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Carbon Dioxide Sinks

 A carbon dioxide sink or CO2 sink is a carbon reservoir that is increasing in size, and is the opposite of a carbon "source". The main natural sinks are the oceans and growing vegetation. Both remove carbon from the atmosphere by incorporating it into biomass such as plankton and trees.


Carbon sequestration is the term describing processes that remove carbon from the atmosphere. A variety of means of artificially capturing and storing carbon, as well as of enhancing natural sequestration processes, are being explored. This is intended to help mitigate global warming. Preservation of natural sinks is extremely important to the future overall health of our planet.



Enormous amounts of carbon are naturally stored in the forest by trees and other plants, as well as in the forest soil. As part of photosynthesis, plants absorb carbon dioxide from the atmosphere, store the carbon as sugar, starch and cellulose, while oxygen is released back into the atmosphere. A young forest, composed of rapidly growing trees, absorbs carbon dioxide, but the sink effect exists only when they grow in size. Furthermore, forests, particularly new ones, may not be straightforward carbon sinks. Although a forest is a net CO2 sink over time, the plantation of new forests may also initially be a source of carbon dioxide emission when carbon from the soil is released into the atmosphere.  Mature forests, made up of a mix of various aged trees as well as dead and decaying matter, may be carbon neutral above ground. In the soil, however, the gradual buildup of slowly decaying organic material will continue to accumulate carbon, thereby acting as a sink.

Over the life of an individual tree or other forest plant, the carbon capturing (sequestering) and releasing is neutral. As the plant grows carbon is absorbed from the atmosphere and then released back into the atmosphere as the plant matures, dies, and rots. Most forests are a mix of old and new trees or plants, and carbon is stored and released continuously depending on the plant and the phase of its life at the time. Also, a severe forest fire will quickly release absorbed carbon back into the atmosphere.

The dead trees, plants, and moss in peat bogs undergo slow anaerobic decomposition below the surface of the bog. This process is slow enough that in many cases the bog grows rapidly and fixes more carbon from the atmosphere than is released. Over time, the peat grows deeper. Peat bogs inter approximately one-quarter of the carbon stored in land plants and soils [1].

Under some conditions, forests and peat bogs may become sources of CO2. This can happen, for example, when a forest is flooded by the construction of a hydroelectric dam. The rotting vegetation is a source of CO2 and methane comparable in magnitude to the amount of carbon released by a fossil-fuel powered plant of equivalent power.


Oceans are natural carbon dioxide sinks, and are the largest active carbon sinks. As the level of carbon dioxide increases in the atmosphere, the level in the oceans also increases, creating potentially disastrous acidic oceans. Ocean water can hold a variable amount of dissolved CO2 depending on temperature and pressure. Phytoplankton in the oceans, like trees, use photosynthesis to extract carbon from CO2. They are the starting point of the marine food chain. Plankton and other marine organisms extract CO2 from the ocean water and convert it to the mineral calcite, CaCO3, to build their skeletons and shells. This removes CO2 from the water, allowing more to dissolve in from the atmosphere. These calcite skeletons and shells, along with the organic carbon of the organisms, eventually fall to the bottom of the ocean when the organisms die. The carbon or plankton cells have to sink to the deep water in 2000 to 4000 meters to be sequestered for ca. 1000 years.


The carbon-sequestration potential of soils (by increasing soil organic matter) is substantial; below-ground organic carbon storage is more than twice above-ground storage. Soils' organic carbon levels in many agricultural areas have been severely depleted. Improving the humus levels of these soils would both improve soil quality and increase the amount of carbon sequestered in these soils.

Grasslands contribute huge quantities of soil organic matter over time, mostly in the form of roots, and much of this organic matter can remain unoxidized for long periods. Since the 1850s, a large proportion of the world's grasslands have been tilled and converted to croplands, allowing the rapid oxidation of large quantities of soil organic carbon. No-till agricultural systems can increase the amount of carbon stored in soil, and conversion to pastureland, particularly with good management of grazing, can sequester even more carbon in the soil.

 Carbon Sinks and the Kyoto Protocol

The protocols hold that, since growing vegetation absorbs carbon dioxide, countries that have large areas of forest (or other vegetation) can deduct a certain amount from their emissions, thus making it easier for them to achieve the desired emission levels. The effectiveness of these provisions is controversial.

