The Carbon Cycle: Implications for Climate Change and Congress

^Pr^ep^ar^{ed f}^o^r^M^e^m^b^ers^an^d^Com^mⁱ^t^t^ees^of^C^ong^r^e^ss

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
planet.
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
Ocean-Atmosphere Flux...........................................................................................................8
Policy Implications..........................................................................................................................9
Figure 1. (a) Storage or Pools (GtC); and (b) Annual Flux or Exchange of Carbon (GtC
per year)........................................................................................................................................3
Table 1. Carbon Stocks in the Atmosphere, Ocean, and Land Surface, and Annual
Carbon Fluxes..............................................................................................................................4
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

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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.

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Figure 1. (a) Storage or Pools (GtC); and
(b) Annual Flux or Exchange of Carbon (GtC per year)
^Source:^{SOCCR; 20}^{07 IPCC W}^{orking Grou}^{p I Report, Table}^7.1^{; and Christophe}^{r L. Sabine et}^al.,^“Curre^nt
^{Status and Past Tre}^{nds of the}^{Global Carbon Cycle,” in C. B. Fiel}^{d and M. R. Raupach, eds.,}^{The Global Carbon}
^{Cycle: Integrating H}^{umans, Climate, and the Natural World}^{(Washington, DC: Island Press, 200}^{4), pp. 1}^7-44.
^Notes:^F^{igure pre}^pared^{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 po}^ol^GtC^from^{the atmosphere}^{the atmosphere}^{atmosphere (GtC/yr)}
^{Atmosphere 780}
^{Ocean 38,1}⁴⁰^92.2^90.5^-1.7^b
^{Land Surfac}^e^a^3,85^{0 59.3}^58.2^-1.1^c
^{(soils plus}
^vegetation)
^{Fossil Carbon}^>6,00^{0 —}^7.2^+7.2
^{(coal, gas o}ⁱ^l^,
^other)
^Sources:^{SOCCR; 20}^{07 IPCC}^{Working Group}^{I Report, Table}^{7.1; Christop}^{her L. Sabine}^{et al., “Curre}^{nt Status}
^{and Past Trends of t}^{he Global Ca}^{rbon Cycle,” in C. B. Field and M.}^{R. Raupach, eds., The Global Carbon Cycle:}
^Integrating^Huma^{ns, Climate, and the Nat}^{ural World (Washington}^{, DC: Island Press, 200}^{4), pp. 1}^7-44.
^a.^{The soil pool contains about 3,20}^{0 GtC, and t}^{he vegetation}^{pool contains about}⁶⁵^{0 GtC.}
^b.^{Gross flu}^xes^{between t}^{he ocea}ⁿ^{and atmosphe}^{re ha}^{ve considerabl}^{e unce}^{rtainty, b}^{ut the}^net^{flux is}^known
^{to within +/-0.3 GtC}^{per y}^{ear (SOCCR, p. 2-3).}
^c.^{The net flu}^x^between^{the land s}^u^{rface and t}^{he atmos}^phere^{is kno}^{wn to within +/-0.7}^GtC^{per yea}^r
^{(Jonathan A. Foley and Navin}^{Ramankutty, “A Primer}^{on the}^Terr^{estrial Carbon Cycle: What We}^Don’t
^{Know But Should,” in C. B. Fie}^ld^{and M. R. Raupach, eds.,}^The^{Global Carbon C}^{ycle: Integrating H}^uman^s,
^{Climate, and the Natural World}^(W^{ashington, DC: Island Press, 20}⁰⁴^{), 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
change.

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¹²^J^o^r^g^e^L^.^Sa^r^m^ie^{nto a}^{nd N}ⁱ^c^o^la^s^G^r^ube^r^,^“^Sⁱⁿ^k^s^f^o^r^Aⁿ^thr^o^p^o^g^e^nic^C^a^r^bon,”^Phy^s^ic^s^To^day⁽^A^ug^us^{t 200}²⁾^{: pp}^.³⁰^-³^6.

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
that occurring?
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
climate change.

