The Carbon Cycle: Key Component of the Climate System, with Implications for Policy
The Carbon Cycle: Key Component of the
August 10, 2022
Climate System, with Implications for Policy
Jonathan D. Haskett
Large quantities of carbon are actively exchanged between the atmosphere and the other carbon
Analyst in Environmental
storage pools, including the oceans, vegetation, and soils on the land surface. The exchange, or
Policy
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 to
atmospheric carbon dioxide (CO
2), to the entire global carbon cycle. Despite this relatively small
contribution to the atmospheric carbon, the resulting 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 climate change?
The answer is that some of the CO2 released to the atmosphere by human activities is not transferred to oceans, vegetation,
and soils quickly enough to prevent CO2 concentrations in the atmosphere from increasing over time. Human activities are
transferring fossil carbon—which took millions of years to accumulate—from a large, long-term carbon storage pool into the
atmosphere over a relatively short time span, thereby affecting the global carbon cycle. As a result, the atmosphere contains
approximately 46% more CO2 today than prior to the beginning of the Industrial Revolution. As the CO2 concentration of the
atmosphere increases, the degree to which the atmosphere traps incoming radiation from the sun increases, which in turn
contributes to further warming of the planet.
The increase in atmospheric CO2 concentration is mitigated to some extent by two relatively large (as compared to the
atmosphere) reservoirs of carbon—the global oceans and the land surface. The global oceans and the land surface are
considered net sinks for carbon because they currently take up more carbon than they release. 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 (e.g., carbon capture and sequestration). If the oceans, vegetation, and soils did not
act as sinks, then the concentration of CO2 in the atmosphere would increase even more rapidly than has been observed.
The behavior and components of the global carbon cycle can be influenced by congressional policy decisions. Legislation
introduced in the 117th Congress includes the climate policy goal of achieving net-zero CO2 emissions—where emissions of
CO2 are balanced by removals from the atmosphere. Achieving this policy goal depends, in part, on the behavior of
components of the global carbon cycle, specifically the land and ocean sinks, and on human activities influencing the cycle.
Policy options to achieve net-zero CO2 emissions will require changes in the carbon cycle that may include reductions in
emissions from fossil fuel combustion and land-use change, increasing the uptake of CO2 by land surface and ocean carbon
sinks, and increasing the capacity for removal and long-term storage of CO2 from the atmosphere by technical means.
Large uncertainties exist in the future behavior of important components of the global carbon cycle, including the levels of
emissions of CO2 to the atmosphere, and future levels of uptake or release of CO2 from the land surface and oceans.
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Contents
Introduction ..................................................................................................................................... 1
Carbon Storage, Sources, and Sinks ................................................................................................ 2
Carbon Flux, or Exchange, with the Atmosphere ............................................................................ 5
How Fast Carbon Is Exchanged ................................................................................................ 6
Land Surface-Atmosphere Flux ................................................................................................ 7
Ocean-Atmosphere Flux ........................................................................................................... 8
The Carbon Budget Imbalance .................................................................................................. 9
Policy Implications ........................................................................................................................ 10
Figures
Figure 1. Panel (a): Storage or Pools (GtC); and Panel (b): Annual Flux or Exchange of
Carbon (GtC per year) .................................................................................................................. 4
Tables
Table 1. Carbon Stocks in the Atmosphere, Ocean, and Land Surface, and Annual
Carbon Fluxes .............................................................................................................................. 5
Contacts
Author Information ......................................................................................................................... 11
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The Carbon Cycle: Key Component of the Climate System, with Implications for Policy
Introduction
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 large carbon reservoirs: the global oceans and the planet’s land
surface. A key question is how CO2 emissions from human activities are changing the global
carbon cycle—the exchange, or flux, of carbon between the atmosphere, oceans, and land
surface—and how the changes affect the rate of CO2 buildup in the atmosphere.1
There is a scientific consensus that human disturbances, or perturbations, to the carbon cycle are a
main factor driving climate change over the past 50 years.2 For the period 0-1850 CE, the global
carbon cycle was roughly in balance, and the concentration of CO2 in the atmosphere was fairly
constant at approximately 280 parts per million (ppm).3 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 are now greater than 400 ppm, having
increased by over 46% since the Industrial Revolution.4
An understanding of the global carbon cycle has shifted from being of mainly academic interest
to being also of policy interest. Policymakers are grappling with, for example, how the United
States could achieve a state of net-zero CO2 emissions, in which emissions to the atmosphere are
balanced by removals. This is likely to require the implementation of human methods of carbon
dioxide removal (CDR)5 and would depend inherently on continued uptake of carbon by the
oceans and land surface. Yet how much CO2 forests or farmland are capable of taking up in the
future, and for how long, is not clear. How the oceans and the land surface carbon reservoirs will
behave in the future—how much CO2 they will take up or release and at what rate—are topics of
active scientific inquiry.
1 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. For more information, see CRS Report R43185, Ocean
Acidification, by Harold F. Upton and Peter Folger.
2 The Fourth National Climate Assessment (NCA4) of the U.S. Global Change Research Program addresses human
causation of climate change. “This assessment concludes, based on extensive evidence, that it is extremely likely that
human activities, especially emissions of greenhouse gases, are the dominant cause of the observed warming since the
mid-20th century. For the warming over the last century, there is no convincing alternative explanation supported by the
extent of the observational evidence.” Carbon dioxide is recognized as the greenhouse gas with the greatest influence
on climate. See also Intergovernmental Panel on Climate Change (IPCC), “Summary for Policymakers,” in Climate
Change 2021: The Physical Science Basis—Contribution of Working Group I to the Sixth Assessment Report of the
Intergovernmental Panel on Climate Change, 2021, p. 7 (hereinafter IPCC AR6 WGI SPM 2021).
3 IPCC, “Chapter 2: Changing State of the Climate System” in Climate Change 2021: The Physical Science Basis—
Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change,
2021, p. 300 (hereinafter 2021 IPCC Working Group I Report).
4 U.S. Global Change Research Program, Second State of the Carbon Cycle Report (SOCCR2): A Sustained Assessment
Report, 2018, p. 24 (hereinafter SOCCR2). See also NOAA, “Climate Change: Atmospheric Carbon Dioxide,”
https://www.climate.gov/news-features/understanding-climate/climate-change-atmospheric-carbon-dioxide; and IPCC
AR6 WGI SPM 2021, p. 4.
5 U.S. Global Change Research Program (USGRP), Climate Science Special Report: Fourth National Climate
Assessment, Volume I, 2017, p. 399.
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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.6 More
than half of the human-caused increase in the earth’s radiation balance is due to CO2, the primary
focus of this report.7 According to the Intergovernmental Panel on Climate Change (IPCC), CO2
is the most important greenhouse gas released to the atmosphere from human activities.8
Carbon Storage, Sources, and Sinks
The atmosphere, oceans, vegetation, and soils on the land surface all store carbon (Figure 1,
panel (a)). Geological reservoirs also store carbon in a variety of forms, including fossil fuels,
such as oil, gas, and coal.9 A portion of these fossil fuel reservoirs are considered recoverable
reserves.10 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 soils11 (Figure 1, panel (a); Table 1). The total carbon stored in the live plant
biomass of the earth is less12 than the 870 billion metric tons of carbon13 (870 GtC)14 present in
the atmosphere. Carbon contained in the oceans, vegetation, and soils on the land surface is
linked to the atmosphere through natural processes such as photosynthesis, respiration,
decomposition, and gas exchange. In contrast, carbon in fossil fuels is linked to the atmosphere
through the extraction and combustion of fossil fuels.
