U.S. Forest Carbon Data: In Brief
Updated July 15, 2021
Congressional Research Service
https://crsreports.congress.gov
R46313
Congressional Research Service

U.S. Forest Carbon Data: In Brief

Introduction
The flux—or flow—of carbon dioxide (CO2) and other greenhouse gases into the atmosphere is
the dominant contributor to the observed warming trend in global temperatures.1 Trees, however,
store (sequester) CO2 from the atmosphere, accruing significant stores of carbon over time. Trees
also release some CO2 back into the atmosphere (e.g., emissions). This process is known as the
forest carbon cycle.
The forest carbon cycle starts with the sequestration and accumulation of atmospheric CO2 due to
tree growth. The accumulated carbon is stored in five different pools in the forest ecosystem:
aboveground biomass (e.g., leaves, trunks, and limbs), belowground biomass (e.g., roots),
deadwood, litter (e.g., fal en leaves and stems), and soils. As trees or parts of trees die, the carbon
cycles through those different pools, specifical y from the living biomass pools to the deadwood,
litter, and soil pools. The length of time carbon stays in each pool varies considerably, ranging
from months (litter) to mil ennia (soil). The cycle continues as carbon flows out of the forest
ecosystem and returns to the atmosphere through several processes, including respiration,
combustion (e.g., fire), and decomposition. Carbon also leaves the forest ecosystem through
timber harvests, by which it enters the product pool. This carbon is stored in harvested wood
products (HWPs) while they are in use but eventual y wil return to the atmosphere upon the
wood products’ disposal and eventual decomposition, which could take several decades or more.
In total, there are seven pools of forest carbon: five in the forest ecosystem and two in the product
pool (HWPs in use and HWPs in disposal sites).
Carbon is always moving through the pools of forested ecosystems. The size of the various pools
and the rate at which carbon moves through them vary considerably over time. The amount of
carbon sequestered in a forest relative to the amount of carbon released into the atmosphere is
constantly changing with tree growth, death, and decomposition. If the total amount of carbon
released into the atmosphere by a given forest over a given period is greater than the amount of
carbon sequestered in that forest, the forest is a net source of carbon emissions to the atmosphere.
If the forest sequesters more carbon than it releases into the atmosphere, the forest is a net sink of
carbon.
These forest carbon dynamics are driven in large part by different anthropogenic and ecological
disturbances. Anthropogenic disturbances are planned activities, such as timber harvests, whereas
ecological disturbances are unplanned, such as weather events (e.g., hurricanes, ice storms,
droughts), insect and disease infestations, and wildfires. General y, disturbances result in tree
mortality, causing the transfer of carbon from the living pools to the deadwood, litter, soil, and
product pools, and/or eventual y to the atmosphere. If a disturbed site regenerates as forest, the
carbon releases caused by the disturbance general y are offset over time. If, however, the site
changes to a different land use (e.g., agriculture), the carbon releases may not be offset.
Congressional debates over climate policy have often included ideas for optimizing carbon
sequestration in forests as a potential mitigation strategy for global warming. To facilitate those
debates, this report provides data on the amount of carbon that is stored in and flows through U.S.
forests. Since the early 1990s, the U.S. Environmental Protection Agency (EPA) has prepared an
annual Inventory of U.S. Greenhouse Gas Emissions (Inventory), which has included an

1 Other greenhouse gases include methane (CH4), nitrous oxide (N2O), and several fluorinated gases. D. J. Wuebbles et
al., “Executive Summary,” in Climate Science Special Report: Fourth National Climate Assessment (NCA), Volume II,
U.S. Global Change Research Program, 2018.
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U.S. Forest Carbon Data: In Brief

accounting of carbon in U.S. forests in the Land Use, Land-Use Change, and Forestry (LULUCF)
sector.2 Estimates of forestland area and forest inventory data are used to estimate carbon stocks,
or the amount of carbon stored in a pool. Carbon flux is then measured by comparing changes in
forest carbon stocks over time. This report includes data for the most recent year available as wel
as data for every five years back to 1990, as available.3 Figure 1 introduces some of the terms
and units for measuring and reporting carbon that are used throughout this report. An
accompanying report, CRS Report R46312, Forest Carbon Primer, addresses basic questions
concerning carbon sequestration in forests and provides an overview of forest carbon accounting
methodologies.
Figure 1. Carbon Terms and Units