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^p^r^o^cesses,^su^ch^{as ph}^o^t^o^s^yⁿ^t^h^esi^s^,^respⁱ^r^atⁱ^oⁿ^,^d^eca^y^,^an^d^{gas exch}^an^{ge b}^e^t^w^e^eⁿ^t^h^{e o}^cean^su^rf^{ace a}ⁿ^d^t^h^{e l}^o^w^e^r
^a^t^m^o^s^phe^r^e^.
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¹⁵²⁰⁰^{7 I}^P^C^C^W^o^r^k^ing^G^r^{oup I}^R^e^p^o^r^t^,^p.⁵¹^4.
¹⁶^{Ice co}^{re d}^a^t^aⁱⁿ^di^cat^{e t}^h^at^CO²^c^o^nc^eⁿ^tr^a^tio^ns^r^a^ng^e^d^be^tw^e^eⁿ¹⁸⁰^a^{nd 3}⁰⁰^p^p^m^ov^e^r^the^pa^s^t⁶^50,⁰⁰^{0 y}^e^a^r^s^,^a^nd
^be^tw^e^eⁿ^{275 a}ⁿ^d²⁸^{5 p}^p^m^f^r^o^m^A^D^{1000 t}^o^A^D¹⁷⁵^{0 (}²⁰^{07 I}^P^C^C^W^o^r^k^ing^G^r^{oup I}^R^e^por^t,^p.¹³^{7 a}ⁿ^d^p.⁴³⁵⁾^.^Se^e
^a^l^s^o^E.T^.^Su^nd^quis^t^aⁿ^d^K^.^Vis^s^e^r^,^“^T^he^G^e^olog^ic^Hⁱ^s^t^or^y^of^the^C^a^r^b^oⁿ^C^y^c^l^e^,^”^{in H}^e^inrⁱ^c^h^D^.^H^o^lla^{nd a}ⁿ^{d K}^a^r^l^K^.
^T^u^re^kⁱ^aⁿ^(e^ds^.),^T^r^ea^tⁱ^s^{e on}^Geo^c^h^e^mi^st^ry^(A^m^s^te^rda^m^{, N}^e^the^r^la^nds^{: Els}^e^vⁱ^e^r^L^t^{d., 20}⁰⁴⁾^{, p}^.⁴⁴³^.
¹⁷²⁰⁰^{7 I}^P^C^C^W^o^r^k^ing^G^r^{oup I}^R^e^p^o^r^t^,^pp.⁵^14-⁵¹^5.
¹⁸^T^h^atⁱ^s^,ⁱ^t^do^{es no}^t^react^wⁱ^t^h^o^t^h^e^{r ch}^emⁱ^c^al^{s i}ⁿ^t^h^{e at}^m^o^sp^h^e^re.^T^hⁱ^s^coⁿ^t^rast^{s w}ⁱ^t^h^o^t^h^e^{r green}^h^o^u^s^{e gases,}^su^ch^as
^met^h^an^{e (CH}⁴^),^w^hⁱ^c^h^react^{s w}ⁱ^t^h^t^h^{e h}^y^d^r^o^x^lⁱ^oⁿ^(OH^-⁾^to^pr^o^duc^e^w^a^te^r^a^{nd a}^m^e^th^y^l^g^r^{oup (}^C^H³⁾^;^a^{nd n}^itr^o^u^s^ox^ide
^(N²^{O), w}^h^ic^{h is de}^c^o^m^pose^d^toⁿⁱ^tric^ox^ide⁽^N^O⁾ⁱⁿ^the^a^t^m^o^s^phe^r^e^by^{its re}^a^c^{tion w}^{ith ultra}^v^iole^t^lig^ht.
¹⁹²⁰⁰^{7 I}^P^C^C^W^o^r^k^ing^G^r^{oup I}^R^e^p^o^r^t^,^p.⁵¹^5.

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
fire.
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