Some countries want to be able to trade in emission rights in carbon emission markets, to make it possible for one country to buy the benefit of carbon dioxide sinks in another country. It is said that such a market mechanism will help find cost-effective ways to reduce greenhouse emissions. There is as yet no carbon audit regime for all such markets globally, and none is specified in the Kyoto Protocol. Each nation is on its own to verify actual carbon emission reductions, and to account for carbon sequestration using some less formal method.

In the Clean Development Mechanism, only afforestation and reforestation are eligible to produce CERs (Clean Emissions Reductions)  in the first commitment period of the Kyoto Protocol (2008-2012). Forest conservation activities or activities avoiding deforestation, which would result in emission reduction through the conservation of existing carbon stocks, are not eligible at this time. Also agricultural carbon sequestration is not possible yet.

 Carbon Storage in the United States

Carbon dioxide in the atmosphere has been increasing steadily since at least 1958 (Keeling 1984). Predictions of future climate change as a consequence of increasing atmospheric carbon dioxide vary widely. Under a scenario of equivalent doubling of atmospheric carbon dioxide by the middle of the next century, most predictions show an increase in average global temperature of between 2 and 5 degrees centigrade and an increase in average global precipitation of between 7 and 15 percent (Schneider 1989). These prospective changes have generated interest in strategies to reduce emissions of carbon dioxide to the atmosphere, or to offset emissions by storing additional carbon in forests.

Across the entire Earth, the total amount of carbon in the atmosphere has been estimated at 720 billion metric tons, the total amount of carbon in terrestrial biomass is about 560 billion metric tons, and the total amount of carbon in terrestrial soils is about 1,500 billion metric tons (Solomon and others 1985). Although oceans store a far greater amount of carbon than terrestrial ecosystems, our ability to manage terrestrial ecosystems is greater and likely to have a greater mitigation effect.

Forest ecosystems in the United States contain approximately 57.8 billion tons (52.5 billion metric tons) of carbon above and below the ground. This is about 4 percent of all the carbon stored in the world's forests. The area of U.S. forests is 731 million acres, or 5 percent of the world's forest area.

The average forest in the United States contains 158 thousand pounds per acre (17.7 kg/m2) of organic carbon. Trees, including tree roots, account for 31 percent of all forest ecosystem carbon (fig. 2). Live and standing dead trees contain 17.7 billion tons (16.1 billion metric tons) of carbon, or an average of 49 thousand pounds per acre (5.5 kg/m2). Of this total, 51 percent is in live tree sections classified as growing stock volume, 24 percent is in other live solid wood above the ground, 17 percent is in the roots, 6 percent is in standing dead trees, and 3 percent is in the foliage.

The largest proportion of carbon in the average U.S. forest is found in the soil, which contains 59 percent of the carbon in the forest ecosystem, or approximately 93 thousand pounds per acre (10.4 kg/m2). About 9 percent of all carbon is found in litter, humus, and coarse woody debris on the forest floor, and about 1 percent is found in the understory vegetation. By adding carbon in tree roots to the carbon in the soil, the average proportion of carbon below the ground in the United States is estimated to be 64 percent.

Forest ecosystems are capable of storing large quantities of carbon in solid wood and other organic matter. Forests may add to the pool of carbon dioxide in the atmosphere through burning of forest lands, deforestation, or decomposition of wood products and byproducts. Forests may also reduce the amount of carbon dioxide in the atmosphere through increases in biomass and organic matter accumulation. Young, growing forests take up carbon at high rates, while carbon uptake in mature forests is balanced by carbon release from decaying vegetation. The end use of timber harvested from forests is an important factor in evaluating the contributions of forestry to the global carbon cycle. If the end uses of forest products are in long-term durable goods such as furniture or timber bridges, the carbon is stored in those materials. If the end use is for paper products that are rapidly used and discarded to decay, then the carbon is released to the atmosphere. Carbon in waste from the manufacturing process and discarded wood products may be sequestered in landfills for long periods of time. Because of the relation between forests and atmospheric carbon dioxide, there are opportunities to manage forests in ways that would result in storage of additional carbon and thus reduce atmospheric carbon dioxide. Major forestry opportunities include increasing forest area, increasing the productivity of existing forest lands, reducing forest burning and deforestation, increasing biomass production and utilization, planting trees in urban environments, and increasing use of wood in durable products.

