U.S. Forest Carbon Data: In Brief
Updated July 26, 2022
Congressional Research Service
https://crsreports.congress.gov
R46313
Congressional Research Service

U.S. Forest Carbon Data: In Brief

Introduction
The fluxâ€”or flowâ€”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., fallen leaves and stems), and soils. As trees or parts of trees die, the carbon
cycles through those different pools, specifically 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 millennia (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 eventually will return to the atmosphere upon the
wood productsâ€™ 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. Generally, disturbances result in tree
mortality, causing the transfer of carbon from the living pools to the deadwood, litter, soil, and
product pools, and/or eventually to the atmosphere. If a disturbed site regenerates as forest, the
carbon releases caused by the disturbance generally 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., â€œExecutive Summary,â€ 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 well
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, 1990-2020,
EPA430-R-22-003, April, 2022. Hereinafter referred to as EPA Inventory (2022).
3 The EPA Inventory (2022) reported changes to figures across the entire time series (1990-2021), related to updated
and refined methodologies and correcting for errors in previous iterations (see Chapter 9â€”Recalculations and
Improvements 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 61.0 billion metric tons (BMT) of carbon in 2021
(see Figure 2 and Table 1).4 The majority of forest carbon was stored in the forest ecosystem
pools (96%); 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 54% of total forest
carbon in 2021. The next-largest pool was aboveground biomass, which contained approximately
26% of the total. Each of the other pools stored 6% or less of the total carbon.
Since 1990, U.S. forest carbon stocks have increased 10%. Nearly all forest pools have gained
more carbon as of 2021. The exceptions are the litter and soil pools, which each continue to store
around the same amount of carbon for each year of reported data. Although forest carbon stocks
have increased, the rate of increase has slowed across recent years.
Since 1990, the size of U.S. forests has remained mostly constant. 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, â€œLand Use, Land-Use Change, and Forestry (LULUCF),â€ in EPA Inventory, 2022, April, 2022. Hereinafter
referred to as EPA Inventory (2022): Ch. 6â€”LULUCF.
5 The 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, GTO-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
2020 (EPA Inventory (2022): Ch. 6â€”LULUCF, p. 10). For more information, see CRS Report R46976, U.S. Forest
Ownership and Management: Background and Issues for Congress, by Katie Hoover and Anne A. Riddle.
6 The 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., â€œImputing Forest Carbon Stock Estimates from Inventory Plots to a Nationally Continuous Coverage,â€
Carbon Balance and Management, vol. 8, no. 1 [2013]). The 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 Ecosystems of Alaska, professional paper 1826, 2016, and EPA Inventory (2022): Ch. 6â€”LULUCF, p. 28).
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U.S. Forest Carbon Data: In Brief

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

Source: Data from EPA, Table 6-10 in Chapter 6, â€œLand Use, Land-Use Change, and Forestry,â€ U.S. National
Greenhouse Gas Inventory, April 2022.
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
(million metric tons [MMT] of carbon [C])
2021
Pool
1990
1995
2000
2005
2010
2015
2020
MMT C
% of Total
Forest Ecosystem
53,148
54,039
54,909
55,721
56,538
57,369
58,156
58,316
96%
Soil (Mineral and Organic)
32,813
32,811
32,810
32,810
32,811
32,816
32,814
32,816
54%
Aboveground Biomass
12,062
12,687
13,294
13,874
14,445
15,020
15,579
15,688
26%
Litter
3,838
3,845
3,852
3,834
3,829
3,828
3,809
3,810
6%
Belowground Biomass
2,375
2,502
2,625
2,743
2,858
2,973
3,085
3,106
5%
Deadwood
2,060
2,194
2,328
2,460
2,595
2,732
2,868
2,896
5%
Harvested Wood Products (HWP)
1,895
2,061
2,218
2,353
2,462
2,567
2,695
2,718
4%
HWP in Use
1,249
1,326
1,395
1,447
1,471
1,490
1,530
1,536
3%
HWP in Disposal
646
735
823
906
991
1,076
1,165
1,182
2%
Total C Stock
55,043
56,100
57,128
58,074
59,000
59,936
60,851
61,034
100%
Sources: Data from EPA, Table 6-10 in Chapter 6, â€œLand Use, Land-Use Change, and Forestry,â€ U.S. National Greenhouse Gas Inventory, April 2022.
Notes: Data reflect carbon stocks for managed forestland remaining forestland in Alaska and the conterminous 48 states and do not include Hawaii or the U.S.
territories. The EPA inventory (2022) reported changes to figures across the entire time series (1990-2021), 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; 2021 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.,
2019) is the change between stock estimates for that year (2019) and the following year (2020).
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 typically called 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 2020, having sequestered 767
MMT CO2 equivalents (or 209 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 13% of the gross
greenhouse gas emissions from the United States in 2020.8
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 (59%). 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. Overall, the annual net flux of carbon into U.S. forests is small
relative to the amount of carbon forests store. For example, U.S. forests gained an additional 209
MMT of carbon between 2019 and 2020, but that represents only a 0.3% increase to the total
forest carbon stock (61.0 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 2020, U.S. forests were a net carbon sink. However, the net amount of carbon
sequestered by U.S. forests varies annually. 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 (2022): Ch. 6â€”LULUCF.
8 In 2020, gross U.S. greenhouse gas emissions were 5.98 billion metric tons of CO2 equivalents, not including any
emissions related to the LULUCF sector (Table 2-1, EPA Inventory (2022): Ch. 2: Trends in Greenhouse Gas
Emissions, p. 5).
9 David N. Wear and John W. Coulston, â€œFrom Sink to Source: Regional Variation in U.S. Forest Carbon Futures,â€
Scientific Reports, vol. 5, no. 16518 (2015). Hereinafter referred to as Wear and Coulston, 2015.
10 EPA Inventory (2022): Ch. 6â€”LULUCF, p. 28.
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 specifically associated with wildfires. The Inventory reports that wildfires,
including prescribed fires, resulted in emissions of 237 MMT CO2 equivalents in 2020.12 Annual
emissions from wildfire vary significantly, because wildfire activity varies annually. For example,
the Inventory reports that wildfire-related emissions in the previous year (2019) were significantly
lower: 83 MMT CO2 equivalents.
Figure 3. U.S. Forest Carbon Sequestration by Pool
(million metric tons [MMT] of carbon dioxide equivalents [CO2 eq.] per year)

