to the atmosphere through natural processes such as
photosynthesis and respiration. In contrast, carbon in fossil fuels is linked to the
atmosphere through the extraction and combustion of fossil fuels. The atmosphere
has a fairly uniform concentration of CO , although it shows minor variations by
2
season (about 1%) — due to photosynthesis and respiration — and by latitude.8
Carbon dioxide released from fossil fuel combustion mixes readily into the
atmospheric carbon pool, where it undergoes exchanges with the ocean and land
surface carbon pools. Thus, where fossil fuels are burned makes relatively little
3 See The First State of the Carbon Cycle Report (SOCCR): The North American Carbon
Budget and Implications for the Global Carbon Cycle, U.S. Science Program Synthesis and
Assessment Product 2.2, ed. Anthony W. King, Lisa Dilling, Gregory P. Zimmerman, David
M. Fairman, Richard A. Houghton, Gregg Marland, Adam Z. Rose, and Thomas J. Wilbanks
(November 2007), p. 2, at [http://cdiac.ornl.gov/SOCCR/final.html], hereafter referred to
as SOCCR. Also see the Intergovernmental Panel on Climate Change, “Working Group I
Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate
Change,” Climate Change 2007: the Physical Science Basis (2007), at [http://ipcc-wg1.
ucar.edu/wg1/wg1-report.html], hereafter referred to as 2007 IPCC Working Group I Report.
4 2007 IPCC Working Group I Report (Summary for Policymakers).
5 Carbon in the Earth’s crust is mainly in the form of carbonates, and is linked to the
atmosphere by natural processes, such as erosion and weathering, and by metamorphism
over geologic time scales. In contrast, the key source of fossil carbon for the purposes of
this report are fossil fuels, which are now linked to the atmosphere almost entirely via
human activities.
6 One metric ton of carbon is equivalent to 3.67 metric tons of CO . A metric ton (or tonne)
2
is 2,204.6 pounds. One billion metric tons of carbon is one gigatonne, or GtC.
7 William H. Schlesinger, Biogeochemistry: an Analysis of Global Change, 2nd Ed. (San
Diego, CA: Academic Press, 1997), p. 360. Hereafter referred to as Schlesinger, 1997.
8 Schlesinger, 1997, p. 56. Larger fluctuations by season occur in the northern hemisphere.

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difference to the concentration of CO in the atmosphere; emissions in any one region
2
affect the concentration of CO everywhere else in the atmosphere.9
2
Figure 1. (a) Storage or Pools (GtC); and (b) Annual Flux
or Exchange of Carbon (GtC per year)
Note: Figure prepared by CRS.
Sources: SOCCR; 2007 IPCC Working Group I Report, Table 7.1; and Christopher L. Sabine et al.,
“Current Status and Past Trends of the Global Carbon Cycle,” in C. B. Field and M. R. Raupach, eds.,
The Global Carbon Cycle: Integrating Humans, Climate, and the Natural World (Washington, DC:
Island Press, 2004), pp. 17-44.
9 Concentrations of CO are slightly higher in the northern hemisphere compared to the
2
southern hemisphere, by several parts per million, because most of the emissions of CO2
from human activities are in the north.

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Table 1. Carbon Stocks in the Atmosphere, Ocean,
and Land Surface, and Annual Carbon Fluxes
Annual
Annual
flux
flux
(GtC/yr)
(GtC/yr)
Net to the
Storage
From the
To the
atmosphere
pool
GtC
atmosphere
atmosphere
(GtC/yr)
Atmosphere
780
Ocean
38,140
92.2
90.5
-1.7b
Land
3,850
59.3
58.2
-1.1c
Surfacea
(soils plus
vegetation)
Fossil
>6,000
—
7.2
+7.2
Carbon
(coal, gas
oil, other)
Sources: SOCCR; 2007 IPCC Working Group I Report, Table 7.1; Christopher L. Sabine et al.,
“Current Status and Past Trends of the Global Carbon Cycle,” in C. B. Field and M. R. Raupach, eds.,
The Global Carbon Cycle: Integrating Humans, Climate, and the Natural World (Washington, DC:
Island Press, 2004), pp. 17-44.