Pacific Coast States, including Alaska, contain the highest average carbon in forest soils, 64 percent of the total. The lowest proportion of soil carbon is found in the Rocky Mountain States, with 49 percent of the total. Soil carbon is closely related to temperature and precipitation, with higher amounts of soil carbon found in regions with cooler temperatures and higher precipitation. The cooler temperatures slow the oxidation of soil carbon, while higher rainfall tends to produce more vegetation and thus the fine roots and litter that are the main sources of organic soil carbon.

Carbon in the forest floor varies by region in a way similar to carbon in the soil. Western and Northern States contain the most carbon on the forest floor, and Southern States contain the least.

There is a clear pattern of increasing forest carbon from Southern to Northern States . The two main factors are climate and average age of the forests. The cooler, wetter climates favor higher retention of carbon on the forest floor and in the soil, and northern forests tend to be older and less frequently disturbed than forests in the South.

Carbon Storage by Forest Type

There are significant differences in carbon storage among forest types. For example, selected eastern softwood types show large differences in total carbon storage and the relative storage by forest ecosystem component. Loblolly pine plantations are younger on average, so there is less carbon in the trees, and since they are mostly located in the South, the soil carbon is lower. Spruce - fir, common in the Northeast, has higher total carbon as a result of the large amount of carbon stored in the soil. Douglas - fir contains the highest average carbon because of the large quantity stored in the trees. Pinyon - juniper has the lowest amount of carbon because it occurs in dry climates that support lower vegetation densities.

Changes in Carbon Storage

U.S. forests are constantly changing. The total area of forest land declined by 4 million acres between 1977 and 1987 (Waddell and others 1989). Most of the loss was from forest clearing for urban and suburban development, highways, and other rights-of-way. Many more million acres were cleared for agricultural use, but this loss was roughly balanced by agricultural land that was planted with trees or allowed to revert naturally to forest. In addition to land-use changes, each year about 4 million acres of timberland are harvested for timber products and regenerated to forests, 4 million acres are damaged by wildfire, and 2.5 million acres are damaged by insects and diseases (estimates based on various unpublished Forest Service data sources). And of course, all forest lands change continually as trees and other vegetation germinate, grow, and die.

Changes in carbon storage in the forest ecosystem are primarily related to changes in carbon storage in live trees. The rate of accumulation of carbon in live trees is greatest in the forest areas where trees typically have the fastest volume growth, the Southeast and the Pacific Northwest. On average, live trees are accumulating carbon at a rate of 1,252 pounds per acre per year (0.14 kg/m2/yr), a rate of increase of 2.7 percent of the amount stored in live trees.

The accumulation of carbon in live and dead trees totals 508 million tons (461 million metric tons) per year, while the total removal of tree carbon from U.S. forests resulting from timber harvest, landclearing, and fuelwood use amounts to 391 million tons (355 million metric tons, fig. 8). A comparison of accumulation and removal suggests that U.S. forest trees are storing additional carbon at a rate of 117 million tons (106 million metric tons) per year. This is equivalent to about 9 percent of the annual U.S. emission of carbon to the atmosphere (1.2 billion metric tons) per year (Boden and others 1 990).

Trees dying annually because of insects, diseases, fire, and weather contain about 83 million tons (75 million metric tons) of carbon. Only a portion of tree mortality was deducted from accumulation in the comparison of accumulation and remov-al since much of the carbon remains in the forest ecosystem for some time as standing dead trees, coarse woody debris on the forest floor, and eventually other organic matter in the forest ecosystem.

There are significant regional differences in relative and total estimates of carbon accumulation, removal, and mortality. For softwoods, Pacific coast forests are accumulating the most carbon annually, followed by the Southeast, South Central, and Rocky Mountain regions (fig. 9). Because softwood removal is so low relative to growth in the Rocky Mountains, the increase in carbon storage in softwood species is much greater there than elsewhere. Mortality is the highest in the Rocky Mountains and on the Pacific coast. In the South Central region, tree removal is causing a net loss of carbon storage in softwood trees.