Source: Data from EPA, Tables 6-8 and 6-24 in Chapter 6, â€œLand Use, Land-Use Change, and Forestry,â€ U.S.
National Greenhouse Gas Inventory, April, 2022.
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
sequestrationâ€”and not carbon flux as in the fol owing tablesâ€”the values on the x-axis are positive numbers.
Litter was a net source of carbon in 2015 (32 MMT CO2 eq/yr) and soil was a net source of carbon in four years:
1990 (2 MMT CO2 eq/yr), 1995 (1 MMT CO2 eq/yr), 2000 (<1 MMT CO2 eq/yr), and 2015 (2 MMT CO2 eq/yr);
these figures 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 Hawaii 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 2020 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 Primer, by Katie Hoover and Anne A. Riddle.
12 Table 6-11 and Table 6-15 in EPA Inventory (2022): Ch. 6â€”LULUCF.
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Table 2. U.S. Forest Carbon Flux by Pool, Carbon Dioxide Equivalents
(million metric tons [MMT] per year, CO2 equivalents)
2020
Carbon Pool
1990
1995
2000
2005
2010
2015
CO2 Eq.
% of Total
Forest Ecosystem
-749
-745
-721
-680
-706
-636
-684
89%
Aboveground Biomass
-518
-507
-491
-472
-477
-468
-455
59%
Deadwood
-105
-102
-99
-95
-95
-93
-90
12%
Belowground Biomass
-109
-110
-110
-109
-112
-109
-113
15%
Litter
-19
-27
-21
-4
-19
32
-22
3%
Soil
2
1
0
-1
-2
2
-4
1%
Harvested Wood Products (HWP)
-124
-112
-93
-106
-69
-92
-84
11%
HWP in Disposal
-55
-52
-32
-43
-7
-27
-20
3%
HWP in Use
-69
-61
-62
-63
-62
-64
-64
8%
Total Carbon Flux
-872
-857
-815
-786
-775
-728
-767
â€”
Sources: Data from EPA, Tables 6-10 and 6-26 in Chapter 6, â€œLand Use, Land-Use Change, and Forestry,â€ in U.S. National Greenhouse Gas Inventory, April, 2022.
Notes: Negative flux values indicate more carbon was removed than was released in that year (e.g., carbon sink)â€”or net sequestration; 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 Hawaii or the U.S. territories. The EPA
inventory (2022) 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 2020 is the
most recent year flux data are available. Columns may not add due to rounding.

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Table 3. U.S. Forest Carbon Flux by Pool
(million metric tons [MMT] of carbon [C] per year)
2019
Carbon Pool
1990
1995
2000
2005
2010
2015
MMT C
% of Total
Forest Ecosystem
-204
-203
-197
-186
-193
-174
-186
89%
Aboveground Biomass
-141
-138
-134
-129
-130
-128
-124
59%
Deadwood
-29
-28
-27
-26
-26
-25
-25
12%
Belowground Biomass
-30
-30
-30
-30
-31
-30
-31
15%
Litter
-5
-7
-6
-1
-5
9
-6
3%
Soil
1
0
0
0
-1
0
-1
1%
Harvested Wood Products (HWP)
-34
-31
-26
-29
-19
-25
-23
11%
HWP in Disposal
-15
-14
-9
-12
-2
-7
-6
3%
HWP in Use
-19
-17
-17
-17
-17
-18
-17
8%
Total Carbon Flux
-238
-234
-222
-214
-211
-199
-209
â€”
Sources: Data from EPA, Tables 6-9 and 6-25 in Chapter 6, â€œLand Use, Land-Use Change, and Forestry,â€ in U.S. National Greenhouse Gas Inventory, April, 2022.
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 Hawaii or the U.S. territories. The EPA inventory (2022) 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 2020 is the most recent year flux 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 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 CRSâ€™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
R46313 Â· VERSION 8 Â· UPDATED
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