a. The soil pool contains about 3,200 GtC, and the vegetation pool contains about 650 GtC.
b. Gross fluxes between the ocean and atmosphere have considerable uncertainty, but the net flux is
known to within +/-0.3 GtC per year (SOCCR, p. 2-3).
c. The net flux between the land surface and the atmosphere is known to within +/-0.7 GtC per year
(Jonathan A. Foley and Navin Ramankutty, “A Primer on the Terrestrial Carbon Cycle: What We
Don’t Know But Should,” in C. B. Field and M. R. Raupach, eds., The Global Carbon Cycle:
Integrating Humans, Climate, and the Natural World (Washington, DC: Island Press, 2004), p. 281.
The oceans, vegetation, and soils truly exchange carbon with the atmosphere
constantly on daily and seasonal time cycles (Figure 1b). In contrast, carbon from
fossil fuels is not exchanged with the atmosphere, but is transferred in a one-way
direction from geologic storage, at least within the time scale of human history.10
Some of the CO currently in the atmosphere may become fossil fuel someday, after
2
it is captured by vegetation, buried under heat and pressure, and converted into coal,
for example, but the process takes millions of years. How much of the fossil fuel
carbon ends up in the atmosphere, instead of the oceans, vegetation, and soils, and
over what time scale, is driving much of the debate over what type of action to take
to ameliorate global warming.
How much carbon is stored in each pool — especially the atmospheric pool —
is important in global warming because as more CO is added to the atmosphere, its
2
10 An exception to this is the concept of carbon capture and sequestration, whereby the
geologic time scale cycle of carbon storage is “short circuited” by capturing CO at its
2
source — a fossil-fueled electricity generating plant for example — and injecting it
underground into geologic reservoirs.

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heat-trapping capacity becomes greater.11 Each storage pool — oceans, soils, and
vegetation — is considered a sink for carbon because each pool takes up carbon from
the atmosphere. Conversely, each storage pool is also a source of carbon for the
atmosphere, because of the constant exchange or flux between the atmosphere and
the storage pools. For example, vegetation in the northern hemisphere is a sink for
atmospheric carbon during the spring and summer months due to the process of
photosynthesis. In the fall and winter it is a source for atmospheric carbon because
the process of respiration returns carbon to the atmosphere from the vegetation pool.
The pool of fossil carbon is only a source, not a sink, except over geologic time
scales, as described above. How much carbon is transferred between the atmosphere
and the sources and sinks is a topic of scientific scrutiny because the mechanisms are
still not understood completely. Whether a storage pool will be a net sink or a net
source for carbon in the future depends very much on the balance of mechanisms that
drives its behavior, and how those mechanisms may change.12
Carbon Flux, or Exchange, with the Atmosphere
Carbon actively exchanges (fluxes) between the atmosphere and the other
storage pools, or stocks, of carbon. Over 90 GtC is exchanged each year between the
atmosphere and the oceans, and close to 60 GtC is exchanged between the
atmosphere and the land surface annually. (See Table 1.)13 Human activities —
primarily land-use change and fossil fuel combustion — contribute slightly less than
9 GtC to the atmosphere each year.14 If the human contribution of CO is removed
2
from the global carbon cycle, then the average net flux — the amount of CO released
2
to the atmosphere versus the amount taken up by the oceans, soils, and vegetation —
is close to zero. Most scientists conclude that for 10,000 years prior to 1750, the net
flux was less than 0.1 GtC per year when averaged over decades.15 That small value
for net flux is reflected by the relatively stable concentration of CO in the
2
atmosphere — between 260 and 280 ppm — for the 10,000 years prior to 1750.16
11 See CRS Report RL33849, Climate Change: Science and Policy Implications, by Jane A.
Leggett, for an explanation of the heat-trapping properties, or radiative forcing, of CO and
2
other greenhouse gases.