Most of the hardwood resource in located in the Eastern United States. The Northeast has the largest excess of hardwood carbon accumulation over removal, but there are also large increases in hardwood carbon storage occurring in the Southeast and on the Pacific coast (fig. 10).


Annual mortality--The volume of sound wood in tree died from natural causes during a specific year.

Annual removals--The net volume of trees removed from the inventory during a specified year by harvesting, cultural operations such as timber stand improvement, or land clearing.

Cull tree--A live tree, 5.0 inches in diameter at breast height (d.b.h.) or larger, that is unmerchantable for saw logs prospectively because of rot, roughness, or species. (See definitions for rotten and rough trees.)

Forest land--Land at least 10 percent stocked by trees of any size, including land that formerly had such tree cover and that will be naturally or artificially regenerated. Forest land includes transition zones, such as areas between heavily forested and nonforested lands that are at leas t 10 percent stocked with forest trees and forest areas adjacent to urban and built-up lands. Also included are pinyon-juniper and chaparral areas in the West and afforested areas. The minimum area for classification of forest land is 1 acre. Roadside, streamside, and shelterbelt strips of timber must have a crown width of at least 120 feet to qualify as forest land. Unimproved roads and trails, streams, and clearings in forest areas are classified as forest if less than 120 feet wide.

Forest type--A classification of forest land based on the species presently forming a plurality of the live-tree stocking.

Major eastern forest-type groups:

White-red-jack-pine--Forests in which eastern white pine, red pine, or jack pine, singly or in combination, make up a plurality of the stocking. Common associates include hemlock, aspen, birch, and maple.

Spruce-fir--Forests in which spruce or true firs, sir in combination, make up a plurality of the stocking. Common associates include white-cedar, tamarack, maple, birch, and hemlock.

Longleaf-slash pine--Forests in which longleaf or pine, singly or in combination, make up a plurality of stocking. Common associates include other southern pines, oak, and gum.

Loblolly-shortleaf pine--Forests in which loblolly shortleaf pine, or southern yellow pines, except longleaf or slash pine, singly or in combination, make up a plurality of the stocking. Common associates include oak, hickory and gum.

Oak-pine--Forests in which hardwoods (usually upland oaks) make up a plurality of the stocking, but in which pine or eastern redcedar makes up 25-50 percent of the stocking. Common associates include gum, hickory yellow-poplar.

Oak-hickory--Forests in which upland oaks or hickory, singly or in combination, make up a plurality of the stocking except where pines make up 25-50 percent, in which case the stand is classified as oak-pine. Common associates include yellow-poplar, elm, maple, and black walnut.

Oak-gum-cypress--Bottomland forests in which tupelo, blackgum, sweetgum, oaks, or southern cypress, singly or in combination, make up a plurality of the stocking except where pines make up 25-50 percent, in which case the stand is classified as oak-pine. Common associates include cottonwood, willow, ash, elm, hackberry, and maple.

Elm-ash-cottonwood--Forests in which elm, ash, or cottonwood, singly or in combination, make up a plurality of the stocking. Common associates include willow, sycamore, beech, and maple.

Maple-beech-birch--Forests in which maple, beech, or yellow birch, singly or in combination, make up a plurality of the stocking. Common associates include hemlock, elm, basswood, and white pine.

Aspen-birch--Forests in which aspen, balsam poplar, paper birch, or gray birch, singly or in combination, make up a plurality of the stocking. Common associates include maple and balsam fir.

Major western forest-type groups:

Douglas-fir--Forests in which Douglas-fir makes up plurality of the stocking. Common associates include western hemlock, western redcedar, the true firs, redwood, ponderosa pine, and larch.

Hemlock-Sitka spruce--Forests in which western hemlock or Sitka spruce, or both, make up a plurality of the stocking. Common associates include Douglas-fir, silver fir, and western redcedar.

Redwood--Forests in which redwood makes up a plurality of the stocking. Common associates include Douglas-fir, grand fir, and tanoak.

Ponderosa pine--Forests in which ponderosa pine makes up a plurality of the stocking. Common associates include Jeffrey pine, sugar pine, limber pine, Arizona pine, Apache pine, Chihuahua pine, Douglas-fir, incense-cedar, and white fir.

