The Carbon Cycle: Implications for Climate Change and Congress
Prepared for Members and Committees of Congress
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 (CO2), to the global 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 CO2 to the atmosphere each year, why is 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 CO2 concentrations in
the atmosphere from increasing. Humans 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 CO2 today than prior to the beginning of the industrial
revolution (380 ppm vs 280 ppm). As the CO2 concentration grows it increases the degree to
which the atmosphere traps incoming radiation from the sun (radiative forcing), warming the
The increase in atmospheric CO2 concentration is mitigated to some extent by 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 CO2 in the 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 CO2 from the atmosphere. Congress may also opt to 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 CO2 are in an
experimental 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.
Introduc tion ..................................................................................................................................... 1
Carbon Storage, Sources, and Sinks................................................................................................2
Carbon Flux, or Exchange, with the Atmosphere............................................................................5
Land Surface-Atmosphere Flux................................................................................................6
Figure 1. (a) Storage or Pools (GtC); and (b) Annual Flux or Exchange of Carbon (GtC
Table 1. Carbon Stocks in the Atmosphere, Ocean, and Land Surface, and Annual
Author Contact Information..........................................................................................................10
Congress is considering several legislative strategies that would reduce U.S. emissions of
greenhouse gases—primarily carbon dioxide (CO2)—and/or increase uptake and storage of CO2
from the atmosphere. Both approaches are viewed by 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
CO2 in the atmosphere has been fairly 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 CO2 have 1
risen by over 35% since the industrial revolution, and are now greater than 380 ppm.
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 CO2 forests are capable of taking up in the future is
largely a scientific question. More broadly, a cap-and-trade system that limits emissions, and is
designed to keep atmospheric CO2 below a specific concentration, would depend inherently on
continued uptake of carbon by the oceans and land surface. How those two carbon reservoirs will
behave in the future—how much CO2 they will take up or release and at what rate—are also 2
topics of active scientific inquiry.
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 CO2, which alone is responsible for over half of the change in 3
Earth’s radiation balance. Moreover, according to the Intergovernmental Panel on Climate
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
2 In addition to its climate warming effect, the buildup of CO2 in the atmosphere is also changing the chemistry of the
ocean’s surface waters, a phenomenon known as ocean acidification, which could harm aquatic life.
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
Change (IPCC), CO2 is the most important greenhouse gas released to the atmosphere from 4
human activities. 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).
The atmosphere, oceans, vegetation, and soils on the land surface all store carbon. (See Error!
Reference source not found.a.) Geological reservoirs also store carbon in the form of fossil fuels; 5
for example, oil, gas, and coal. 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 Error! Reference source not found.a and Table 1.) The 6
atmosphere itself contains nearly 800 billion metric tons of carbon (800 GtC), which is more 7
carbon than all of the Earth’s living vegetation contains. 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 CO2, although it shows minor variations by season (about 1%)—due to 8
photosynthesis and respiration—and by latitude. 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 difference to the concentration of CO2 in the atmosphere; emissions in any one region affect 9
the concentration of CO2 everywhere else in the atmosphere.
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 10
time scale of human history. Some of the CO2 currently in the atmosphere may become fossil
fuel someday, after 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.
42007 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
6 One metric ton of carbon is equivalent to 3.67 metric tons of CO2. A metric ton (or tonne) 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.
9 Concentrations of CO2 are slightly higher in the northern hemisphere compared to the southern hemisphere, by
several parts per million, because most of the emissions of CO2 from human activities are in the north.
10An 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 CO2 at its source—a fossil-fueled electricity generating plant for
example—and injecting it underground into geologic reservoirs.
Figure 1. (a) Storage or Pools (GtC); and
(b) Annual Flux or Exchange of Carbon (GtC per year)
Source: 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.
Notes: Figure prepared by CRS.
Table 1. Carbon Stocks in the Atmosphere, Ocean,
and Land Surface, and Annual Carbon Fluxes
Annual flux (GtC/yr) Annual flux (GtC/yr) to Net to the
Storage pool GtC from the atmosphere the atmosphere atmosphere (GtC/yr)
Ocean 38,140 92.2 90.5 -1.7b
Land Surfacea 3,850 59.3 58.2 -1.1c
Fossil Carbon >6,000 — 7.2 +7.2
(coal, gas oil,
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.
How much carbon is stored in each pool—especially the atmospheric pool—is important in
global warming because as more CO2 is added to the atmosphere, its heat-trapping capacity 11
becomes greater. 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 12
11See 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 CO2 and other greenhouse gases.
12Jorge L. Sarmiento and Nicolas Gruber, “Sinks for Anthropogenic Carbon,” Physics Today (August 2002): pp. 30-36.