12 Jorge L. Sarmiento and Nicolas Gruber, “Sinks for Anthropogenic Carbon,” Physics
Today (August 2002): pp. 30-36.
13 These massive exchanges of CO between the atmosphere, oceans, and land surface result
2
mostly from natural processes, such as photosynthesis, respiration, decay, and gas exchange
between the ocean surface and the lower atmosphere.
14 About 80% of human-related CO emissions results from fossil fuel combustion, and 20%
2
from land use change (primarily deforestation). Fossil fuel burning and industrial activities
release approximately 7.2 GtC per year, land use change releases about 1.6 GtC per year
(2007 IPCC Working Group I Report, pp. 501, 514-515).
15 2007 IPCC Working Group I Report, p. 514.
16 Ice core data indicate that CO concentrations ranged between 180 and 300 ppm over the
2
past 650,000 years, and between 275 and 285 ppm from AD 1000 to AD 1750 (2007 IPCC
Working Group I Report, p. 137 and p. 435). See also E.T. Sundquist and K. Visser, “The
(continued...)

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Currently the atmospheric concentration of CO is approximately 100 ppm
2
higher than it was before 1750 because human activities are adding carbon to the
atmosphere faster than the oceans, land vegetation, and soils remove it. The
relatively rapid addition of CO to the atmosphere has tipped the balance so that the
2
oceans and the land surface take up more CO per year on average than they release,
2
yet atmospheric concentrations of CO continue to rise. (See Table 1.) Why is that
2
occurring?
The short answer is timing; CO from fossil fuel combustion and land use
2
changes is being released to the atmosphere faster than the oceans, vegetation, and soil
can take it up, so CO is accumulating in the atmosphere. About 45% of the CO
2
2
released from fossil fuel combustion and land use activities during the 1990s has
remained in the atmosphere, while the remainder has been taken up by the oceans,
vegetation, or soils on the land surface.17 Carbon dioxide is nonreactive18 in the
atmosphere and has a relatively long residence time, although eventually most of it
will return to the ocean and land sinks. About 50% of a single pulse of CO will be
2
removed within 30 years, a further 30% removed in within a few centuries, and the
remaining 20% may persist in the atmosphere for thousands of years.19 If CO2
emissions continue or grow, however, atmospheric concentrations of CO will likely
2
also continue to increase, with serious implications for future climate change.
As the CO concentration grows it increases radiative forcing (the degree to
2
which the atmosphere traps incoming radiation from the sun), warming the planet.
At present, the oceans and land surface are acting as sinks for CO emitted from
2
fossil fuel combustion and deforestation, but as they accumulate more carbon the
capacity of the sinks — and the rate at which they accumulate carbon — may change.
It is also likely that climate change itself (e.g., higher temperatures, a more intense
hydrologic cycle) may alter the balance between sources and sinks, due to changes
in the complicated feedback mechanisms between the atmosphere, oceans, and land
surface.20 How carbon sinks will behave in the future is a prominent question for
both scientists and policy makers.
Land Surface-Atmosphere Flux. Most estimates of the carbon cycle
indicate that the land surface (vegetation plus soils) accumulates more carbon per
year than it emits to the atmosphere.21 (See Figure 1b and Table 1.) The land
surface thus acts as a net sink for CO at present. Some policy makers advocate
2
16 (...continued)
Geologic History of the Carbon Cycle,” in Heinrich D. Holland and Karl K. Turekian (eds.),
Treatise on Geochemistry (Amsterdam, Netherlands: Elsevier Ltd., 2004), p. 443.
17 2007 IPCC Working Group I Report, pp. 514-515.