The nearly constant concentration of CO2 in the atmosphere has small annual and latitudinal
fluctuations (approximately 1%), caused by photosynthesis and respiration.15 Carbon dioxide
released from fossil fuel combustion mixes into the atmospheric carbon pool, where it undergoes
exchanges with the ocean and land surface carbon pools. Thus, where fossil fuels are burned
6 This report updates CRS Report RL34059, The Carbon Cycle: Implications for Climate Change and Congress, by
Peter Folger (available to congressional clients upon request). This report draws on language, discussion, and
information that appeared in the earlier report while adding new language, information, and analysis on the science and
the legislative context of the carbon cycle.
7 SOCCR2, p. 44. See also U.S. Global Change Research Program, 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, 2007, p. 2 (hereinafter SOCCR).
8 Methane, black carbon, and organic carbon pollution are also part of the carbon cycle and have roles in human-
induced climate change. Methane probably accounts for about an additional 16% of the change in the earth’s radiation
balance.
9 The earth stores carbon mainly as carbonate minerals. Carbonate minerals are linked to the atmosphere by natural
processes, such as erosion and weathering, and by metamorphism over geologic timescales.
10 Not all coal, natural gas, and oil fossil fuel resources present in the earth’s crust are recoverable. The subset portion
of those resources that have been verified and could be economically recovered are known as reserves.
11 Rattan Lal, “Managing Soils and Ecosystems for Mitigating Anthropogenic Carbon Emissions and Advancing
Global Food Security,” BioScience, vol. 60 (2010), p. 708 (hereinafter Lal 2010). Also see Mojtaba Fakhraee et al., “A
Largely Invariant Marine Dissolved Organic Carbon Reservoir across Earth’s History,” Proceedings of the National
Academy of Sciences, vol. 118, no. 40 (2021).
12 Lal 2010, p. 708. See also William H. Schlesinger, Biogeochemistry: An Analysis of Global Change, 2nd ed (San
Diego: Academic Press, 1997), p. 360 (hereinafter Schlesinger 1997).
13 IPCC, “Global Carbon and Other Biogeochemical Cycles and Feedbacks” in 2021 IPCC Working Group I Report,
2021, p. 700.
14 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 1 gigatonne, or GtC.
15 Schlesinger 1997, p. 56. Larger fluctuations by season occur in the Northern Hemisphere.
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makes relatively little difference to the average concentration of CO2 in the global atmosphere;
emissions in any one region affect the concentration of CO2 globally.16
The oceans, vegetation, and soils exchange carbon with the atmosphere constantly on daily and
seasonal time cycles (see Figure 1, panel (b)). 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 timescale of human history. An understanding of the dynamics of the carbon
cycle—and the ways in which human-sourced carbon emissions are apportioned among the
atmosphere, oceans, vegetation, and soils—can inform and support deliberations on whether to
take action to ameliorate global warming and what type of action that might be.
16 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.
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Figure 1. Panel (a): Storage or Pools (GtC); and Panel (b): Annual Flux or
Exchange of Carbon (GtC per year)
Source: IPCC, “Global Carbon and Other Biogeochemical Cycles and Feedbacks” in 2021 IPCC Working
Group I Report, 2021, p. 700.
Notes: Figure prepared by CRS. One GtC refers to 1 bil ion metric tons of carbon.
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Table 1. Carbon Stocks in the Atmosphere, Ocean, and Land Surface,
and Annual Carbon Fluxes
Annual flux
Annual flux
Net to the
(GtC/yr) from the
(GtC/yr) to the
atmosphere
Storage pool
GtC
atmosphere
atmosphere
(GtC/yr)
Atmosphere
870
—
—
—
Ocean
37,273
79.5
77.6
-1.9
Land surface
2,150
142
136.7
-5.3
(soils plus vegetation)
Fossil carbon reserves
928
—
—
+9.4
(coal, gas, oil, other)
Source: IPCC, “Global Carbon and Other Biogeochemical Cycles and Feedbacks” in 2021 IPCC Working
Group I Report, 2021, p. 700.
Notes: There is some uncertainty associated with each of the estimated values in the table. Determining the
values for carbon stocks and fluxes among stocks is an area of active, ongoing scientific research.
The amount of carbon stored in the atmospheric pool is important because as more CO2 is added
to the atmosphere, its heat-trapping capacity becomes greater.17 Each storage pool—oceans, soils,
and vegetation—is considered a sink for carbon because each pool takes up carbon from the
atmosphere. 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. In contrast to the oceans, soils, and vegetation, the pool
of fossil carbon is only a source, not a sink, except over geologic timescales.
Carbon Flux, or Exchange, with the Atmosphere
Over 77 GtC is exchanged each year between the atmosphere and the oceans, and over 136 GtC is
exchanged between the atmosphere and the land surface annually (see Table 1).18 Human
activities—primarily land-use change (e.g., deforestation) and burning of fossil fuels—contribute
less than 12 GtC to the atmosphere each year.19 If the human contribution of CO2 is subtracted
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, a
balanced state of natural net-zero CO2 emissions. Research indicates that the average decadal net
flux was less than 0.1 GtC per year for about 10,000 years leading up to 1750.20 That small value
17 See CRS Report RL33849, Climate Change: Science and Policy Implications, by Jane A. Leggett (available to
congressional clients upon request), for an explanation of the heat-trapping properties, or radiative forcing, of CO2 and
other greenhouse gases.
18 These exchanges of CO2 among the atmosphere, oceans, and land surface result mostly from natural processes, such
as photosynthesis, respiration, decay, and gas exchange between the ocean and land surface, and the lower atmosphere.
19 Land-use change (mainly deforestation) accounts for approximately 15% of human-related CO2 emissions, while
fossil fuel combustion accounts for approximately 85% of such emissions. See 2021 IPCC Working Group I Report, p.
700. See also IPCC, Climate Change 2007: The Physical Science Basis—Contribution of Working Group I to the
Fourth Assessment Report of the Intergovernmental Panel on Climate Change, 2007, pp. 514-515 (hereinafter 2007
IPCC Working Group I Report). Fossil fuel burning and industrial activities release about 9.4 GtC per year; land-use
change releases about 1.6 GtC per year (2021 IPCC Working Group I Report, p. 700).
20 2007 IPCC Working Group I Report, p. 514.
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for net flux is reflected by the relatively stable concentration of CO2 in the atmosphere—between
260 ppm and 280 ppm—for about the 10,000 years prior to 1750.21
How Fast Carbon Is Exchanged
Currently the atmospheric concentration of CO2 is approximately 130 ppm higher than it was
before 1750 (410 ppm versus 280 ppm), primarily because human activities are adding carbon to
the atmosphere faster than the oceans, land vegetation, and soils can remove it. The relatively
rapid addition of CO2 to the atmosphere has tipped the balance so that even though the oceans
and the land surface take up more CO2 per year, on average, than they release, atmospheric
concentrations of CO2 continue to rise (see Table 1). During the 1990s 55% of CO2 emissions
from land-use activities and fossil fuel combustion were taken up by the oceans, vegetation, or
soils on the land surface, with 45% persisting in the atmosphere,22 a trend that was also confirmed
for the period 2010-2019.23 CO2 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.24 When a
pulse25 of CO2 is added to the atmosphere, in 30 years about half will be withdrawn; thereafter,
removal proceeds more slowly, with centuries required to remove an additional 30% and
millennia before the final 20% is removed.26 If CO2 emissions continue or increase, however,
adding to the cumulative CO2 emissions since the start of the Industrial Revolution, atmospheric
concentrations of CO2 will also continue to rise. This will increase radiative forcing, the degree to
which the atmosphere traps incoming radiation from the sun. There is a scientific consensus that
there is an almost linear relationship between global temperature increases and such cumulative
human-caused emissions of CO2,27 and a likely result is a continued warming of the planet.
At present the oceans and land surface are acting as sinks for CO2 emitted from fossil fuel
combustion, deforestation, and other human sources, 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 cycle28) may
alter the balance between sources and sinks due to changes in the complicated feedback
mechanisms among the atmosphere, oceans, and land surface.29 How carbon sinks will behave in
the future is a prominent question for both scientists and policymakers.