²⁰^Se^e^C^R^S^R^e^por^t^R^L³³⁸^49,^C^lim^a^t^e^C^h^a^nge^:^Sc^ie^nc^e^an^{d P}^o^lic^y^I^m^plic^ati^ons^,^by^J^a^ne^A^.^L^e^gg^e^{tt, f}^o^r^m^o^r^e
ⁱⁿ^f^o^rm^atⁱ^oⁿ^oⁿ^clⁱ^m^at^{e f}^e^ed^b^ack^mech^anⁱ^s^m^s^.
²¹²⁰⁰^{7 I}^P^C^C^W^o^r^k^ing^G^r^{oup I}^R^e^p^o^r^t^,^p.⁵¹^5.
²²^For^m^o^r^e^inf^o^r^m^a^{tion on}^s^e^que^s^t^r^a^{tion i}ⁿ^t^h^e^a^g^rⁱ^c^u^ltur^a^{l a}^{nd f}^o^r^e^s^t^r^y^se^c^t^or^s^,^s^e^e^C^R^S^R^e^por^{t R}^L³¹⁴³²^,^Ca^r^b^oⁿ
^S^e^q^u^e^stra^tⁱ^on^{in Fo}^rests^,^by^R^o^s^s^W^.^G^o^r^t^e^,^a^{nd C}^R^S^R^e^p^o^r^t^R^L³³⁸⁹⁸^,^C^lim^ate^C^h^an^ge^:^The^Role^o^f^the^U^.^S.
^A^g^ri^cu^l^t^u^r^{e S}^ect^o^r^,^by^R^e^né^e^J^ohns^on^.
²³²⁰⁰^{7 I}^P^C^C^W^o^r^k^ing^G^r^{oup I}^R^e^p^o^r^t^,^p.5¹⁸^.
²⁴²⁰⁰^{7 I}^P^C^C^W^o^r^k^ing^G^r^{oup I}^R^e^p^o^r^t^,^p.⁵¹^7.
²⁵²⁰⁰^{7 I}^P^C^C^W^o^r^k^ing^G^r^{oup I}^R^e^p^o^r^t^{, T}^a^ble^7.^1.
²⁶²⁰⁰^{7 I}^P^{CC W}^o^rk^ing^G^r^{oup I Re}^p^o^rt,^p.⁵²^{2. H}^o^w^e^v^e^{r, SO}^{CCR (p.}^{5) n}^o^te^s^tha^t^ra^te^s^of^f^o^re^s^t^c^l^e^a^rⁱⁿ^g^{in t}^h^e^tro^p^ic^s^,
ⁱⁿ^cl^udⁱⁿ^{g M}^e^xi^co^,^exceed^rat^e^{s o}^f^reco^ver^y^an^d^coⁿ^c^l^u^d^e^{s t}^h^at^t^r^o^pⁱ^cal^regi^oⁿ^s^d^o^mⁱⁿ^at^ed^b^y^raiⁿ^f^o^rest^{s o}^r^o^t^h^e^{r f}^o^rest
^ty^pe^s^ar^e^a^ne^{t s}^our^c^e^of^c^a^r^{bon t}^o^the^a^t^m^o^s^phe^r^e^.

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
States.
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
carbon.

²⁷^Ho^w^e^v^e^{r, a recen}^{t stu}^d^yⁱⁿ^dⁱ^{cates th}^{at th}^eⁿ^o^r^th^ern^latitu^d^e^f^o^{rests tak}^e^u^p^{less carb}^oⁿ^th^aⁿ^p^r^ev^io^u^s^ly^e^s^ti^m^a^ted^,^an^d