18 That is, it does not react with other chemicals in the atmosphere. This contrasts with other
greenhouse gases, such as methane (CH ), which reacts with the hydroxl ion (OH-) to
4
produce water and a methyl group (CH ); and nitrous oxide (N O), which is decomposed to
3
2
nitric oxide (NO) in the atmosphere by its reaction with ultraviolet light.
19 2007 IPCC Working Group I Report, p. 515.
20 See CRS Report RL33849, Climate Change: Science and Policy Implications, by Jane A.
Leggett, for more information on climate feedback mechanisms.
21 2007 IPCC Working Group I Report, p. 515.

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strategies for increasing the amount of CO taken up and stored, or sequestered, by
2
soils and plants, typically through land use changes, such as agricultural or forestry
practices.22 How effective those strategies are likely to be depends, in part, on how
the carbon cycle behaves in the future, particularly the land-atmosphere flux. How
the land-atmosphere flux may change, and how land use practices will change in the
future is not clear.
The land use change component has the largest uncertainty of any component
in the overall carbon cycle.23 Most scientists agree, however, that in the past two
decades tropical deforestation has been responsible for the largest share of CO2
released to the atmosphere from land use changes.24 Tropical deforestation and other
land use changes released approximately 1.6 GtC per year to the atmosphere in the
1990s, and may be contributing similar amounts of carbon to the atmosphere today.25
Even though deforestation releases more carbon than is captured by forest regrowth
within some regions, net forest regrowth in other regions takes up sufficient carbon
so the land surface acts as a global net sink of approximately 1 GtC per year. By
some estimates, even tropical lands, despite widespread deforestation, may be
carbon-neutral or even net carbon sinks; tropical systems take up substantial carbon
to offset what is lost through deforestation and fire.26
What used to be known as “the missing sink” component in the overall global
carbon cycle is now understood to be that part of the terrestrial ecosystem responsible
for the net uptake of carbon from the atmosphere to the land surface (especially high-
latitude, or boreal, forests).27 Scientists now prefer the term “residual land sink” to
“missing sink” as it portrays the residual — or left over — part of the global carbon
cycle calculation once the other components are accounted for (fossil fuel emissions,
land-use emissions, atmospheric increase, and ocean uptake).28 Precisely which
mechanisms are responsible for the residual land sink is a topic of scientific inquiry.
One mechanism postulated for many years has been the fertilizing effect of increased
atmospheric CO concentrations on plant growth. Most models predict enhanced
2
growth and carbon sequestration by plants in response to rising CO levels; however,
2
22 For more information on sequestration in the agricultural and forestry sectors, see CRS
Report RL31432, Carbon Sequestration in Forests, by Ross W. Gorte, and CRS Report
RL33898, Climate Change: The Role of the U.S. Agriculture Sector, by Renee Johnson.
23 2007 IPCC Working Group I Report, p.518.
24 2007 IPCC Working Group I Report, p. 517.
25 2007 IPCC Working Group I Report, Table 7.1.
26 2007 IPCC Working Group I Report, p. 522. However, SOCCR (p. 5) notes that rates of
forest clearing in the tropics, including Mexico, exceed rates of recovery and concludes that
tropical regions dominated by rainforests or other forest types are a net source of carbon to
the atmosphere.
27 However, a recent study indicates that the northern latitude forests take up less carbon
than previously estimated, and tropical forests take up more. See Britton B. Stephens, et al.,
“Weak northern and strong tropical land carbon uptake from vertical profiles of atmospheric
CO ,” Science, Vol. 316 (June 22, 2007): pp. 1732-1735.