Source: CRS, adapted from Maria Janowiak et al., Considering Forest and Grassland Carbon in Land Management,
U.S. Department of Agriculture, Forest Service, GTR-WO-95, June 2017, p. 4.
Notes: Because much of the data for this report are based on international standards, this report uses the
metric system for consistency purposes. Forest carbon stocks are reported as measures of carbon, whereas

2 Environmental Protection Agency (EPA), Inventory of U.S. Greenhouse Gas Emissions and Sinks, 1990 -2019,
EPA430-R-21-005, April, 2021. Hereinafter referred to as EPA Inventory (2021).
3 T he EPA Inventory (2021) reported changes to figures across the entire time series (1990-2020), related to updated
and refined methodologies and correcting for errors in previous iterations (see pp. 35 -37 for more information).
Consequently, figures reported here may not be consistent with figures reported in earlier versions of this report or
other CRS products.
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greenhouse gas emissions and removals (e.g., sequestration) are reported as measures of carbon dioxide or
carbon dioxide equivalents (to facilitate comparisons with other greenhouse gases). As a chemical element, the
mass of carbon (C) is based on its molecular weight. Carbon dioxide (CO2) is a compound consisting of one part
carbon and two parts of the element oxygen (O). The conversion factor between C and CO2 is the ratio of their
molecular weights. The molecular weight of carbon is 12 atomic mass units (amu), and the molecular weight of
CO2 is 44 amu, which equals a ratio of 3.67. The same method is used to convert measurements of other
greenhouse gases to carbon dioxide equivalents (CO2 eq.).
U.S. Forest Carbon Stocks
According to the Inventory, U.S. forests stored 58.7 bil ion metric tons (BMT) of carbon in 2020
(see Figure 2 and Table 1).4 The majority of forest carbon was stored in the forest ecosystem
pools (95%); the remainder was stored in the product pool (i.e., harvested wood products, HWP).
The largest pool of carbon was forest soils, which contained approximately 53% of total forest
carbon in 2020. The next-largest pool was aboveground biomass, which contained approximately
26% of the total. Each of the other pools stored less than 6% of the total carbon.
Since 1990, U.S. forest carbon stocks have increased 10%. Nearly al forest pools have gained
more carbon as of 2020. The exceptions are the litter and soil pools, which each continue to store
around the same amount of carbon each year. Overal forest carbon stocks have increased
annual y (Figure 2), meaning U.S. forests have been a net carbon sink, absorbing more carbon
out of the atmosphere than they release (discussed in “Carbon Emissions and Sinks from U.S.
Forests,” below). About one-third of the United States is forested.5 These forested areas vary
considerably by location, climate, vegetation type, and disturbance histories, among other factors.
Because of this variation, U.S. forests contain varying amounts of carbon stored in varying
proportions across the different forest pools. Accordingly, the amount of carbon within a certain
area, or carbon density, also varies.6

4 Chapter 6, “Land Use, Land-Use Change, and Forestry (LULUCF),” in EPA Inventory, 2021, April, 2021. Hereinafter
referred to as EPA Inventory (2021): Ch. 6—LULUCF.
5 T he total land area of the United States is approximately 936 million hectares (2.3 billion acres, not including the U.S.
territories). Estimates for U.S. forestland area vary primarily based on how forestland is defined. See for example, S.
Oswalt et al., Forest Resources of the United States (FROTUS), 2017 , USDA Forest Service, GT O-WO-97, March
2019, estimating 310 million hectares (766 million acres) of forestland. In contrast, the EPA Inventory defines forest
area more narrowly and estimates 289 million hectares (714 million acres) of managed and unmanaged forestland in
2019 (EPA Inventory (2021): Ch. 6—LULUCF, p. 10).
6 T he forests in the Pacific Northwest and Great Lakes regions contain the highest carbon density in the conterminous
United States, though the distribution of carbon across the different pools varies between those regions (see Barry
Wilson et al., “Imputing Forest Carbon Stock Estimates from Inventory Plots to a Nationally Continuous Coverage,”
Carbon Balance and Managem ent, vol. 8, no. 1 [2013]). T he forests in Alaska also are estimated to contain significant
stocks of carbon (see U.S. Geological Survey, Baseline and Projected Future Carbon Storage and Greenhouse-Gas
Fluxes in Ecosystem s of Alaska, professional paper 1826, 2016, and EPA Inventory (2021): Ch. 6—LULUCF, p. 27).
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U.S. Forest Carbon Data: In Brief