Western white pine--Forests in which western pine makes up a plurality of the stocking. Common associates include western redcedar, larch, white fir, Douglas-fir, lodgepole pine, and Engelmann spruce.

Lodgepole pine--forests in which lodgepole pine makes up a plurality of the stocking. Common associates include alpine fir, western white pine, Engelmann spruce, aspen, and larch.

Larch--Forests in which western larch makes up a' plurality of the stocking. Common associates include Douglas-fir, grand fir, western redcedar, and wester pine.

Fir-spruce--Forests in which true firs, Engelmann or Colorado blue spruce, singly or in combination, make up a plurality of the stocking. Common associates include mountain hemlock and lodgepole pine.

Western hardwoods--Forests in which aspen, red or other western hardwoods, singly or in combination make up a plurality of the stocking.

Pinyonjuniper--Forests in which pinyon pine or juniper, or both, make up a plurality of the stocking.

Growing stock--A classification of timber inventory that includes live trees of commercial species meeting specified standards of quality or vigor. Cull trees are excluded. When associated with volume, includes only trees 5.0 inches and larger.

Hardwood--A dicotyledonous tree, usually broad-leaved and deciduous.

Industrial wood--All commercial roundwood product except fuelwood.

Net annual growth--The net increase in the volume trees during a specified year. Components include the increment in net volume of trees at the beginning of the specific year surviving to its end, plus the net volume reaching the minimum size class during the year, minus the volume of trees that died during the year, and minus the volume of trees that became cull trees during the year.

Net volume in cubic feet--The gross volume in cubic feet less deductions for rot, roughness, and poor form. Volume is computed for the central stem from a 1-foot-high stump to the point where the diameter of the outside bark equals 4 inches, or to the point where the central stem breaks into limbs.

Nonstocked area--Timberland less than 10 percent stocked with growing stock trees.

Other forest land--Forest land other than timberland and reserved timberland. It includes available and reserve unproductive forest land that is incapable of producing. annually 20 cubic feet per acre of industrial wood under natural conditions because of adverse site conditions Such as sterile soils, dry climate, poor drainage, high elevation, steepness, or rockiness.

Other removals--Unutilized wood volume from cut or otherwise killed growing stock, from nongrowing stock sources on timberland (for example, precommercial thinnings), or from timberland clearing. Does not include volume removed from inventory through reclassification of timberland to reserved timberland.

Other sources--Sources of roundwood products that are nongrowing stock. These include salvable dead trees, rough and rotten trees, trees of noncommercial species, trees less than 5.0 inches d.b.h., tops, and roundwood harvested from nonforest land (for example, fence rows).

Productivity class--A classification of forest land in items of potential annual cubic-foot volume growth per acre at culmination of mean annual increment in fully stocked natural stands.

Reserved timberland--Forest land that would otherwise be classified as timberland except that it is withdrawn from timber utilization by statute or administrative regulation.

Rotten tree--A live tree of commercial species that does not contain a saw log now or prospectively primarily because of rot (that is, when rot accounts for more than 50 percent of the total cull volume).

Rough tree- (a) A live tree of commercial species that does not contain a saw log now or prospectively primarily because of roughness (that is, when sound cull due to such factors as poor form, splits, or cracks accounts for more than 50 percent of the total cull volume) or (b) a live tree of noncommercial species.

Softwood--A coniferous tree, usually evergreen, having needles or scalelike leaves.

Stocking--The degree of occupancy of lands by trees, measured by basal area or number of trees by size and spacing, or both, compared to a stocking standard; that is, the basal area or number of trees, or both, required to fully utilize the growth potential of the land.

Timberland--Forest land that is producing or is capable of producing crops of industrial wood and not withdrawn from timber utilization by statute or administrative regulation. (Note: Areas qualifying as timberland are capable of producing in excess of 20 cubic feet per acre per year of industrial wood in natural stands. Currently inaccessible and inoperable areas are included.)

Unreserved forest land--Forest land that is not withdrawn from use by statute or administrative regulation.

Weight--The weight of wood and bark, oven-dry basis (approximately 12 percent moisture content).


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The Registry of Nature Habitats
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