2
28 SOCCR, p. 25.

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results of experiments have been mixed. Many experiments show enhanced growth
from increased CO concentrations — at least initially — but nutrient and water
2
availability and other limitations to growth are common. Long-term observations of
biomass change and growth rates suggest that fertilization effects are too small to
account for the residual land sink, at least in the United States.29
In North America, particularly the United States, the land-atmosphere flux is
strongly tilted towards the land, with approximately 0.5 GtC per year accumulating
in terrestrial sinks.30 That amount constitutes a large fraction — possibly 40% — of
the global terrestrial carbon sink.31 According to some estimates, approximately half
of the North American terrestrial carbon sink stems from regrowth of forests on
abandoned U.S. farmland.32 Woody encroachment — the increase in woody biomass
occurring mainly on former grazing lands — is thought to be another potentially large
terrestrial sink, possibly accounting for more than 20% of the net North American
sink (although the actual number is highly uncertain).33 Wood products (e.g.,
furniture, house frames, etc.), wetlands, and other smaller, poorly understood carbon
sinks are responsible for accumulating the remaining carbon in North America.
Most of the North American terrestrial carbon sink, such as the forest regrowth
component, is sometimes referred to as the unmanaged, or background, carbon cycle.
Very little carbon is sequestered by deliberate action.34 The future behavior of the
unmanaged terrestrial carbon sink is another consideration for lawmakers. Whether
the United States will continue its trajectory as a major terrestrial carbon sink is
highly uncertain, and little evidence suggests that the terrestrial ecosystem sinks will
increase in the future; some current sinks may even become sources for carbon.35
Policy makers may also need to evaluate how management practices, such as
afforestation, conservation tillage, and other techniques, would increase the net flux
of carbon from the atmosphere to the land surface.36 How forests, rangelands, and
29 Sarmiento and Gruber, p. 31.
30 SOCCR, p. 29. This includes fluxes to and from land vegetation and soils, and excludes
emissions from fossil fuel combustion, cement manufacturing, and other industrial
processes.
31 SOCCR, p. 32. However, SOCCR reports that the magnitude of the global terrestrial
carbon sink is highly uncertain.
32 SOCCR, p. VII.
33 SOCCR, Table 3.1; 2007 IPCC Working Group I Report, p. 527.
34 SOCCR, p. 27.
35 SOCCR, p. 27. Sinks may convert to sources, for example, if melting permafrost under
warming conditions releases large amounts of methane currently trapped in frozen tundra;
or increased wildfires from increased drought releases large amounts of CO . See
2
Christopher B. Field, et al., “Feedbacks of terrestrial ecosystems to climate change,” Annual
Review of Environment and Resources, vol. 32 (July 5, 2007): pp. 7.1-7.29.
36 For more information on agricultural and forestry practices and carbon management, see
CRS Report RL34042, Environmental Services Markets: Farm Bill Proposals, by Renee
(continued...)

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croplands are managed in the future for carbon sequestration may become an
important factor in the overall land-atmosphere flux.
Ocean-Atmosphere Flux. Similar to the land surface, the oceans today
accumulate more carbon than they emit to the atmosphere each year, acting as a net
sink for about 1.7 GtC per year. (See Figure 1b and Table 1.) If the land surface
and oceans were not acting as net sinks, the CO concentration in the atmosphere
2
would be increasing at a faster rate than observed. More than the land surface, the
oceans have a huge capacity to store carbon. Ultimately, the oceans could store more
than 90% of all the carbon released to the atmosphere by human activities, but the
process takes thousands of years.37 Policy makers may be more concerned about CO2
accumulating in the oceans now, its impact on ocean chemistry and marine life (e.g.,
ocean acidification), and its behavior as a net sink over the next few decades.
Carbon dioxide enters the oceans by dissolving into seawater at the ocean
surface, at a rate controlled by the difference in CO concentration between the
2
atmosphere and the sea surface.38 Because the surface waters39 of the ocean have a
relatively small volume — and thus a limited capacity to store CO — how much
2
CO is stored in the oceans over the time scale of decades depends on ocean mixing
2
and the transport of CO from the surface to intermediate and deep waters. Mixing
2
between surface waters and deeper portions of the ocean is a sluggish process; for
example, the oldest ocean water in the world — found in the North Pacific — has not
flowed to the ocean surface for about 1,000 years.40 Thus the slow rate of ocean
mixing, and slow transport of CO from the surface to the ocean depths, is of possible
2
concern to policymakers because it influences the effectiveness of the ocean sink for
CO , and because CO added to the surface waters of the ocean increases its acidity.
