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Emissions of Greenhouse Gases Report
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Land Use
Overview | Forest Lands and Harvested Wood Pools | Croplands and Grasslands |
Urban Trees, Yard Trimmings, and Food Scraps |
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Overview |
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In 2007, land use, land-use change, and forests accounted for net carbon sequestration of 1,062.6 MMTCO2e (Table 30 below), representing 17.7 percent of total U.S. CO2 emissions.
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Net carbon sequestration from land use, land-use change, and forestry activities in 2007 was about 26 percent greater than in 1990 (Figure 29 on right). The increase resulted primarily from a higher average annual rate of net carbon accumulation in forest carbon stocks. |
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Sequestration from land use, land-use change, and forestry peaked in 2004 at 1,294.6 MMTCO2e. By 2006 it had fallen to 1,050.5 MMTCO2e, and in 2007 it rose slightly to 1,062.6 MMTCO2e. |
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Because forest land is the predominant category, its fluctuations drive the total. It is the category with the largest change since 1990 in absolute terms (249
MMTCO2e). |
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The largest percentage changes from 1990 to 2007 were
seen for urban trees, which increased by 61 percent,
and for grasslands remaining grasslands, which
decreased by 91 percent.
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Among the categories of estimated greenhouse gas
emissions, land-use change has a relatively high
degree of uncertainty. As discussed on the following
pages, the estimated values are highly dependent on
the estimation methods used. Thus, countries that submit
their emissions estimates to the UNFCCC generally
provide two estimates: without land-use change
(gross emissions) and with land-use change (net methane emissions). |
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figure data
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| Forest Lands and Harvested Wood Pools |
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Carbon sequestration attributed to forest land remaining forest land in 2007 totaled 910.1 MMTCO2e (Figure 30 on right and Table 31 below). According to the EPA, that total has a 95-percent probability of being between 736 and 1,083 MMTCO2e. |
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Changes in underlying data from the USDA Forest Service Forestry Inventory Analysis Database (FIADB) have led to changes in estimates across the time series. Most States have added new data or modified existing data in the inventory. |
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Version 3.0 of the FIADB includes more use of moving
averages, which affect extrapolations of stocks and stock changes. The major changes in estimates for
1990-2007 relative to 1990-2006 is a spike in the
2002-2006 estimates for forest carbon. |
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According to the USDA, average carbon densities have been updated from 331 million grams per hectare (Mg/ha) to 179 Mg/ha for Alaska and from 89 Mg/ha to 91 Mg/ha for the lower 48 States. |
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The largest changes for the 1990-2007 period are in the estimates for soil organic carbon and dead wood, which increased by 153 percent and 139 percent, respectively. |
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figure data
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| Croplands and Grasslands |
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For 2007, aggregate carbon flux for the four agri-cultural categories shown in Table 32 below was 45.1 MMTCO2e. |
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Land converted to grassland, cropland remaining cropland, and grassland remaining grassland sequestered 26.7, 19.7, and 4.7 MMTCO2e, respectively, in 2007. Land converted to cropland emitted 5.9 MMTCO2e (Figure 31 on right and Table 32 below). |
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In the USDA National Resources Inventory (NRI), land is classified as cropland remaining cropland in a given year if it has been used as cropland for 20 years. |
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In previous inventory reports, land use data for 1997 were used for all subsequent years; however, the information on land use from 1997 to 2003. In Table 32 below, the data for 2004-2007 are extrapolations, except for the data on liming of soils, which extend through 2006. |
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Annual area data (rather than 5-year increments) were used to estimate soil carbon stock changes for the current inventory, leading to more accurate estimates; and each NRI point was simulated separately, instead of simulating clusters of common cropping rotation histories and soil characteristics in a county. |
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NRI area data were reconciled with the forest area estimates in the FIADB data and incorporated into the estimation of soil carbon stock changes, leading to adjustments in grassland areas in the NRI data, including land converted to grassland. |
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| Urban Trees, Yard Trimmings, and Food Scraps |
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Urban trees, yard trimmings, and food scraps
sequestered 107.4 MMTCO2e in 2007 (Figure 32 on right and
Table 33 below). |
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Updated data from Municipal Solid Waste Generation,
Recycling, and Disposal in the United States: 2007 Facts
and Figures,12 used in this report—including revisions
to the amounts of food scraps generated in 2000
and 2004-2007—resulted in an average 1.0-percent
decrease in carbon storage across the time series for
yard trimmings and food scraps. |
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Carbon sequestration in yard trimmings showed the
largest change from 1990 to 2007, with a decrease of 70
percent attributable to an increase in municipal collection
and composting of yard trimmings. |
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figure data
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Notes and Sources |
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