21 IPCC, Climate Change 2001: The Scientific Basis: Contribution of Working Group I to the Third Assessment Report
of the Intergovernmental Panel on Climate Change, 2001, p. 203.
22 2007 IPCC Working Group I Report, pp. 514-515.
23 IPCC, “Global Carbon and Other Biogeochemical Cycles and Feedbacks” in 2021 IPCC Working Group I Report,
2021, p. 676.
24 That 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 hydroxyl 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.
25 In this context, a pulse of CO2 is an addition of this gas to the atmosphere, where the definition of pulse is “a
variation, characterized by a rise, limited duration, and decline of a quantity whose value normally is constant” in
Webster’s New World Dictionary of the English Language, Second College Edition (New York: Simon and Schuster,
1980).
26 2007 IPCC Working Group I Report, p. 514.
27 IPCC, “Summary for Policy Makers,” in 2021 IPCC Working Group I Report, 2021, p. 28.
28 The hydrologic cycle is defined as “the sequence of conditions through which water passes from vapor in the
atmosphere through precipitation upon land or water surfaces and ultimately back into the atmosphere as a result of
evaporation and transpiration.” See “hydrologic cycle” in Merriam-Webster.com Dictionary, Merriam-Webster,
https://www.merriam-webster.com/dictionary/hydrologic%20cycle.
29 See CRS Report RL34266, Climate Change: Science Highlights, by Jane Leggett (available to congressional clients
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Land Surface-Atmosphere Flux
The IPCC estimates of the carbon cycle indicate that the land surface (vegetation plus soils)
accumulates approximately 5.3 GtC more carbon per year than it emits to the atmosphere30 (see
Figure 1, panel (b); and Table 1). Recent research indicates that during the past 60 years, the
land surface sink for CO2 has increased,31 as the growth rate of atmospheric CO2 has increased,
among other factors.32 That means that the land surface acts as a net sink for CO2 at present.
Some policymakers advocate strategies for increasing the amount of CO2 taken up and stored, or
sequestered, by soils and plants, typically through land-use change, such as agricultural or
forestry practices.33 How effective these land-use practices will be for large-scale and long-term
carbon sequestration is not clear.
There are large uncertainties associated with the land-use component of the global carbon cycle,
although these uncertainties have been reduced.34 There is uncertainty regarding which
environmental factors are foremost in influencing the behavior of the land surface sink.35 Studies
combining satellite land cover change with biomass data suggest that tropical deforestation has
been responsible for the largest share of CO2 released to the atmosphere from land-use changes.36
Tropical deforestation and other land-use changes released approximately 3.6 GtC per year to the
atmosphere in the 1990s and approximately 3.8 GtC per year for the period 2011-2020.37 Even
though deforestation emits more carbon than is taken up by forest growth within some regions,38
net forest regrowth takes up sufficient carbon such that the land surface acts as a global net sink.
upon request), for more information on climate feedbacks.
30 IPCC, “Global Carbon and Other Biogeochemical Cycles and Feedbacks” in 2021 IPCC Working Group I Report,
2021, p. 700.
31 IPCC, “Global Carbon and Other Biogeochemical Cycles and Feedbacks” in 2021 IPCC Working Group I Report,
2021, p. 694.
32 Pierre Friedlingstein et al., “Global Carbon Budget 2021,” Earth System Science Data, vol. 14, no. 4 (2022), p. 1917
(hereinafter Friedlingstein 2022).
33 For more information on sequestration in the agricultural and forestry sectors, see CRS Report R46312, Forest
Carbon Primer, by Katie Hoover and Anne A. Riddle; CRS Report R46313, U.S. Forest Carbon Data: In Brief, by
Katie Hoover and Anne A. Riddle; CRS In Focus IF11693, Agricultural Soils and Climate Change Mitigation, by
Genevieve K. Croft; and CRS In Focus IF11404, Greenhouse Gas Emissions and Sinks in U.S. Agriculture, by
Genevieve K. Croft.
34 IPCC, “Carbon and Other Biogeochemical Cycles” in Climate Change 2013: The Physical Science Basis—
Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change,
2013, p. 490.
35 D.N. Huntzinger et al., “Uncertainty in the Response of Terrestrial Carbon Sink to Environmental Drivers
Undermines Carbon-Climate Feedback Predictions,” Scientific Reports, vol. 7, no. 1 (2017), p. 4765. See also Simone
Fatichi et al., “Modelling Carbon Sources and Sinks in Terrestrial Vegetation,” New Phytologist, vol. 221, no. 2 (2019),
p. 652.
36 Veronique De Sy et al., “Tropical Deforestation Drivers and Associated Carbon Emission Factors Derived from
Remote Sensing Data,” Environmental Research Letters, vol. 14, no. 9 (2019). See also IPCC, Climate Change 2013:
The Physical Science Basis—Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental
Panel on Climate Change, 2013, p. 50.
37 Friedlingstein 2022.
38 William H. Schlesinger and Emily S. Bernhardt, Biogeochemistry: An Analysis of Global Change, 4th ed. (Academic
Press, an imprint of Elsevier, 2020), p. 458.
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Ocean-Atmosphere Flux
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. Like the land surface, the oceans
today accumulate more carbon than they emit to the atmosphere each year, acting as a net sink of
about 1.9 GtC per year.39 (See Figure 1, panel (b); and Table 1.) The oceans have a much larger
capacity to store carbon than the land surface. Ultimately, over the course of millennia, the oceans
could take up all but about 7% of carbon emitted to the atmosphere from fossil fuel combustion
and land-use change.40 Policymakers are likely more concerned about CO2 accumulating in the
oceans now and its behavior as a net sink over the next few decades. Also, the additional CO2 is
increasing the acidity of the ocean’s surface water, which may affect marine life.41
Uptake of CO2 by the ocean occurs in stages. First, CO2 dissolves in ocean water at the surface.42
Higher CO2 concentrations increase the rate of this reaction. Once dissolved, the CO2 may be
taken up biologically through phytoplanktonic photosynthesis, or chemically through the
formation of calcium carbonate.
The quantity of uptake of CO2 by the ocean is determined by a number of factors, some of which
are affected by increases or decreases in the concentration of atmospheric CO2, and by global
temperature. CO2, which diffuses into the ocean, reacts with water to form carbonic acid, which
then reacts with calcium to form calcium carbonate.43 This allows the ocean to absorb more CO2
than is simply dissolved in the water. This chemical ability of the ocean to absorb CO2 is known
as the buffering capacity.
These uptake processes take place in the surface layers of the ocean, a small component of the
ocean’s total volume.44 This surface water is moved into the ocean’s depths by a slow process of
mixing whereby it may be a millennium before the water returns to the surface.45 The vertical
mixing of the ocean, by the ocean waters’ extensive movement around the globe, is a critical
component of the ocean sink.46
This extensive mixing of the ocean, due to overturning circulation by the large ocean “conveyor
belt” currents such as the Atlantic Meridional Overturning Current (AMOC), brings absorbed
atmospheric carbon into the deep ocean, where it is sequestered from the atmosphere for long
periods of time.47 The scale and speed of this circulation contribute to the effectiveness of the
39 IPCC, “Global Carbon and Other Biogeochemical Cycles and Feedbacks” in 2021 IPCC Working Group I Report,
2021, p. 700.
40 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 D.
Archer et al., “Dynamics of Fossil Fuel CO2 Neutralization by Marine CaCO3,” Global Biogeochemical Cycles, vol. 12,
no. 2 (1998), p. 259.
41 CRS Report R40143, Ocean Acidification, by Harold F. Upton and Peter Folger.
42 SOCCR, p. 26. Turbulence, wave action, and wind also influence the rate at which CO2 dissolves in seawater.
SOCCR, p. 26.