2
2
In addition to the vertical mixing of the ocean, large-scale circulation of the
oceans around the globe is a critical component for determining the effectiveness of
36 (...continued)
Johnson; CRS Report RL33898, Climate Change: the Role of the U.S. Agricultural Sector,
by Renee Johnson; and CRS Report RL31432, Carbon Sequestration in Forests, by Ross
W. Gorte.
37 CO forms carbonic acid when dissolved in water. Over time, the solid calcium carbonate
2
(CaCO ) on the seafloor will react with, or neutralize, much of the carbonic acid that entered
3
the oceans as CO from the atmosphere. See David Archer, et al., “Dynamics of fossil fuel
2
CO neutralization by marine CaCO ,” Global Biogeochemical Cycles, vol. 12, no. 2 (June
2
3
1998): pp. 259-276.
38 SOCCR, p. 26. In addition to the relative difference in CO concentration between
2
atmosphere and ocean, the rate of CO dissolution also depends on factors such as wave
2
action, wind, and turbulence.
39 The surface waters or surface layer of the ocean is commonly characterized as the top
layer of the ocean that is well mixed by waves, tides, and weather events, and is separated
from the deep ocean by a difference in density. The depth of the surface layer varies, but
probably averages 100-200 meters deep. See [http://www.windows.ucar.edu/tour/link=/
earth/Water/ocean_currents.html].
40 Sarmiento and Gruber, p. 31.

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the ocean sink.41 Surface waters carrying anthropogenic CO descend into the ocean
2
depths primarily in the North Atlantic and the Southern Oceans, part of the so-called
oceanic “conveyor belt.”42 Some model simulations suggest that the Southern Ocean
around Antarctica accounts for nearly half of the net air-sea flux of anthropogenic
carbon.43 From that region, a large portion of dissolved CO is transported north
2
towards the subtropics. Despite its importance as a CO sink, the Southern Ocean is
2
poorly understood, and at least one study suggests that its capacity for absorbing
carbon may be weakening.44
As CO is added to the surface of the ocean from the atmosphere, it increases
2
the acidity of the sea surface waters, with possible impacts to the biological
production of organisms, such as corals. Corals, and calcifying phytoplankton and
zooplankton, are susceptible to increased acidity as their ability to make shells in the
water column is inhibited or possibly reversed, leading to dissolution.45 Some reports
indicate that sea surface pH has dropped by 0.1 pH units since the beginning of the
industrial revolution.46 One report suggests that pH levels could drop by 0.5 pH units
by 2100, and suggests further that the magnitude of ocean acidification can be
predicted with a high level of confidence.47 The same report states, however, that
research on the impacts of high concentrations of CO on marine organisms is in its
2
infancy.
The oceans appear to be a larger net sink for carbon than the land surface at
present. As with the land surface, however, a consideration for policy makers is the
future behavior of the ocean sink, particularly the Southern Ocean, given its
importance to the net ocean-atmosphere CO flux. In contrast to the terrestrial carbon
2
sink, where management practices such as afforestation and conservation tillage may
increase the amount of carbon uptake, it is unclear how the ocean carbon sink can be
managed in a similar fashion. Some proposed techniques for increasing ocean
sequestration of carbon, such as iron fertilization48 and deep ocean injection of CO ,
2
41 SOCCR, p. 26.
42 Sarmiento and Gruber, p. 31.
43 Sarmiento and Gruber, p. 31.
44 Corinne Le Quere et al., “Saturation of the Southern Ocean CO sink due to recent climate
2
change,” Science, vol. 316 (June 22, 2007): pp. 1735-1737.
45 2007 IPCC Working Group I Report, p. 529.