^tropic^a^l^f^o^re^s^t^s^ta^k^e^{up m}^o^re^{. Se}^e^Britton^{B. S}^t^e^phe^ns^{, e}^t^a^l^.,^“^W^e^a^{k nort}^h^e^r^{n a}^{nd s}^t^r^ong^tr^opic^a^l^la^nd^c^a^r^bon^upta^k^e
^f^r^o^m^v^e^rtic^a^l^prof^ile^{s of}^a^t^m^o^sph^e^ric^CO²^,”^Sc^ie^nc^e^{, V}^o^l.³¹^{6 (}^J^uⁿ^e^22,²⁰⁰⁷⁾^{: p}^p^.¹⁷^32-¹⁷³⁵^.
²⁸^{SOCCR, p}^.²⁵^.
²⁹^Sa^r^m^ie^{nto a}^{nd G}^r^ube^r^,^p^.³¹^.
³⁰^{SOCCR, p}^.²⁹^{. T}^h^{is i}ⁿ^clu^d^e^{s f}^l^u^x^{es to}^an^d^f^r^o^m^lan^d^v^e^g^e^t^a^tioⁿ^an^d^so^ils,^an^d^ex^clu^d^e^{s em}^issioⁿ^s^f^r^o^m^f^o^{ssil f}^u^el
^c^o^m^bus^{tion, c}^e^m^eⁿ^{t m}^a^nuf^a^c^t^ur^ing^,^a^{nd o}^t^he^rⁱⁿ^d^u^s^t^rⁱ^a^l^pr^oc^e^s^s^e^s^.
³¹^{SOCCR, p}^.³²^{. Ho}^w^e^v^e^{r, SOCCR rep}^o^r^{ts t}^h^{at t}^h^{e m}^a^gⁿ^itu^d^e^o^f^th^{e g}^l^o^b^a^{l terrestrial carb}^oⁿ^sin^k^is^hⁱ^g^h^ly^uⁿ^certain^.
³²^{SOCCR, p}^.^V^I^I^.
³³^SO^{CCR, T}^a^ble^3.1;²⁰⁰⁷^I^P^CC^W^o^rk^ing^G^r^{oup I Re}^port^,^p^.⁵²⁷^.
³⁴^{SOCCR, p}^.²⁷^.
³⁵^SO^{CCR, p.}²⁷^{. Si}^nk^s^m^a^y^c^onv^e^r^{t to s}^ourc^e^s^{, f}^o^{r e}^x^a^m^ple^,^if^m^e^lti^ng^pe^rm^a^f^ros^t^unde^{r w}^a^r^m^ing^c^onditio^ns^re^le^a^s^e^s
^la^r^g^e^am^ounts^of^m^e^thaⁿ^e^c^u^r^r^e^ntly^tr^a^ppe^{d in f}^r^oz^eⁿ^t^uⁿ^d^r^a^{; or}^inc^r^e^a^s^e^d^w^ild^fⁱ^r^e^s^f^r^om^inc^r^e^a^s^e^{d dr}^oug^{ht r}^e^le^a^s^e^s
^la^r^g^e^am^ounts^of^C^O²^{. Se}^e^C^h^rⁱ^s^t^ophe^r^B^.^Fie^l^{d, e}^t^a^l^.^,^“^F^e^e^dba^c^k^s^of^te^r^r^e^s^t^rⁱ^a^l^e^c^o^sy^s^t^em^s^{to c}^lim^a^t^e^c^h^aⁿ^g^e^,”^Annu^al
^R^evi^{ew o}^f^Eⁿ^vi^ron^m^en^t^aⁿ^d^R^e^sou^r^ces^{, v}^o^{l. 3}²⁽^J^uly^5,²⁰⁰⁷⁾^:^p^p^.^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
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 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