43 American Chemical Society, “Ocean Chemistry,” https://www.acs.org/content/acs/en/climatescience/oceansicerocks/
oceanchemistry.html.
44 SOCCR, p. 26.
45 Jorge L. Sarmiento and Nicolas Gruber, “Sinks for Anthropogenic Carbon,” Physics Today, vol. 55, no. 8 (2002), p.
30 (hereinafter Sarmiento and Gruber 2002).
46 SOCCR, p. 26.
47 Sarminento and Gruber, 2002.
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ocean sink. The North Atlantic and near Antarctica are sites of the downward movement of frigid,
dense, carbon-rich surface water into the deep ocean.48
Recent research has shown that increases in atmospheric CO2 due to human activity have started
to affect the buffering capacity of the ocean, although there is no scientific consensus that this has
changed the uptake of CO2 by the ocean.
Some researchers have raised concerns that continued increases in atmospheric CO2 and global
temperatures could decrease the ocean’s capacity to absorb CO2 in the later part of this century.
For example, increasing concentrations of atmospheric CO2 reduce the buffering capacity of the
ocean, which could lead to a decrease in CO2 absorption by the ocean. In addition, CO2 is less
soluble in water at higher temperatures, and increased temperatures could also contribute to
decreased ocean CO2 uptake. If overturning circulation or mixing of ocean water were to decrease
during this time,49 a smaller quantity of carbon initially absorbed by surface water would be
transported to the deep ocean. Ocean uptake of CO2 could decrease for these reasons; however,
models indicate that these changes are more likely to occur under future scenarios of higher CO2
emissions.
The Carbon Budget Imbalance
The global carbon budget is the accounting of the carbon added to the global carbon cycle by
human activity, often measured since the start of the Industrial Revolution (defined as the year
1750 CE). The global carbon budget includes CO2 emissions from fossil fuel use (EFOS); CO2
emissions from land use change (ELUC); the increased CO2 concentration in the atmosphere
(GATM); the removal of CO2 from the atmosphere by the ocean sink (SOCEAN); and removal of CO2
from the atmosphere by the land-use sink (SLAND). In the global carbon cycle, these quantities
balance and sum to zero. However, our knowledge of the pools and fluxes of carbon in the global
carbon cycle is imperfect, and when the estimates of these quantities do not sum to zero, there is a
carbon budget imbalance (BIM) with the following equation:50
BIM = EFOS + ELUC – (GATM + SOCEAN + SLAND)
In the past, this imbalance was referred to as the “missing carbon” and was thought to be largely
due to the difficulty of measuring the uptake of carbon by terrestrial ecosystems—for example, in
the Northern Hemisphere.51 However, the average imbalance from recent calculations is now
about 3% of emissions. This small percentage of imbalance (individual years sometimes differ) in
the global carbon cycle suggests the partitioning of emissions among the components of the
carbon cycle almost completely accounts for carbon in the global carbon budget.52 These low
values for the carbon budget imbalance support increased confidence in the scientific
understanding of the global carbon cycle. As a recent scientific journal article on the global
carbon budget noted, “Therefore, the near-zero mean and trend in the budget imbalance is seen as
48 Sarmiento and Gruber 2002; see also SOCCR, p. 26.
49 See CRS Report R47021, Federal Involvement in Ocean-Based Research and Development, by Caitlin Keating-
Bitonti, pp. 23, 28. As the planet warms, there is an increase in glacial melt and an increase in the addition of fresh
water to the North Atlantic. This addition decreases the salinity and the density of the water, making it less heavy with
less of a tendency to sink. The sinking of water in the North Atlantic is the driver of the AMOC, and a decrease in
density slows the rate of the AMOC, reducing its capacity to carry absorbed atmospheric carbon to the deep ocean.
50 Friedlingstein 2022.
51 Britton B. Stephens et al., “Weak Northern and Strong Tropical Land Carbon Uptake from Vertical Profiles of
Atmospheric CO2,” Science, vol. 316 (2007), p. 1732.
52 Friedlingstein 2022.
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evidence of a coherent community understanding of the emissions and their partitioning on those
timescales.”53
It is not currently possible to attribute, with certainty, the source of the discrepancy producing
these imbalances. Uncertainties in the measurements of the ocean and land surface sinks have
been suggested, but a scientific consensus on the source of the imbalance has not been reached.
Policy Implications
The behavior of the global carbon cycle can be affected by the policy decisions and legislative
actions of Congress. Assessing policy options in the context of the broader carbon cycle
summarized above may be useful to project the potential effects of, and interactions among,
various options. The vast array of policy choices that can affect GHG emissions and uptake may
be direct and intentional, or indirect and collateral in their impact on other policy objectives.
There is a scientific consensus that in order to reduce the risk of increasing impacts of climate
change, it is necessary to stabilize global temperatures, and that reaching a state of net-zero CO2
emissions is necessary to achieve this stabilization.54 Net zero means that emissions of CO2 are
balanced by removals of CO2 and there is no net addition of CO2 to the atmosphere, a situation
that existed before the start of the Industrial Revolution. There have been legislative proposals
introduced during the 117th Congress aimed at achieving net-zero CO2 emissions domestically.55
Additionally, having the United States achieve net-zero CO2 emissions by 2050 is a stated climate
policy goal of the Biden Administration.56
Any of the policy options to achieve net-zero CO2 will require changes to the carbon cycle. These
changes may include the following:
Reducing emissions of CO2 from fossil fuels and land-use change.
Techniques for reducing CO2 emissions from fossil fuel use and land-use
change are well understood and span a range of approaches, including
energy efficiency and use of renewable energy sources to reduce
emissions from fossil fuels, and maintaining indigenous forests and
reducing deforestation to mitigate emissions from land-use change.
Increasing the uptake of CO2 by the land surface through management.
Techniques for doing so are well understood and include increasing
forested land and changing cultivation practices to increase soil carbon.
Increasing the uptake of CO2 by the oceans.57
Techniques for doing so are not developed, are the subject of ongoing
research, and have given rise to concerns about their feasibility and
potential environmental impacts.
Increasing the capacity for removal and long-term storage of CO2 from the
atmosphere.
53 Friedlingstein 2022.
54 IPCC AR6 WGI SPM 2021, p. 28.
55 H.R. 1512, H.R. 5179.
56 Executive Order 14008, “Tackling the Climate Crisis at Home and Abroad,” 86 Federal Register 7619 (“Sec. 201.
Policy ... put the United States on a path to achieve net-zero emissions, economy-wide, by no later than 2050”).
57 CRS Report R47172, Geoengineering: Ocean Iron Fertilization, by Caitlin Keating-Bitonti.
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Techniques to develop additional carbon dioxide removal (CDR)
capacity through technologies such as direct air capture (DAC)—
essentially the development of an additional human-created carbon sink
that removes CO2 from the atmosphere for long-term sequestration—are
under development and have not been demonstrated at scale.
The success of policies in achieving net zero is dependent, in part, on the behavior of the carbon
cycle, specifically on the behavior of the land and ocean sinks. If the sink capacity of the ocean or
the land carbon pools were to decrease, then achieving net zero would potentially require greater
levels of emissions reductions, greater levels of CDR capacity, or a combination of the two. Such
levels might not be necessary if the sink capacity of the ocean and the land continue to increase.
The ongoing scientific effort to understand and anticipate the behavior of the global carbon cycle,
now and in the future, can help to inform congressional deliberations on these and other climate
policies.
Author Information
Jonathan D. Haskett
Analyst in Environmental Policy
Disclaimer
This document was prepared by the Congressional Research Service (CRS). CRS serves as nonpartisan
shared staff to congressional committees and Members of Congress. It operates solely at the behest of and
under the direction of Congress. Information in a CRS Report should not be relied upon for purposes other
than public understanding of information that has been provided by CRS to Members of Congress in
connection with CRS’s institutional role. CRS Reports, as a work of the United States Government, are not
subject to copyright protection in the United States. Any CRS Report may be reproduced and distributed in
its entirety without permission from CRS. However, as a CRS Report may include copyrighted images or
material from a third party, you may need to obtain the permission of the copyright holder if you wish to
copy or otherwise use copyrighted material.