Figure 2. U.S. Forest Carbon Stocks by Pool
(bil ion metric tons [BMT] of carbon [C])

Source: Data from EPA, Table 6-10 in Chapter 6, “Land Use, Land-Use Change, and Forestry,” U.S. National
Greenhouse Gas Inventory, April 2021.
Notes: Harvested wood products (HWPs) includes both HWPs in use and HWPs in disposal sites.

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Table 1. U.S. Forest Carbon Stocks by Pool
(mil ion metric tons [MMT] of carbon [C])
2020
Pool
1990
1995
2000
2005
2010
2015
MMT C
% of Total
Forest Ecosystem
50,913
51,808
52,681
53,489
54,302
55,125
55,933
95%
Soil (Mineral and Organic)
31,079
31,078
31,078
31,081
31,083
31,081
31,080
53%
Aboveground Biomass
11,810
12,424
13,019
13,584
14,144
14,707
15,260
26%
Litter
3,656
3,665
3,673
3,655
3,645
3,644
3,638
6%
Belowground Biomass
2,319
2,459
2,594
2,723
2,851
2,979
3,103
5%
Deadwood
2,049
2,182
2,316
2,446
2,580
2,716
2,852
5%
Harvested Wood Products (HWP)
1,895
2,061
2,218
2,353
2,462
2,567
2,669
5%
HWP in Use
1,249
1,326
1,395
1,447
1,471
1,490
1,532
3%
HWP in Disposal
646
735
823
906
991
1,076
1,167
2%
Total C Stock
52,808
53,870
54,899
55,842
56,764
57,692
58,632
100%
Sources: Data from EPA, Table 6-10 in Chapter 6, “Land Use, Land-Use Change, and Forestry,” U.S. National Greenhouse Gas Inventory, April 2021.
Notes: Data reflect carbon stocks for managed forestland remaining forestland in Alaska and the conterminous 48 states and do not in clude Hawai or the U.S.
territories. The EPA inventory (2021) reported changes to figures across the entire time series (1990-2020), related to updated and refined methodologies and
correcting for errors in previous iterations (see pp. 35-37 for more information). Consequently, figures reported here may not be consistent with figures reported in
earlier versions of this report or other CRS products. The years were selected to show carbon stocks over time at regular intervals; 2020 is the most recent year for
which data are available. Columns may not add due to rounding.

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Carbon Emissions and Sinks from U.S. Forests
Carbon flux is the net annual change in carbon stocks. The flux estimate for any given year (e.g.,
2018) is the change between stock estimates for that year (2018) and the following year (2019).
Negative flux values indicate more carbon was removed from the atmosphere and sequestered
than was released in that year (e.g., net carbon sink); net negative flux is typical y cal ed net
sequestration (or sometimes just sequestration). Positive flux values indicate more carbon was
released than was sequestered in that year (e.g., net carbon source).
According to the Inventory, U.S. forests were a net carbon sink in 2019, having sequestered 791
MMT CO2 equivalents (or 216 MMT of carbon) that year (see Figure 3 for net sequestration by
MMT CO2 equivalents, Table 2 for flux data by MMT CO2 equivalents, and Table 3 for flux data
by MMT of carbon).7 This total represents an offset of approximately 12% of the gross
greenhouse gas emissions from the United States in 2019.8
The net sink reflects carbon accumulation on existing forestland and carbon accumulation
associated with land converted to forestland within the past 20 years. Most of the sink is
associated with existing forests (85%). Within the carbon pools, most of the flux is associated
with aboveground biomass (57%). The carbon flux into the living biomass pools (above- and
belowground) reflects net carbon accumulation from the atmosphere. The carbon flux into the
other pools represents the movement of carbon from the living biomass pools into the nonliving
pools (e.g., deadwood, litter), primarily through the decomposition process.
Although soils store significant amounts of carbon, the carbon accumulates slowly over long
periods of time, so the annual flux is minimal. In some years, soils are a net source of carbon to
the atmosphere. In some years, litter may be a net source to the atmosphere, particularly in years
of increased wildfire activity. Overal , the annual net flux of carbon into U.S. forests is smal
relative to the amount of carbon forests store. For example, U.S. forests gained an additional 216
MMT of carbon between 2019 and 2020, but that represents only a 0.4% increase to the total
forest carbon stock (58.7 BMT of carbon). In addition, the total stock of carbon stored in forests
is equivalent to the sum of several decades of U.S. greenhouse gas emissions.9
From 1990 to 2019, U.S. forests were a net carbon sink. However, the net amount of carbon
sequestered by U.S. forests varies annual y. As stated earlier, interannual variation depends
largely on the size, duration, and severity of unplanned disturbances, which disrupt forest
ecosystems. For example, wildfire activity in Alaska drives a significant portion of the
interannual variability, due in part to fluctuations in the size of the area in the state affected by
wildfire each year and because more of the carbon in Alaska is stored in pools (e.g., litter) that are
likely to be combusted in a fire as compared to other states.10 Other factors influencing the net
flux of carbon in U.S. forests over the time series include management activities (e.g., timber
harvests) and land use trends (e.g., afforestation or deforestation).11