³⁶^For^m^o^r^e^inf^o^r^m^a^{tion on}^a^g^rⁱ^c^u^ltur^a^{l a}ⁿ^d^f^o^r^e^s^t^r^y^pr^a^c^tic^e^s^a^{nd c}^a^r^bon^m^a^na^g^e^m^e^{nt, s}^e^e^C^R^S^R^e^por^{t R}^L³⁴⁰^42,
^Eⁿ^vi^roⁿ^m^ent^a^l^S^e^rvi^{ces Ma}^rket^{s i}ⁿ^t^h^e²⁰⁰⁸^F^a^{rm B}ⁱ^l^l^,^by^R^e^né^e^Johns^on;^C^R^S^R^e^p^o^r^t^R^L³³⁸⁹^8,^C^lim^ate^C^h^an^ge^:
^T^h^{e R}^o^l^e^o^f^t^h^{e U.}^S^.^A^g^ri^cu^l^t^u^r^e^S^ect^o^r^{, by}^R^e^né^e^J^ohns^{on; a}^{nd C}^R^{S R}^e^por^t^R^L³¹⁴³²^,^Car^b^oⁿ^Se^q^u^e^s^tratio^{n i}ⁿ
^Fo^rests^{, by}^Ros^s^W^.^G^o^rte^.
³⁷^CO²^f^o^r^m^s^c^a^r^bonic^a^cⁱ^{d w}^h^eⁿ^di^s^s^o^lv^e^d^{in w}^a^te^r^.^O^v^e^r^ti^m^e^{, the}^s^o^lid^c^a^l^cⁱ^um^c^a^r^bona^te⁽^C^a^C^O³⁾^{on the}^s^e^a^f^loor
^w^{ill react}^w^ith^{, o}^r^ne^utr^a^lⁱ^z^e^,^m^u^c^h^o^f^t^h^{e carbo}ⁿⁱ^{c aci}^{d t}^h^at^ent^e^{red t}^h^{e o}^cean^{s as CO}²^f^r^o^m^t^h^{e at}^m^o^sp^h^e^re.^S^{ee Davi}^d
^A^r^ch^er,^et^al^.^,^“D^yⁿ^a^mⁱ^c^{s o}^f^f^o^ssi^l^f^u^el^CO²^ne^utr^a^liz^a^tio^{n by}^m^a^rⁱⁿ^e^C^a^C^O³^,”^G^l^obal^Bio^g^e^o^c^h^e^m^ic^{al C}^y^c^l^e^s^{, v}^o^l.^12,
^{no. 2}⁽^J^une¹⁹⁹⁸⁾^{: pp}^.²^59-²⁷^6.
³⁸^{SOCCR, p}^.²⁶^{. I}ⁿ^ad^d^itioⁿ^t^o^t^h^e^relativ^{e d}ⁱ^ff^eren^c^eⁱⁿ^CO²^coⁿ^cent^r^atⁱ^oⁿ^b^e^t^w^een^at^m^o^sp^h^e^{re an}^d^ocean^,^t^h^{e rat}^e^o^f
^CO²^dis^s^o^lu^tio^{n a}^l^s^o^de^peⁿ^d^s^oⁿ^f^a^c^t^or^s^s^u^c^h^a^s^w^a^ve^a^c^{tion, w}ⁱ^nd,^a^{nd t}^u^r^b^uleⁿ^c^e^.
³⁹^T^h^{e su}^rf^a^ce^w^a^t^e^{rs o}^r^su^rf^a^{ce l}^a^{yer o}^f^t^h^{e o}^ceanⁱ^s^co^mm^oⁿ^l^y^ch^ar^act^eri^zed^{as t}^h^{e t}^o^p^l^a^{yer o}^f^t^h^{e ocean}^t^h^atⁱ^s^w^e^l^l
^mⁱ^xed^b^y^w^a^ves,^tⁱ^d^es,^an^d^w^eat^h^e^{r even}^t^s^,^an^dⁱ^s^sep^a^rat^e^d^f^r^o^m^t^h^{e d}^eep^o^cean^b^y^{a d}ⁱ^ff^eren^{ce i}ⁿ^d^eⁿ^sⁱ^t^y^.^T^h^{e d}^e^p^t^h
^o^f^t^h^{e su}^rf^{ace l}^a^{yer vari}^e^s^,^b^u^t^p^r^o^b^a^b^l^y^avera^g^es^100-²^{00 m}^e^te^r^s^de^e^p^{. Se}^e^h^ttp:/^/w^w^w^.^w^indow^s^.^uc^a^r^.e^du/^tour^/^link⁼^/
^eart^h^/^W^at^er/^o^cean^_^c^u^rren^t^s^.^h^t^m^l^.
⁴⁰^Sa^r^m^ie^{nto a}^{nd G}^r^ube^r^,^p^.³¹^.
⁴¹^{SOCCR, 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
weakening.
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