46 2007 IPCC Working Group I Report, p. 529. pH is a measure of the concentration of
hydrogen ions in solution. A lower pH means an increase in acidity, or a higher
concentration of hydrogen ions. A change of one pH unit is a factor of 10 different than the
next higher or lower unit. For example, a pH of 4.0 is 10 times the acidity than a pH of 5.0.
47 Ken Caldeira, et al., “Ocean acidification due to increasing atmospheric carbon dioxide,”
The Royal Society, Policy Document 12/05 (June 2005), 60 pages; at [http://www.royalsoc.
ac.uk/].
48 The deliberate introduction of iron into the surface ocean to stimulate marine
phytoplankton growth, which would increase carbon sequestration from the atmosphere via
photosynthesis. The Southern Ocean, in particular, is deficient in iron as a nutrient such that
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are in an experimental phase and have unknown long-term environmental
consequences.49
Policy Implications
Huge amounts of carbon are exchanged among the atmosphere, the land surface,
and the oceans each year. Although humans are responsible for only a small fraction
of the total exchange, that small amount affects the global system by adding a
significant net flux of CO to the atmosphere. Before the industrial revolution — and
2
the large-scale combustion of fossil fuels, land-clearing and deforestation activities
— the average net flux of CO to the atmosphere hovered around zero for nearly
2
10,000 years. Because of the human contribution to the net flux, the amount of CO2
in the atmosphere is now nearly 100 ppm (35%) higher today than it has been for the
past 650,000 years.50
Congress is exploring legislative strategies that would alter the human
component of the global carbon cycle. Strategies that limit emissions from fossil fuel
combustion would reduce the current one-way transfer of fossil carbon to the
atmosphere; what took millions of years to accumulate geologically is being released
in only a few hundred years. Capturing CO before it is released to the atmosphere
2
and injecting it back into geological reservoirs — carbon capture and sequestration
— is one possible strategy to “short circuit” the geologic process and return the
carbon underground over a much shorter time scale. CO injection into the
2
subsurface has been used for decades to enhance recovery of oil. However, large-
scale geologic sequestration of CO for storage is currently in a pilot testing stage.
2
Less than half of the total amount of CO released from burning fossil fuels over
2
the past 250 years remains in the atmosphere, because two huge sinks for carbon —
the global oceans and the land surface — take up more carbon than they release at
present. Congress is exploring if and how management practices, such as
afforestation, conservation tillage, and other techniques, might increase the net flux
of carbon from the atmosphere to land surface. How the ocean sink could be
managed to store more carbon is unclear. Iron fertilization and deep ocean injection
of CO are in an experimental stage, and their promise for long-term enhancement
2
of carbon uptake by the oceans is not well understood.
Also of possible concern to Congress is how the ocean and land surface sinks
will behave over the coming decades and longer, and whether they will continue to
take up more carbon than they release. For example, carbon emissions may be
48 (...continued)
the introduction of iron could stimulate phytoplankton growth. Several experiments have
been conducted or are underway to further explore this process, for example, Stephane
Blain, et al., “Effect of natural iron fertilization on carbon sequestration in the Southern
Ocean,” Nature, vol. 446, no. 7139 (April 26, 2007): pp. 1070-1074.
49 For more information about injection of CO into the deep oceans, see CRS Report
2
RL33801, Carbon Capture and Sequestration (CCS), by Peter Folger.
50 Urs Siegenthaler et al., “Stable carbon cycle — climate relationship during the Late
Pleistocene,” Science, vol. 310 (Nov. 25, 2005): pp. 1313-1317.

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capped so as to keep atmospheric CO concentrations below a prescribed level at
2
some future date, but changes in the magnitude, or even the direction, of the ocean
or land-surface sinks may affect whether those target concentrations can be achieved.
Congress may wish to incorporate what is known about the carbon cycle into its
legislative strategies. Congress may also wish to evaluate whether the global carbon
cycle is sufficiently well understood that the consequences of long-term policies
aimed at mitigating global climate change are fully appreciated.

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