7 EPA Inventory (2021): Ch. 6—LULUCF.
8 In 2019, gross U.S. greenhouse gas emissions were 6.56 billion metric tons of CO2 equivalents, not including any
emissions related to the LULUCF sector (T able 2-1, EPA Inventory (2021): Ch. 2: Trends in Greenhouse Gas
Em issions, p. 5).
9 David N. Wear and John W. Coulston, “From Sink to Source: Regional Variation in U.S. Forest Carbon Futures,”
Scientific Reports, vol. 5, no. 16518 (2015). Hereinafter referred to as Wear and Coulston, 2015.
10 EPA Inventory (2021): Ch. 6—LULUCF, p. 27.
11 Afforestation is the conversion of non-forestland to forest; deforestation is the conversion of forestland to non -
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U.S. Forest Carbon Data: In Brief

Although the Inventory reflects the net carbon flux associated with forest disturbances through
annual changes in the carbon stock, recent iterations of the Inventory also have included the
estimated emissions specifical y associated with wildfires. The Inventory reports that wildfires,
including prescribed fires, resulted in emissions of 142 MMT CO2 equivalents in 2018, the most
recent year available.12 Annual forest carbon emissions from wildfire vary significantly, because
wildfire activity varies annual y. For example, the Inventory reports that wildfire-related
emissions in the previous year (2005) were significantly lower: 74 MMT CO2 equivalents.
Figure 3. U.S. Forest Carbon Sequestration by Pool
(mil ion metric tons [MMT] of carbon dioxide equivalents [CO2 eq.] per year)

Source: Data from EPA, Tables 6-8 and 6-24 in Chapter 6, “Land Use, Land-Use Change, and Forestry,” U.S.
National Greenhouse Gas Inventory, April, 2021.
Notes: Harvested wood products (HWPs) includes both HWPs in use and HWPs in disposal sites. The figure
reflects the net amount of carbon sequestered by forests, after accounting for the amount of carbon released by
forests in that year (e.g., the net amount of the carbon sink). Because this figure reflects net carbon
sequestration—and not carbon flux as in the fol owing tables—the values on the x-axis are positive numbers. Soil
was a net source of carbon in 1990 (<1 MMT CO2 eq/yr), and litter was a net source of carbon in 2005 (26 MMT
CO2 eq/yr) and 2015 (10 MMT CO2 eq/yr); they are not reflected in the bars above. Data reflect sequestration
estimates for forest remaining forestland and land converted to forestland (forest ecosystem pools only) for
managed forestland in Alaska and the conterminous 48 states and do not include Hawai or the U.S. territories.
The years were selected to show carbon sequestration rates over time at regular intervals; 1990 is the first year
data are available, and 2019 is the most recent year data are available.

