U.S. Forest Carbon Data: In Brief



U.S. Forest Carbon Data: In Brief
Updated June 6, 2023
Congressional Research Service
https://crsreports.congress.gov
R46313
Congressional Research Service



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Contents
Introduction ..................................................................................................................................... 1
U.S. Forest Carbon Stocks............................................................................................................... 3
Carbon Emissions and Sinks from U.S. Forests .............................................................................. 5
Wildfire Emissions .................................................................................................................... 9

Figures
Figure 1. Carbon Terms and Units ................................................................................................... 2
Figure 2. U.S. Forest Carbon Stocks by Pool .................................................................................. 3
Figure 3. U.S. Forest Carbon Sequestration by Pool ....................................................................... 5

Tables
Table 1. U.S. Forest Carbon Stocks by Pool ................................................................................... 4
Table 2. U.S. Forest Carbon Flux by Pool, Carbon Dioxide Equivalents ....................................... 7
Table 3. U.S. Forest Carbon Flux by Pool ....................................................................................... 8
Table 4. Wildfire Emissions and Acres Impacted, 2017-2021 ......................................................... 9

Contacts
Author Information .......................................................................................................................... 9

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., 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 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. 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
accounting of carbon in U.S. forests in the Land Use, Land-Use Change, and Forestry (LULUCF)

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

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
See Figure 1 for terms and units for measuring and reporting carbon. 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
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.).

2 Environmental Protection Agency (EPA), Inventory of U.S. Greenhouse Gas Emissions and Sinks, 1990-2021,
EPA430-R-23-002, April 2023. Hereinafter referred to as EPA Inventory (2023).
3 The EPA Inventory (2023) 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 9—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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U.S. Forest Carbon Data: In Brief

U.S. Forest Carbon Stocks
According to the Inventory, U.S. forests stored 59.7 billion metric tons (BMT) of carbon in 2022
(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 52% of total forest
carbon in 2022. The next-largest pool was aboveground biomass, which contained approximately
27% of the total. Each of the other pools stored 7% or less of the total carbon.
Figure 2. U.S. Forest Carbon Stocks by Pool

Source: Data from EPA, Table 6-10 in Chapter 6, “Land Use, Land-Use Change, and Forestry,” U.S. National
Greenhouse Gas Inventory
, April 2023.
Notes: Harvested wood products (HWPs) includes both HWPs in use and HWPs in disposal sites.
Since 1990, U.S. forest carbon stocks have increased 12%. Nearly all forest pools have gained
more carbon as of 2022. 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, “Land Use, Land-Use Change, and Forestry (LULUCF),” in EPA Inventory, 2023. Hereinafter referred to
as EPA Inventory (2023): Ch. 6—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). The EPA Inventory estimates 290 million hectares (716 million acres) of managed and unmanaged
forestland in 2020 (EPA Inventory (2023): Ch. 6—LULUCF, p. 10). Estimates for U.S. forestland area vary, based on
how forestland is defined; other sources estimate 310 million hectares (766 million acres) of forestland. For more
information, see CRS Report R46976, U.S. Forest Ownership and Management: Background and Issues for Congress.
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., “Imputing Forest Carbon Stock Estimates from Inventory Plots to a Nationally Continuous Coverage,”
Carbon Balance and Management, vol. 8, no. 1 [2013]). The forests in Alaska also are estimated to contain significant
stocks of carbon.
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Table 1. U.S. Forest Carbon Stocks by Pool
(million metric tons [MMT] of carbon [C])
2022
Pool
1990
1995
2000
2005
2010
2015
2020
MMT C
% of Total
Forest Ecosystem
51,354
52,308
53,237
54,098
54,952
55,810
56,623
56,951
95%
Soil (Mineral and Organic)
31,235
31,232
31,230
31,229
31,229
31,232
31,232
31,233
52%
Aboveground Biomass
11,899
12,573
13,226
13,849
14,453
15,051
15,635
15,861
27%
Litter
3,929
3,936
3,943
3,922
3,913
3,911
3,888
3,888
7%
Belowground Biomass
2,344
2,481
2,614
2,740
2,862
2,982
3,098
3,143
5%
Deadwood
1,948
2,086
2,224
2,359
2,496
2,634
2,771
2,827
5%
Harvested Wood Products (HWP)
1,895
2,061
2,218
2,353
2,462
2,567
2,695
2,749
5%
HWP in Use
1,249
1,326
1,395
1,447
1,471
1,490
1,530
1,549
3%
HWP in Disposal
646
735
823
906
991
1,076
1,165
1,200
2%
Total C Stock
53,249
54,369
55,455
56,451
57,414
58,376
59,318
59,701
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 2023.
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 (2023) reported changes to figures across the entire time series (1990-2022), related to updated and refined methodologies and
correcting for errors in previous iterations (see pp. 38-40 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; 2022 is the most recent year for
which data are available. Columns may not add due to rounding.