Congressional Research Service
R47214 · VERSION 1 · NEW
11
August 10, 2022
Climate System, with Implications for Policy
Jonathan D. Haskett
Large quantities of carbon are actively exchanged between the atmosphere and the other carbon
Analyst in Environmental
storage pools, including the oceans, vegetation, and soils on the land surface. The exchange, or
Policy
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 to
atmospheric carbon dioxide (CO
2), to the entire global carbon cycle. Despite this relatively small
contribution to the atmospheric carbon, the resulting 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 climate change?
The answer is that some of the CO2 released to the atmosphere by human activities is not transferred to oceans, vegetation,
and soils quickly enough to prevent CO2 concentrations in the atmosphere from increasing over time. Human activities are
transferring fossil carbon—which took millions of years to accumulate—from a large, long-term carbon storage pool into the
atmosphere over a relatively short time span, thereby affecting the global carbon cycle. As a result, the atmosphere contains
approximately 46% more CO2 today than prior to the beginning of the Industrial Revolution. As the CO2 concentration of the
atmosphere increases, the degree to which the atmosphere traps incoming radiation from the sun increases, which in turn
contributes to further warming of the planet.
The increase in atmospheric CO2 concentration is mitigated to some extent by two relatively large (as compared to the
atmosphere) reservoirs of carbon—the global oceans and the land surface. The global oceans and the land surface are
considered net sinks for carbon because they currently take up more carbon than they release. 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 (e.g., carbon capture and sequestration). If the oceans, vegetation, and soils did not
act as sinks, then the concentration of CO2 in the atmosphere would increase even more rapidly than has been observed.
The behavior and components of the global carbon cycle can be influenced by congressional policy decisions. Legislation
introduced in the 117th Congress includes the climate policy goal of achieving net-zero CO2 emissions—where emissions of
CO2 are balanced by removals from the atmosphere. Achieving this policy goal depends, in part, on the behavior of
components of the global carbon cycle, specifically the land and ocean sinks, and on human activities influencing the cycle.
Policy options to achieve net-zero CO2 emissions will require changes in the carbon cycle that may include reductions in
emissions from fossil fuel combustion and land-use change, increasing the uptake of CO2 by land surface and ocean carbon
sinks, and increasing the capacity for removal and long-term storage of CO2 from the atmosphere by technical means.
Large uncertainties exist in the future behavior of important components of the global carbon cycle, including the levels of
emissions of CO2 to the atmosphere, and future levels of uptake or release of CO2 from the land surface and oceans.
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Contents
Introduction ..................................................................................................................................... 1
Carbon Storage, Sources, and Sinks ................................................................................................ 2
Carbon Flux, or Exchange, with the Atmosphere ............................................................................ 5
How Fast Carbon Is Exchanged ................................................................................................ 6
Land Surface-Atmosphere Flux ................................................................................................ 7
Ocean-Atmosphere Flux ........................................................................................................... 8
The Carbon Budget Imbalance .................................................................................................. 9
Policy Implications ........................................................................................................................ 10
Figures
Figure 1. Panel (a): Storage or Pools (GtC); and Panel (b): Annual Flux or Exchange of
Carbon (GtC per year) .................................................................................................................. 4
Tables
Table 1. Carbon Stocks in the Atmosphere, Ocean, and Land Surface, and Annual
Carbon Fluxes .............................................................................................................................. 5
Contacts
Author Information ......................................................................................................................... 11
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The Carbon Cycle: Key Component of the Climate System, with Implications for Policy
Introduction
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 large carbon reservoirs: the global oceans and the planet’s land
surface. A key question is how CO2 emissions from human activities are changing the global
carbon cycle—the exchange, or flux, of carbon between the atmosphere, oceans, and land
surface—and how the changes affect the rate of CO2 buildup in the atmosphere.1
There is a scientific consensus that human disturbances, or perturbations, to the carbon cycle are a
main factor driving climate change over the past 50 years.2 For the period 0-1850 CE, the global
carbon cycle was roughly in balance, and the concentration of CO2 in the atmosphere was fairly
constant at approximately 280 parts per million (ppm).3 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 are now greater than 400 ppm, having
increased by over 46% since the Industrial Revolution.4
An understanding of the global carbon cycle has shifted from being of mainly academic interest
to being also of policy interest. Policymakers are grappling with, for example, how the United
States could achieve a state of net-zero CO2 emissions, in which emissions to the atmosphere are
balanced by removals. This is likely to require the implementation of human methods of carbon
dioxide removal (CDR)5 and would depend inherently on continued uptake of carbon by the
oceans and land surface. Yet how much CO2 forests or farmland are capable of taking up in the
future, and for how long, is not clear. How the oceans and the land surface carbon reservoirs will
behave in the future—how much CO2 they will take up or release and at what rate—are topics of
active scientific inquiry.
1 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. For more information, see CRS Report R43185, Ocean
Acidification, by Harold F. Upton and Peter Folger.
2 The Fourth National Climate Assessment (NCA4) of the U.S. Global Change Research Program addresses human
causation of climate change. “This assessment concludes, based on extensive evidence, that it is extremely likely that
human activities, especially emissions of greenhouse gases, are the dominant cause of the observed warming since the
mid-20th century. For the warming over the last century, there is no convincing alternative explanation supported by the
extent of the observational evidence.” Carbon dioxide is recognized as the greenhouse gas with the greatest influence
on climate. See also Intergovernmental Panel on Climate Change (IPCC), “Summary for Policymakers,” in Climate
Change 2021: The Physical Science Basis—Contribution of Working Group I to the Sixth Assessment Report of the
Intergovernmental Panel on Climate Change, 2021, p. 7 (hereinafter IPCC AR6 WGI SPM 2021).
3 IPCC, “Chapter 2: Changing State of the Climate System” in Climate Change 2021: The Physical Science Basis—
Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change,
2021, p. 300 (hereinafter 2021 IPCC Working Group I Report).
4 U.S. Global Change Research Program, Second State of the Carbon Cycle Report (SOCCR2): A Sustained Assessment
Report, 2018, p. 24 (hereinafter SOCCR2). See also NOAA, “Climate Change: Atmospheric Carbon Dioxide,”
https://www.climate.gov/news-features/understanding-climate/climate-change-atmospheric-carbon-dioxide; and IPCC
AR6 WGI SPM 2021, p. 4.
5 U.S. Global Change Research Program (USGRP), Climate Science Special Report: Fourth National Climate
Assessment, Volume I, 2017, p. 399.
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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.6 More
than half of the human-caused increase in the earth’s radiation balance is due to CO2, the primary
focus of this report.7 According to the Intergovernmental Panel on Climate Change (IPCC), CO2
is the most important greenhouse gas released to the atmosphere from human activities.8
Carbon Storage, Sources, and Sinks
The atmosphere, oceans, vegetation, and soils on the land surface all store carbon (Figure 1,
panel (a)). Geological reservoirs also store carbon in a variety of forms, including fossil fuels,
such as oil, gas, and coal.9 A portion of these fossil fuel reservoirs are considered recoverable
reserves.10 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 soils11 (Figure 1, panel (a); Table 1). The total carbon stored in the live plant
biomass of the earth is less12 than the 870 billion metric tons of carbon13 (870 GtC)14 present in
the atmosphere. Carbon contained in the oceans, vegetation, and soils on the land surface is
linked to the atmosphere through natural processes such as photosynthesis, respiration,
decomposition, and gas exchange. In contrast, carbon in fossil fuels is linked to the atmosphere
through the extraction and combustion of fossil fuels.