⁴²^Sa^r^m^ie^{nto a}^{nd G}^r^ube^r^,^p^.³¹^.
⁴³^Sa^r^m^ie^{nto a}^{nd G}^r^ube^r^,^p^.³¹^.
⁴⁴^Co^riⁿⁿ^{e L}^e^Qu^{ere et}^al^.^,^“S^at^u^r^atⁱ^oⁿ^o^f^t^h^{e S}^o^u^t^h^e^rn^Ocean^CO²^si^{nk d}^u^e^t^o^recen^t^clⁱ^m^at^{e ch}^an^ge,^”^S^cⁱ^eⁿ^ce,^vo^l^.³¹⁶
⁽^J^une²²^,²⁰⁰⁷⁾^:^p^p^.¹⁷^35-¹⁷^37.
⁴⁵²⁰⁰^{7 I}^P^C^C^W^o^r^k^ing^G^r^{oup I}^R^e^p^o^r^t^,^p.⁵²^9.
⁴⁶²⁰⁰^{7 I}^P^C^C^W^o^r^k^ing^G^r^{oup I}^R^e^p^o^r^t^,^p.⁵²^9.^pH^is^a^m^e^a^s^ur^e^of^{the c}^onc^eⁿ^tr^a^tⁱ^on^of^hy^dr^og^eⁿ^ionsⁱⁿ^s^o^l^u^ti^on.^A
^l^o^w^e^{r p}^H^mean^{s an}ⁱⁿ^{crease i}ⁿ^aci^dⁱ^t^y^,^o^r^{a h}ⁱ^gh^{er c}^oⁿ^cen^t^r^atⁱ^oⁿ^o^f^h^y^d^r^o^g^enⁱ^oⁿ^s^.^A^ch^an^{ge o}^f^oⁿ^e^p^H^unⁱ^tⁱ^s^{a f}^act^o^r
^of¹⁰^dif^f^e^r^eⁿ^{t tha}ⁿ^t^h^e^ne^x^t^hⁱ^g^h^e^r^or^low^e^r^uni^{t. F}^o^r^e^x^a^m^ple^,^a^pH^of^4.0^is¹⁰^tim^e^s^the^a^cⁱ^dity^thaⁿ^a^pH^of⁵^.^0.
⁴⁷^Ken^Cal^d^ei^ra,^et^al^.^,^“Ocean^aci^dⁱ^fⁱ^catⁱ^oⁿ^d^u^e^t^oⁱⁿ^creasiⁿ^{g at}^m^o^sp^h^e^ri^{c carb}^oⁿ^dⁱ^o^xⁱ^d^e,^”^T^h^e^Roy^a^l^Soc^ie^ty^,^Polic^y
^D^o^c^u^m^e^{nt 12/0}⁵⁽^J^une²⁰⁰⁵⁾^,⁶⁰^pa^g^e^s^;^a^t^http:/^/w^w^w^.r^o^y^a^l^s^o^c^.^a^c^.^uk^/.
⁴⁸^T^h^{e d}^e^lⁱ^b^erat^{e i}ⁿ^t^r^o^d^u^c^tⁱ^oⁿ^o^fⁱ^r^onⁱⁿ^t^o^t^h^{e su}^rf^{ace o}^cean^t^o^stⁱ^m^u^l^at^e^m^a^riⁿ^e^p^h^y^t^o^p^l^an^kt^oⁿ^gro^w^t^h^,^w^hⁱ^c^h^w^o^u^l^d
^inc^r^e^a^s^e^c^a^r^{bon s}^e^que^s^t^r^a^tio^{n f}^r^o^m^the^a^t^m^o^s^phe^r^e^vⁱ^a^phot^os^yⁿ^the^s^is^{. T}^h^e^Sout^he^rⁿ^O^cean^,ⁱⁿ^p^a^rtⁱ^c^u^l^ar,ⁱ^s^d^e^fⁱ^ci^en^tⁱⁿ
^ir^{on a}^s^a^nutrⁱ^eⁿ^{t s}^u^c^h^t^h^a^t^theⁱⁿ^t^r^od^uc^tio^{n of}^ir^oⁿ^c^o^uld^s^tim^ula^t^e^phy^toplaⁿ^k^t^oⁿ^g^r^ow^{th. Se}^v^e^r^a^{l e}^xpe^rⁱ^m^e^nts^ha^v^e
^be^eⁿ^c^oⁿ^duc^te^d^or^a^r^e^un^de^r^w^ay^t^o^f^u^r^t^he^r^e^x^plor^e^t^h^is^pr^oc^e^s^s^,^f^o^r^e^x^am^ple^,^Ste^pha^ne^B^l^aⁱ^n,^e^t^a^l^{., “}^E^f^f^e^c^t^of^na^tur^a^l
^iroⁿ^f^e^rtilizatioⁿ^oⁿ^carb^oⁿ^seq^u^estratioⁿⁱⁿ^t^h^{e S}^o^u^t^h^e^rn^Ocean^,”^Na^tu^re^{, v}^o^l^.⁴⁴⁶^,ⁿ^o^.⁷¹³⁹⁽^A^pr^{il 26,}²⁰⁰⁷⁾^:^p^p^.
¹⁰⁷^0-¹⁰^74.
⁴⁹^{For m}^o^re^inf^o^rm^a^{tion a}^b^o^u^{t i}ⁿ^je^c^tion^of^CO²ⁱⁿ^t^o^t^h^e^d^eep^o^cean^s,^{see CRS}^Rep^o^r^t^RL³³⁸⁰¹^,^C^a^r^b^on^C^apt^ur^e^a^nd
^Se^que^s^t^r^a^tⁱ^oⁿ⁽^C^C^S⁾^{, by}^P^e^te^{r F}^o^l^g^e^r^.

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
years.
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.
^P^e^ter^Fo^lg^er
^Sp^{ecialist i}ⁿ^En^er^g^y^aⁿ^d^Natu^r^a^{l Reso}^u^r^{ces P}^o^lic^y
^pf^ol^g^e^r@^cr^s^.^l^o^c.g^o^v^,^7-¹⁵¹⁷

⁵⁰^{Urs S}ⁱ^egen^t^h^al^{er et}^al^.^,^“S^t^a^bl^{e carb}^oⁿ^cy^cl^e—^clⁱ^m^at^{e rel}^a^tⁱ^oⁿ^s^hⁱ^p^d^u^rⁱⁿ^{g t}^h^{e L}^a^t^e^P^l^ei^st^o^cen^e,^”^Sc^ie^nc^e^{, v}^o^l.³¹⁰
⁽^N^ov^{. 25}^{, 2}⁰⁰⁵⁾^:^pp^{. 1}³¹³^-¹³¹^7.