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 13
Table 1.) Human activities—primarily land-use change and fossil fuel combustion—contribute 14
slightly less than 9 GtC to the atmosphere each year. If the human contribution of CO2 is
removed from the global carbon cycle, then the average net flux—the amount of CO2 released 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 15
per year when averaged over decades. That small value for net flux is reflected by the relatively
stable concentration of CO2 in the atmosphere—between 260 and 280 ppm—for the 10,000 years 16
prior to 1750.
Currently the atmospheric concentration of CO2 is approximately 100 ppm 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 CO2 to the atmosphere has
tipped the balance so that the oceans and the land surface take up more CO2 per year on average
than they release, yet atmospheric concentrations of CO2 continue to rise. (See Table 1.) Why is
The short answer is timing; CO2 from fossil fuel combustion and land use changes is being
released to the atmosphere faster than the oceans, vegetation, and soil can take it up, so CO2 is
accumulating in the atmosphere. About 45% of the CO2 released from fossil fuel combustion and
land use activities during the 1990s has remained in the atmosphere, while the remainder has been 1718
taken up by the oceans, vegetation, or soils on the land surface. Carbon dioxide is nonreactive
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 CO2 will be removed within 30
years, a further 30% removed in within a few centuries, and the remaining 20% may persist in the 19
atmosphere for thousands of years. If CO2 emissions continue or grow, however, atmospheric
concentrations of CO2 will likely also continue to increase, with serious implications for future
13These massive exchanges of CO2 between the atmosphere, oceans, and land surface result mostly from natural
processes, such as photosynthesis, respiration, decay, and gas exchange between the ocean surface and the lower
14About 80% of human-related CO2 emissions results from fossil fuel combustion, and 20% 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).
152007 IPCC Working Group I Report, p. 514.
16Ice core data indicate that CO2 concentrations ranged between 180 and 300 ppm over the 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 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.
172007 IPCC Working Group I Report, pp. 514-515.
18That is, it does not react with other chemicals in the atmosphere. This contrasts with other greenhouse gases, such as
methane (CH4), which reacts with the hydroxl ion (OH-) to produce water and a methyl group (CH3); and nitrous oxide
(N2O), which is decomposed to nitric oxide (NO) in the atmosphere by its reaction with ultraviolet light.
192007 IPCC Working Group I Report, p. 515.
As the CO2 concentration grows it increases radiative forcing (the degree to which the
atmosphere traps incoming radiation from the sun), warming the planet. At present, the oceans
and land surface are acting as sinks for CO2 emitted from 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, 20
and land surface. How carbon sinks will behave in the future is a prominent question for both
scientists and policy makers.
Most estimates of the carbon cycle indicate that the land surface (vegetation plus soils) 21
accumulates more carbon per year than it emits to the atmosphere. (See Figure 1b and Table 1.)
The land surface thus acts as a net sink for CO2 at present. Some policy makers advocate
strategies for increasing the amount of CO2 taken up and stored, or sequestered, by soils and 22
plants, typically through land use changes, such as agricultural or forestry practices. 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 23
carbon cycle. 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 24
changes. 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 25
atmosphere today. 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 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
20See CRS Report RL33849, Climate Change: Science and Policy Implications, by Jane A. Leggett, for more
information on climate feedback mechanisms.
212007 IPCC Working Group I Report, p. 515.
22For 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 Renée Johnson.
232007 IPCC Working Group I Report, p.518.
242007 IPCC Working Group I Report, p. 517.
252007 IPCC Working Group I Report, Table 7.1.
262007 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.
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 28
for (fossil fuel emissions, land-use emissions, atmospheric increase, and ocean uptake).
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 CO2 concentrations on plant growth. Most models predict enhanced growth and
carbon sequestration by plants in response to rising CO2 levels; however, results of experiments
have been mixed. Many experiments show enhanced growth from increased CO2
concentrations—at least initially—but nutrient and water 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 29
In North America, particularly the United States, the land-atmosphere flux is strongly tilted 30
towards the land, with approximately 0.5 GtC per year accumulating in terrestrial sinks. That 31
amount constitutes a large fraction—possibly 40%—of the global terrestrial carbon sink.
According to some estimates, approximately half of the North American terrestrial carbon sink 32
stems from regrowth of forests on abandoned U.S. farmland. 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 33
American sink (although the actual number is highly uncertain). 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 34
sequestered by deliberate action. 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 35
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 CO2,” Science, Vol. 316 (June 22, 2007): pp. 1732-1735.
28SOCCR, p. 25.
29Sarmiento and Gruber, p. 31.
30SOCCR, 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.
31SOCCR, p. 32. However, SOCCR reports that the magnitude of the global terrestrial carbon sink is highly uncertain.