The nearly constant concentration of CO2 in the atmosphere has small annual and latitudinal
fluctuations (approximately 1%), caused by photosynthesis and respiration.15 Carbon dioxide
released from fossil fuel combustion mixes into the atmospheric carbon pool, where it undergoes
exchanges with the ocean and land surface carbon pools. Thus, where fossil fuels are burned
6 This report updates CRS Report RL34059, The Carbon Cycle: Implications for Climate Change and Congress, by
Peter Folger (available to congressional clients upon request). This report draws on language, discussion, and
information that appeared in the earlier report while adding new language, information, and analysis on the science and
the legislative context of the carbon cycle.
7 SOCCR2, p. 44. See also U.S. Global Change Research Program, 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, 2007, p. 2 (hereinafter SOCCR).
8 Methane, black carbon, and organic carbon pollution are also part of the carbon cycle and have roles in human-
induced climate change. Methane probably accounts for about an additional 16% of the change in the earth’s radiation
balance.
9 The earth stores carbon mainly as carbonate minerals. Carbonate minerals are linked to the atmosphere by natural
processes, such as erosion and weathering, and by metamorphism over geologic timescales.
10 Not all coal, natural gas, and oil fossil fuel resources present in the earth’s crust are recoverable. The subset portion
of those resources that have been verified and could be economically recovered are known as reserves.
11 Rattan Lal, “Managing Soils and Ecosystems for Mitigating Anthropogenic Carbon Emissions and Advancing
Global Food Security,” BioScience, vol. 60 (2010), p. 708 (hereinafter Lal 2010). Also see Mojtaba Fakhraee et al., “A
Largely Invariant Marine Dissolved Organic Carbon Reservoir across Earth’s History,” Proceedings of the National
Academy of Sciences, vol. 118, no. 40 (2021).
12 Lal 2010, p. 708. See also William H. Schlesinger, Biogeochemistry: An Analysis of Global Change, 2nd ed (San
Diego: Academic Press, 1997), p. 360 (hereinafter Schlesinger 1997).
13 IPCC, “Global Carbon and Other Biogeochemical Cycles and Feedbacks” in 2021 IPCC Working Group I Report,
2021, p. 700.
14 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 1 gigatonne, or GtC.
15 Schlesinger 1997, p. 56. Larger fluctuations by season occur in the Northern Hemisphere.
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makes relatively little difference to the average concentration of CO2 in the global atmosphere;
emissions in any one region affect the concentration of CO2 globally.16
The oceans, vegetation, and soils exchange carbon with the atmosphere constantly on daily and
seasonal time cycles (see Figure 1, panel (b)). 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 timescale of human history. An understanding of the dynamics of the carbon
cycle—and the ways in which human-sourced carbon emissions are apportioned among the
atmosphere, oceans, vegetation, and soils—can inform and support deliberations on whether to
take action to ameliorate global warming and what type of action that might be.
16 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.
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Figure 1. Panel (a): Storage or Pools (GtC); and Panel (b): Annual Flux or
Exchange of Carbon (GtC per year)
Source: IPCC, “Global Carbon and Other Biogeochemical Cycles and Feedbacks” in 2021 IPCC Working
Group I Report, 2021, p. 700.
Notes: Figure prepared by CRS. One GtC refers to 1 bil ion metric tons of carbon.
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Table 1. Carbon Stocks in the Atmosphere, Ocean, and Land Surface,
and Annual Carbon Fluxes
Annual flux
Annual flux
Net to the
(GtC/yr) from the
(GtC/yr) to the
atmosphere
Storage pool
GtC
atmosphere
atmosphere
(GtC/yr)
Atmosphere
870
—
—
—
Ocean
37,273
79.5
77.6
-1.9
Land surface
2,150
142
136.7
-5.3
(soils plus vegetation)
Fossil carbon reserves
928
—
—
+9.4
(coal, gas, oil, other)
Source: IPCC, “Global Carbon and Other Biogeochemical Cycles and Feedbacks” in 2021 IPCC Working
Group I Report, 2021, p. 700.
Notes: There is some uncertainty associated with each of the estimated values in the table. Determining the
values for carbon stocks and fluxes among stocks is an area of active, ongoing scientific research.
The amount of carbon stored in the atmospheric pool is important because as more CO2 is added
to the atmosphere, its heat-trapping capacity becomes greater.17 Each storage pool—oceans, soils,
and vegetation—is considered a sink for carbon because each pool takes up carbon from the
atmosphere. 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. In contrast to the oceans, soils, and vegetation, the pool
of fossil carbon is only a source, not a sink, except over geologic timescales.
Carbon Flux, or Exchange, with the Atmosphere
Over 77 GtC is exchanged each year between the atmosphere and the oceans, and over 136 GtC is
exchanged between the atmosphere and the land surface annually (see Table 1).18 Human
activities—primarily land-use change (e.g., deforestation) and burning of fossil fuels—contribute
less than 12 GtC to the atmosphere each year.19 If the human contribution of CO2 is subtracted
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, a
balanced state of natural net-zero CO2 emissions. Research indicates that the average decadal net
flux was less than 0.1 GtC per year for about 10,000 years leading up to 1750.20 That small value
17 See CRS Report RL33849, Climate Change: Science and Policy Implications, by Jane A. Leggett (available to
congressional clients upon request), for an explanation of the heat-trapping properties, or radiative forcing, of CO2 and
other greenhouse gases.
18 These exchanges of CO2 among the atmosphere, oceans, and land surface result mostly from natural processes, such
as photosynthesis, respiration, decay, and gas exchange between the ocean and land surface, and the lower atmosphere.
19 Land-use change (mainly deforestation) accounts for approximately 15% of human-related CO2 emissions, while
fossil fuel combustion accounts for approximately 85% of such emissions. See 2021 IPCC Working Group I Report, p.
700. See also IPCC, Climate Change 2007: The Physical Science Basis—Contribution of Working Group I to the
Fourth Assessment Report of the Intergovernmental Panel on Climate Change, 2007, pp. 514-515 (hereinafter 2007
IPCC Working Group I Report). Fossil fuel burning and industrial activities release about 9.4 GtC per year; land-use
change releases about 1.6 GtC per year (2021 IPCC Working Group I Report, p. 700).
20 2007 IPCC Working Group I Report, p. 514.
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for net flux is reflected by the relatively stable concentration of CO2 in the atmosphere—between
260 ppm and 280 ppm—for about the 10,000 years prior to 1750.21
How Fast Carbon Is Exchanged
Currently the atmospheric concentration of CO2 is approximately 130 ppm higher than it was
before 1750 (410 ppm versus 280 ppm), primarily because human activities are adding carbon to
the atmosphere faster than the oceans, land vegetation, and soils can remove it. The relatively
rapid addition of CO2 to the atmosphere has tipped the balance so that even though the oceans
and the land surface take up more CO2 per year, on average, than they release, atmospheric
concentrations of CO2 continue to rise (see Table 1). During the 1990s 55% of CO2 emissions
from land-use activities and fossil fuel combustion were taken up by the oceans, vegetation, or
soils on the land surface, with 45% persisting in the atmosphere,22 a trend that was also confirmed
for the period 2010-2019.23 CO2 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.24 When a
pulse25 of CO2 is added to the atmosphere, in 30 years about half will be withdrawn; thereafter,
removal proceeds more slowly, with centuries required to remove an additional 30% and
millennia before the final 20% is removed.26 If CO2 emissions continue or increase, however,
adding to the cumulative CO2 emissions since the start of the Industrial Revolution, atmospheric
concentrations of CO2 will also continue to rise. This will increase radiative forcing, the degree to
which the atmosphere traps incoming radiation from the sun. There is a scientific consensus that
there is an almost linear relationship between global temperature increases and such cumulative
human-caused emissions of CO2,27 and a likely result is a continued warming of the planet.