forestland. For more information on the impacts of land use change on forest carbon, see CRS Report R46312, Forest
Carbon Prim er, by Katie Hoover and Anne A. Riddle.
12 T able 6-11 and T able 6-15 in EPA Inventory (2021): Ch. 6—LULUCF.
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Table 2. U.S. Forest Carbon Flux by Pool, Carbon Dioxide Equivalents
(mil ion metric tons [MMT] per year, CO2 equivalents)
2019
Carbon Pool
1990
1995
2000
2005
2010
2015
CO2 Eq.
% of Total
Forest Ecosystem
-762
-745
-723
-654
-711
-682
-682
86%
Aboveground Biomass
-511
-497
-481
-456
-470
-469
-449
57%
Deadwood
-109
-110
-110
-105
-112
-111
-111
14%
Belowground Biomass
-114
-111
-108
-103
-105
-103
-100
13%
Litter
-28
-27
-23
12
-20
10
-21
3%
Soil
1
0.03
-1
-2
-4
-9
-2
0%
Harvested Wood Products (HWP)
-124
-112
-93
-106
-69
-89
-109
14%
HWP in Disposal
-69
-61
-62
-63
-62
-64
-69
9%
HWP in Use
-55
-52
-32
-43
-7
-25
-39
5%
Total Carbon Flux
-886
-858
-816
-760
-780
-770
-791
—
Sources: Data from EPA, Tables 6-10 and 6-26 in Chapter 6, “Land Use, Land-Use Change, and Forestry,” in U.S. National Greenhouse Gas Inventory, April, 2021.
Notes: Negative flux values indicate more carbon was removed than was released in that year (e.g., carbon sink)—or net sequestration; positive flux values indicate
more carbon was released than was removed in that year (e.g., carbon source). Data reported to the hundredths only for values between -1 and 1. Data reflect flux
estimates for forest remaining forestland and land converted to forestland (forest ecosystem pools only) for managed forestland in Alaska and the conterminous 48
states and do not include Hawai or the U.S. territories. The EPA inventory (2021) reported changes to figures across the entire time series (1990-2020), related to
updated and refined methodologies and correcting for errors in previous iterations (see pp. 35-37 for more information). Consequently, figures reported here may not
be consistent with figures reported in earlier versions of this report or other CRS products. The years were selected to show carbon stocks over time at regular
intervals; 1990 is the first year data are available, and 2019 is the most recent year data are available. Columns may not add due to rounding.

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Table 3. U.S. Forest Carbon Flux by Pool
(mil ion metric tons [MMT] of carbon [C] per year)
2019
Carbon Pool
1990
1995
2000
2005
2010
2015
MMT C
% of Total
Forest Ecosystem
-208
-203
-197
-178
-194
-186
-186
86%
Aboveground Biomass
-139
-136
-131
-124
-128
-128
-122
57%
Deadwood
-30
-30
-30
-29
-30
-30
-30
14%
Belowground Biomass
-31
-30
-29
-28
-29
-28
-27
13%
Litter
-8
-7
-6
3
-5
3
-6
3%
Soil
0
0
0
-1
-1
-2
-1
0%
Harvested Wood Products (HWP)
-34
-31
-26
-29
-19
-24
-30
14%
HWP in Disposal
-19
-17
-17
-17
-17
-18
-19
9%
HWP in Use
-15
-14
-9
-12
-2
-7
-11
5%
Total Carbon Flux
-242
-234
-223
-207
-213
-210
-216
—
Sources: Data from EPA, Tables 6-9 and 6-25 in Chapter 6, “Land Use, Land-Use Change, and Forestry,” in U.S. National Greenhouse Gas Inventory, April, 2021.
Notes: Negative flux values indicate more carbon was removed than was released in that year (e.g., carbon sink); positive flux values indicate more carbon was released
than was removed in that year (e.g., carbon source). Data reflect flux estimates for forest remaining forestland and land converted to forestland (forest ecosystem pools
only) for managed forestland in Alaska and the conterminous 48 states and do not include Hawai or the U.S. territories. The EPA inventory (2021) reported changes to
figures across the entire time series (1990-2020), related to updated and refined methodologies and correcting for errors in previous iterations (see pp. 35-37 for more
information). Consequently, figures reported here may not be consistent with figures reported in earlier versions of this report or other CRS products. The years were
selected to show carbon stocks over time at regular intervals; 1990 is the first year data are available, and 2019 is the most recent year data are available. Columns may
not add due to rounding.

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U.S. Forest Carbon Data: In Brief

Author Information

Katie Hoover
Anne A. Riddle
Specialist in Natural Resources Policy
Analyst in Natural Resources 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 n ot 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
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