CRS-4

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U.S. Forest Carbon Data: In Brief

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.,
2020) is the change between stock estimates for that year (2020) and the following year (2021).
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 2021, having sequestered 794
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 13% of the gross
greenhouse gas emissions from the United States in 2021.8
Figure 3. U.S. Forest Carbon Sequestration by Pool

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 2023.
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.
Litter was a net source of carbon in two years: 2005 (2 MMT CO2 eq/yr) and 2015 (32 MMT CO2 eq/yr); 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 2021 is the most recent year data are available.
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

7 EPA Inventory (2023): Ch. 6—LULUCF.
8 In 2021, gross U.S. greenhouse gas emissions were 6.34 billion metric tons of CO2 equivalents, not including any
emissions related to the LULUCF sector (Table 2-1, EPA Inventory (2023): Ch. 2: Trends in Greenhouse Gas
Emissions,
p. 5).
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U.S. Forest Carbon Data: In Brief

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 216
MMT of carbon between 2020 and 2021, but that represents only a 0.4% increase to the total
forest carbon stock (59.7 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 2021, 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


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 (2023): Ch. 6—LULUCF, p. 29.
11 Afforestation is the conversion of non-forestland to forest; deforestation is the conversion of forestland to non-
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.
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Table 2. U.S. Forest Carbon Flux by Pool, Carbon Dioxide Equivalents
(million metric tons [MMT] per year, CO2 equivalents)
2021
% of
Carbon Pool
1990
1995
2000
2005
2010
2015
2020
CO2 Eq.
Total
Forest Ecosystem
-796
-789
-763
-707
-727
-645
-709
-691
87%
Aboveground Biomass
-555
-540
-524
-499
-496
-481
-474
-464
58%
Deadwood
-112
-109
-105
-100
-99
-96
-93
-91
12%
Belowground Biomass
-112
-113
-113
-110
-113
-109
-114
-113
14%
Litter
-19
-28
-22
2
-18
38
-22
-18
2%
Soil
2
1
1
0
-1
2
-6
-4
1%
Harvested Wood Products (HWP)
-124
-112
-93
-106
-69
-92
-97
-103
13%
HWP in Disposal
-55
-52
-32
-43
-7
-27
-32
-38
5%
HWP in Use
-69
-61
-62
-63
-62
-64
-65
-65
8%
Total Carbon Flux
-920
-901
-857
-813
-796
-737
-806
-794

Sources: Data from EPA, Tables 6-8 and 6-24 in Chapter 6, “Land Use, Land-Use Change, and Forestry,” in U.S. National Greenhouse Gas Inventory, April 2023.
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 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 (2023) 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. 38-40 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 2021 is the
most recent year flux data are available. Columns may not add due to rounding.


CRS-7


Table 3. U.S. Forest Carbon Flux by Pool
(million metric tons [MMT] of carbon [C] per year)
2021
Carbon Pool
1990
1995
2000
2005
2010
2015
2020
MMT C
% of Total
Forest Ecosystem
-217
-215
-208
-193
-198
-176
-193
-188
87%
Aboveground Biomass
-151
-147
-143
-136
-135
-131
-129
-127
59%
Deadwood
-31
-30
-29
-27
-27
-26
-26
-25
12%
Belowground Biomass
-31
-31
-31
-30
-31
-30
-31
-31
14%
Litter
-5
-8
-6
1
-5
10
-6
-5
2%
Soil
1
1
0
0
0
1
-2
-1
1%
Harvested Wood Products (HWP)
-34
-31
-26
-29
-19
-25
-26
-28
13%
HWP in Disposal
-15
-14
-9
-12
-2
-7
-9
-10
5%
HWP in Use
-19
-17
-17
-17
-17
-18
-18
-18
8%
Total Carbon Flux
-251
-246
-234
-222
-217
-201
-220
-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 2023.
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 (2023) 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; 1990 is the first year data are available, and 2021 is the most recent year flux data are available. Columns
may not add due to rounding.


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Wildfire Emissions
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 estimated emissions of 227 MMT CO2 equivalents in
2021.12 Annual emissions from wildfire vary significantly, because wildfire activity varies
annually, as shown in Table 4. Wildfire emissions are driven by the fire’s severity, intensity, and
location, among other factors.
Table 4. Wildfire Emissions and Acres Impacted, 2017-2021

2017
2018
2019
2020
2021
Mil ion metric tons [MMT] per year, CO2 equivalents
138.5
106.0
208.2
205.8
227.0
Mil ions of acres impacted by wildfire
10.0
8.8
4.7
10.1
7.1
Source: Emissions data from EPA, Tables 6-11 and 6-15 in Chapter 6, “Land Use, Land-Use Change, and
Forestry,” in U.S. National Greenhouse Gas Inventory, April 2023. Wildfire data from National Interagency
Coordination Center, Wildland Fire Summary and Statistics, annual reports. For more information, see CRS In
Focus IF10244, Wildfire Statistics.
Notes: Emissions data reflects carbon dioxide (CO2) emissions and emissions from other greenhouse gases,
converted to CO2 equivalents. Wildfire location, severity, and intensity influence wildfire emissions in any given
year. Wildfire acres impacted (e.g., burned) do not reflect the degree of impact to humans and communities or
the degree of impact to forests, soils, and other ecological effects. Further, wildfire response decisions may
influence the number of acres impacted in any given year. For example, wildfire response strategies range from
aggressive suppression to monitoring with minimal to no human intervention, which can allow wildfires to grow
in size before natural forces (e.g., weather) create conditions that cause the wildfire to extinguish. In 2019, more
than half the acreage impacted that year occurred in Alaska, most of which were the result of a limited
suppression response.


Author Information

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



12 Table 6-11 (reporting estimates for CO2 emissions) and Table 6-15 (reporting estimates for non-CO2 emissions) in
EPA Inventory (2023): Ch. 6—LULUCF.
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U.S. Forest Carbon Data: In Brief



Disclaimer
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