32SOCCR, p. VII.
33SOCCR, Table 3.1; 2007 IPCC Working Group I Report, p. 527.
34SOCCR, p. 27.
35SOCCR, 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 CO2. See 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.
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 36
atmosphere to the land surface. How forests, rangelands, and croplands are managed in the
future for carbon sequestration may become an important factor in the overall land-atmosphere
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 CO2 concentration in the
atmosphere 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 37
years. 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 38
controlled by the difference in CO2 concentration between the atmosphere and the sea surface. 39
Because the surface waters of the ocean have a relatively small volume—and thus a limited
capacity to store CO2—how much CO2 is stored in the oceans over the time scale of decades
depends on ocean mixing and the transport of CO2 from the surface to intermediate and deep
waters. Mixing 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 40
ocean surface for about 1,000 years. Thus the slow rate of ocean mixing, and slow transport of
CO2 from the surface to the ocean depths, is of possible concern to policymakers because it
influences the effectiveness of the ocean sink for CO2, and because CO2 added to the surface
waters of the ocean increases its acidity.
In addition to the vertical mixing of the ocean, large-scale circulation of the oceans around the 41
globe is a critical component for determining the effectiveness of the ocean sink. Surface waters
carrying anthropogenic CO2 descend into the ocean depths primarily in the North Atlantic and the
36For more information on agricultural and forestry practices and carbon management, see CRS Report RL34042,
Environmental Services Markets in the 2008 Farm Bill, by Renée Johnson; CRS Report RL33898, Climate Change:
The Role of the U.S. Agriculture Sector, by Renée Johnson; and CRS Report RL31432, Carbon Sequestration in
Forests, by Ross W. Gorte.
37 CO2 forms carbonic acid when dissolved in water. Over time, the solid calcium carbonate (CaCO3) on the seafloor
will react with, or neutralize, much of the carbonic acid that entered the oceans as CO2 from the atmosphere. See David
Archer, et al., “Dynamics of fossil fuel CO2 neutralization by marine CaCO3,” Global Biogeochemical Cycles, vol. 12,
no. 2 (June 1998): pp. 259-276.
38SOCCR, p. 26. In addition to the relative difference in CO2 concentration between atmosphere and ocean, the rate of
CO2 dissolution also depends on factors such as wave action, wind, and turbulence.
39The 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=/
40Sarmiento and Gruber, p. 31.
41SOCCR, p. 26.
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 43
of anthropogenic carbon. From that region, a large portion of dissolved CO2 is transported north
towards the subtropics. Despite its importance as a CO2 sink, the Southern Ocean is poorly
understood, and at least one study suggests that its capacity for absorbing carbon may be 44
As CO2 is added to the surface of the ocean from the atmosphere, it increases 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 45
dissolution. Some reports indicate that sea surface pH has dropped by 0.1 pH units since the 46
beginning of the industrial revolution. 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 47
with a high level of confidence. The same report states, however, that research on the impacts of
high concentrations of CO2 on marine organisms is in its 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 CO2 flux. In
contrast to the terrestrial carbon 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 48
sequestration of carbon, such as iron fertilization and deep ocean injection of CO2, are in an 49
experimental phase and have unknown long-term environmental consequences.
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
42Sarmiento and Gruber, p. 31.
43Sarmiento and Gruber, p. 31.
44Corinne Le Quere et al., “Saturation of the Southern Ocean CO2 sink due to recent climate change,” Science, vol. 316
(June 22, 2007): pp. 1735-1737.
452007 IPCC Working Group I Report, p. 529.
462007 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.
47Ken 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/.
48The 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 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.
49For more information about injection of CO2 into the deep oceans, see CRS Report RL33801, Carbon Capture and
Sequestration (CCS), by Peter Folger.
small amount affects the global system by adding a significant net flux of CO2 to the atmosphere.
Before the industrial revolution—and the large-scale combustion of fossil fuels, land-clearing and
deforestation activities—the average net flux of CO2 to the atmosphere hovered around zero for
nearly 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 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 CO2 before it is released to
the atmosphere 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. CO2 injection into the subsurface has been
used for decades to enhance recovery of oil. However, large-scale geologic sequestration of CO2
for storage is currently in a pilot testing stage.
Less than half of the total amount of CO2 released from burning fossil fuels over 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 CO2 are in
an experimental stage, and their promise for long-term enhancement 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 capped so as to keep atmospheric CO2
concentrations below a prescribed level at 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.
Specialist in Energy and Natural Resources Policy
50Urs Siegenthaler et al., “Stable carbon cycle—climate relationship during the Late Pleistocene,” Science, vol. 310
(Nov. 25, 2005): pp. 1313-1317.