At present the oceans and land surface are acting as sinks for CO2 emitted from fossil fuel
combustion, deforestation, and other human sources, 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 cycle28) may
alter the balance between sources and sinks due to changes in the complicated feedback
mechanisms among the atmosphere, oceans, and land surface.29 How carbon sinks will behave in
the future is a prominent question for both scientists and policymakers.
21 IPCC, Climate Change 2001: The Scientific Basis: Contribution of Working Group I to the Third Assessment Report
of the Intergovernmental Panel on Climate Change, 2001, p. 203.
22 2007 IPCC Working Group I Report, pp. 514-515.
23 IPCC, “Global Carbon and Other Biogeochemical Cycles and Feedbacks” in 2021 IPCC Working Group I Report,
2021, p. 676.
24 That 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 hydroxyl 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.
25 In this context, a pulse of CO2 is an addition of this gas to the atmosphere, where the definition of pulse is “a
variation, characterized by a rise, limited duration, and decline of a quantity whose value normally is constant” in
Webster’s New World Dictionary of the English Language, Second College Edition (New York: Simon and Schuster,
1980).
26 2007 IPCC Working Group I Report, p. 514.
27 IPCC, “Summary for Policy Makers,” in 2021 IPCC Working Group I Report, 2021, p. 28.
28 The hydrologic cycle is defined as “the sequence of conditions through which water passes from vapor in the
atmosphere through precipitation upon land or water surfaces and ultimately back into the atmosphere as a result of
evaporation and transpiration.” See “hydrologic cycle” in Merriam-Webster.com Dictionary, Merriam-Webster,
https://www.merriam-webster.com/dictionary/hydrologic%20cycle.
29 See CRS Report RL34266, Climate Change: Science Highlights, by Jane Leggett (available to congressional clients
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Land Surface-Atmosphere Flux
The IPCC estimates of the carbon cycle indicate that the land surface (vegetation plus soils)
accumulates approximately 5.3 GtC more carbon per year than it emits to the atmosphere30 (see
Figure 1, panel (b); and Table 1). Recent research indicates that during the past 60 years, the
land surface sink for CO2 has increased,31 as the growth rate of atmospheric CO2 has increased,
among other factors.32 That means that the land surface acts as a net sink for CO2 at present.
Some policymakers advocate strategies for increasing the amount of CO2 taken up and stored, or
sequestered, by soils and plants, typically through land-use change, such as agricultural or
forestry practices.33 How effective these land-use practices will be for large-scale and long-term
carbon sequestration is not clear.
There are large uncertainties associated with the land-use component of the global carbon cycle,
although these uncertainties have been reduced.34 There is uncertainty regarding which
environmental factors are foremost in influencing the behavior of the land surface sink.35 Studies
combining satellite land cover change with biomass data suggest that tropical deforestation has
been responsible for the largest share of CO2 released to the atmosphere from land-use changes.36
Tropical deforestation and other land-use changes released approximately 3.6 GtC per year to the
atmosphere in the 1990s and approximately 3.8 GtC per year for the period 2011-2020.37 Even
though deforestation emits more carbon than is taken up by forest growth within some regions,38
net forest regrowth takes up sufficient carbon such that the land surface acts as a global net sink.
upon request), for more information on climate feedbacks.
30 IPCC, “Global Carbon and Other Biogeochemical Cycles and Feedbacks” in 2021 IPCC Working Group I Report,
2021, p. 700.
31 IPCC, “Global Carbon and Other Biogeochemical Cycles and Feedbacks” in 2021 IPCC Working Group I Report,
2021, p. 694.
32 Pierre Friedlingstein et al., “Global Carbon Budget 2021,” Earth System Science Data, vol. 14, no. 4 (2022), p. 1917
(hereinafter Friedlingstein 2022).
33 For more information on sequestration in the agricultural and forestry sectors, see CRS Report R46312, Forest
Carbon Primer, by Katie Hoover and Anne A. Riddle; CRS Report R46313, U.S. Forest Carbon Data: In Brief, by
Katie Hoover and Anne A. Riddle; CRS In Focus IF11693, Agricultural Soils and Climate Change Mitigation, by
Genevieve K. Croft; and CRS In Focus IF11404, Greenhouse Gas Emissions and Sinks in U.S. Agriculture, by
Genevieve K. Croft.
34 IPCC, “Carbon and Other Biogeochemical Cycles” in Climate Change 2013: The Physical Science Basis—
Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change,
2013, p. 490.
35 D.N. Huntzinger et al., “Uncertainty in the Response of Terrestrial Carbon Sink to Environmental Drivers
Undermines Carbon-Climate Feedback Predictions,” Scientific Reports, vol. 7, no. 1 (2017), p. 4765. See also Simone
Fatichi et al., “Modelling Carbon Sources and Sinks in Terrestrial Vegetation,” New Phytologist, vol. 221, no. 2 (2019),
p. 652.
36 Veronique De Sy et al., “Tropical Deforestation Drivers and Associated Carbon Emission Factors Derived from
Remote Sensing Data,” Environmental Research Letters, vol. 14, no. 9 (2019). See also IPCC, Climate Change 2013:
The Physical Science Basis—Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental
Panel on Climate Change, 2013, p. 50.
37 Friedlingstein 2022.
38 William H. Schlesinger and Emily S. Bernhardt, Biogeochemistry: An Analysis of Global Change, 4th ed. (Academic
Press, an imprint of Elsevier, 2020), p. 458.
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Ocean-Atmosphere Flux
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. Like the land surface, the oceans
today accumulate more carbon than they emit to the atmosphere each year, acting as a net sink of
about 1.9 GtC per year.39 (See Figure 1, panel (b); and Table 1.) The oceans have a much larger
capacity to store carbon than the land surface. Ultimately, over the course of millennia, the oceans
could take up all but about 7% of carbon emitted to the atmosphere from fossil fuel combustion
and land-use change.40 Policymakers are likely more concerned about CO2 accumulating in the
oceans now and its behavior as a net sink over the next few decades. Also, the additional CO2 is
increasing the acidity of the ocean’s surface water, which may affect marine life.41
Uptake of CO2 by the ocean occurs in stages. First, CO2 dissolves in ocean water at the surface.42
Higher CO2 concentrations increase the rate of this reaction. Once dissolved, the CO2 may be
taken up biologically through phytoplanktonic photosynthesis, or chemically through the
formation of calcium carbonate.
The quantity of uptake of CO2 by the ocean is determined by a number of factors, some of which
are affected by increases or decreases in the concentration of atmospheric CO2, and by global
temperature. CO2, which diffuses into the ocean, reacts with water to form carbonic acid, which
then reacts with calcium to form calcium carbonate.43 This allows the ocean to absorb more CO2
than is simply dissolved in the water. This chemical ability of the ocean to absorb CO2 is known
as the buffering capacity.
These uptake processes take place in the surface layers of the ocean, a small component of the
ocean’s total volume.44 This surface water is moved into the ocean’s depths by a slow process of
mixing whereby it may be a millennium before the water returns to the surface.45 The vertical
mixing of the ocean, by the ocean waters’ extensive movement around the globe, is a critical
component of the ocean sink.46
This extensive mixing of the ocean, due to overturning circulation by the large ocean “conveyor
belt” currents such as the Atlantic Meridional Overturning Current (AMOC), brings absorbed
atmospheric carbon into the deep ocean, where it is sequestered from the atmosphere for long
periods of time.47 The scale and speed of this circulation contribute to the effectiveness of the
39 IPCC, “Global Carbon and Other Biogeochemical Cycles and Feedbacks” in 2021 IPCC Working Group I Report,
2021, p. 700.
40 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 D.
Archer et al., “Dynamics of Fossil Fuel CO2 Neutralization by Marine CaCO3,” Global Biogeochemical Cycles, vol. 12,
no. 2 (1998), p. 259.
41 CRS Report R40143, Ocean Acidification, by Harold F. Upton and Peter Folger.
42 SOCCR, p. 26. Turbulence, wave action, and wind also influence the rate at which CO2 dissolves in seawater.
SOCCR, p. 26.
43 American Chemical Society, “Ocean Chemistry,” https://www.acs.org/content/acs/en/climatescience/oceansicerocks/
oceanchemistry.html.
44 SOCCR, p. 26.
45 Jorge L. Sarmiento and Nicolas Gruber, “Sinks for Anthropogenic Carbon,” Physics Today, vol. 55, no. 8 (2002), p.
30 (hereinafter Sarmiento and Gruber 2002).
46 SOCCR, p. 26.
47 Sarminento and Gruber, 2002.
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ocean sink. The North Atlantic and near Antarctica are sites of the downward movement of frigid,
dense, carbon-rich surface water into the deep ocean.48
Recent research has shown that increases in atmospheric CO2 due to human activity have started
to affect the buffering capacity of the ocean, although there is no scientific consensus that this has
changed the uptake of CO2 by the ocean.
Some researchers have raised concerns that continued increases in atmospheric CO2 and global
temperatures could decrease the ocean’s capacity to absorb CO2 in the later part of this century.
For example, increasing concentrations of atmospheric CO2 reduce the buffering capacity of the
ocean, which could lead to a decrease in CO2 absorption by the ocean. In addition, CO2 is less
soluble in water at higher temperatures, and increased temperatures could also contribute to
decreased ocean CO2 uptake. If overturning circulation or mixing of ocean water were to decrease
during this time,49 a smaller quantity of carbon initially absorbed by surface water would be
transported to the deep ocean. Ocean uptake of CO2 could decrease for these reasons; however,
models indicate that these changes are more likely to occur under future scenarios of higher CO2
emissions.
The Carbon Budget Imbalance
The global carbon budget is the accounting of the carbon added to the global carbon cycle by
human activity, often measured since the start of the Industrial Revolution (defined as the year
1750 CE). The global carbon budget includes CO2 emissions from fossil fuel use (EFOS); CO2
emissions from land use change (ELUC); the increased CO2 concentration in the atmosphere
(GATM); the removal of CO2 from the atmosphere by the ocean sink (SOCEAN); and removal of CO2
from the atmosphere by the land-use sink (SLAND). In the global carbon cycle, these quantities
balance and sum to zero. However, our knowledge of the pools and fluxes of carbon in the global
carbon cycle is imperfect, and when the estimates of these quantities do not sum to zero, there is a
carbon budget imbalance (BIM) with the following equation:50
BIM = EFOS + ELUC – (GATM + SOCEAN + SLAND)
In the past, this imbalance was referred to as the “missing carbon” and was thought to be largely
due to the difficulty of measuring the uptake of carbon by terrestrial ecosystems—for example, in
the Northern Hemisphere.51 However, the average imbalance from recent calculations is now
about 3% of emissions. This small percentage of imbalance (individual years sometimes differ) in
the global carbon cycle suggests the partitioning of emissions among the components of the
carbon cycle almost completely accounts for carbon in the global carbon budget.52 These low
values for the carbon budget imbalance support increased confidence in the scientific
understanding of the global carbon cycle. As a recent scientific journal article on the global
carbon budget noted, “Therefore, the near-zero mean and trend in the budget imbalance is seen as
48 Sarmiento and Gruber 2002; see also SOCCR, p. 26.
49 See CRS Report R47021, Federal Involvement in Ocean-Based Research and Development, by Caitlin Keating-
Bitonti, pp. 23, 28. As the planet warms, there is an increase in glacial melt and an increase in the addition of fresh
water to the North Atlantic. This addition decreases the salinity and the density of the water, making it less heavy with
less of a tendency to sink. The sinking of water in the North Atlantic is the driver of the AMOC, and a decrease in
density slows the rate of the AMOC, reducing its capacity to carry absorbed atmospheric carbon to the deep ocean.
50 Friedlingstein 2022.
51 Britton B. Stephens et al., “Weak Northern and Strong Tropical Land Carbon Uptake from Vertical Profiles of
Atmospheric CO2,” Science, vol. 316 (2007), p. 1732.
52 Friedlingstein 2022.
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evidence of a coherent community understanding of the emissions and their partitioning on those
timescales.”53
It is not currently possible to attribute, with certainty, the source of the discrepancy producing
these imbalances. Uncertainties in the measurements of the ocean and land surface sinks have
been suggested, but a scientific consensus on the source of the imbalance has not been reached.
Policy Implications
The behavior of the global carbon cycle can be affected by the policy decisions and legislative
actions of Congress. Assessing policy options in the context of the broader carbon cycle
summarized above may be useful to project the potential effects of, and interactions among,
various options. The vast array of policy choices that can affect GHG emissions and uptake may
be direct and intentional, or indirect and collateral in their impact on other policy objectives.
There is a scientific consensus that in order to reduce the risk of increasing impacts of climate
change, it is necessary to stabilize global temperatures, and that reaching a state of net-zero CO2
emissions is necessary to achieve this stabilization.54 Net zero means that emissions of CO2 are
balanced by removals of CO2 and there is no net addition of CO2 to the atmosphere, a situation
that existed before the start of the Industrial Revolution. There have been legislative proposals
introduced during the 117th Congress aimed at achieving net-zero CO2 emissions domestically.55
Additionally, having the United States achieve net-zero CO2 emissions by 2050 is a stated climate
policy goal of the Biden Administration.56
Any of the policy options to achieve net-zero CO2 will require changes to the carbon cycle. These
changes may include the following:
Reducing emissions of CO2 from fossil fuels and land-use change.
Techniques for reducing CO2 emissions from fossil fuel use and land-use
change are well understood and span a range of approaches, including
energy efficiency and use of renewable energy sources to reduce
emissions from fossil fuels, and maintaining indigenous forests and
reducing deforestation to mitigate emissions from land-use change.
Increasing the uptake of CO2 by the land surface through management.
Techniques for doing so are well understood and include increasing
forested land and changing cultivation practices to increase soil carbon.
Increasing the uptake of CO2 by the oceans.57
Techniques for doing so are not developed, are the subject of ongoing
research, and have given rise to concerns about their feasibility and
potential environmental impacts.
Increasing the capacity for removal and long-term storage of CO2 from the
atmosphere.
53 Friedlingstein 2022.
54 IPCC AR6 WGI SPM 2021, p. 28.
55 H.R. 1512, H.R. 5179.
56 Executive Order 14008, “Tackling the Climate Crisis at Home and Abroad,” 86 Federal Register 7619 (“Sec. 201.
Policy ... put the United States on a path to achieve net-zero emissions, economy-wide, by no later than 2050”).
57 CRS Report R47172, Geoengineering: Ocean Iron Fertilization, by Caitlin Keating-Bitonti.
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Techniques to develop additional carbon dioxide removal (CDR)
capacity through technologies such as direct air capture (DAC)—
essentially the development of an additional human-created carbon sink
that removes CO2 from the atmosphere for long-term sequestration—are
under development and have not been demonstrated at scale.
The success of policies in achieving net zero is dependent, in part, on the behavior of the carbon
cycle, specifically on the behavior of the land and ocean sinks. If the sink capacity of the ocean or
the land carbon pools were to decrease, then achieving net zero would potentially require greater
levels of emissions reductions, greater levels of CDR capacity, or a combination of the two. Such
levels might not be necessary if the sink capacity of the ocean and the land continue to increase.
The ongoing scientific effort to understand and anticipate the behavior of the global carbon cycle,
now and in the future, can help to inform congressional deliberations on these and other climate
policies.
Author Information
Jonathan D. Haskett
Analyst in Environmental Policy
Disclaimer
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