Congress is considering several legislative strategies that would reduce U.S. emissions of greenhouse gases—primarily carbon dioxide (CO 2 )—and/or increase uptake and storage of CO 2 from the atmosphere. Both approaches are viewed by many observers as critical to forestalling global climate change caused, in part, by the buildup of greenhouse gases in the atmosphere from human activities. Others point out that the human contribution of carbon to the atmosphere is a small fraction of the total quantity of carbon that cycles naturally back and forth each year between the atmosphere and two huge carbon reservoirs: the global oceans and the planet's land surface. A key question is how CO 2 emissions from human activities are changing the global carbon cycle—the exchange, or flux, of carbon between the atmosphere, oceans, and land surface—and how the changes affect the rate of CO 2 buildup in the atmosphere. Most climate scientists agree that human perturbations to the carbon cycle are a main factor driving climate change over the past 50 years. For most of human history, the global carbon cycle has been roughly in balance, and the concentration of CO 2 in the atmosphere has been fairly constant at approximately 280 parts per million (ppm). Human activities, namely the burning of fossil fuels, deforestation, and other land use activities, have significantly altered the carbon cycle. As a result, atmospheric concentrations of CO 2 have risen by over 35% since the industrial revolution, and are now greater than 380 ppm. An understanding of the global carbon cycle has shifted from being of mainly academic interest to being also of policy interest. Policy makers are grappling with, for example, how to design a cap-and-trade system that accurately accounts for carbon sequestration by components of the land surface sink, such as forests or farmland. A cap-and-trade system that limits emissions, and is designed to keep atmospheric CO 2 below a specific concentration, would depend inherently on continued uptake of carbon by the oceans and land surface. Yet how much CO 2 forests or farmland are capable of taking up in the future, and for how long, is not clear. How the ocean and the land surface carbon reservoirs will behave in the future—how much CO 2 they will take up or release and at what rate—are topics of active scientific inquiry. Thus the scientific understanding of the carbon cycle is integral to many aspects of the current congressional debate over how to mitigate climate change. This report puts the human contribution of carbon to the atmosphere into the larger context of the global carbon cycle. The report focuses almost entirely on CO 2 , which alone is responsible for over half of the change in Earth's radiation balance. According to the Intergovernmental Panel on Climate Change (IPCC), CO 2 is the most important greenhouse gas released to the atmosphere from human activities. The atmosphere, oceans, vegetation, and soils on the land surface all store carbon. (See Figure 1 a .) Geological reservoirs also store carbon in the form of fossil fuels, such as oil, gas, and coal. Of these reservoirs (or pools), dissolved inorganic carbon in the ocean is the largest, followed in size by fossil carbon in geological reservoirs, and by the total amount of carbon contained in soils. (See Figure 1 a and Table 1 .) The atmosphere itself contains nearly 800 billion metric tons of carbon (800 GtC), which is more carbon than all of the Earth's living vegetation contains. Carbon contained in the oceans, vegetation, and soils on the land surface is linked to the atmosphere through natural processes such as photosynthesis and respiration. In contrast, carbon in fossil fuels is linked to the atmosphere through the extraction and combustion of fossil fuels. The atmosphere has a fairly uniform concentration of CO 2 , although it shows minor variations by season (about 1%)—due to photosynthesis and respiration—and by latitude. Carbon dioxide released from fossil fuel combustion mixes into the atmospheric carbon pool, where it undergoes exchanges with the ocean and land surface carbon pools. Thus, where fossil fuels are burned makes relatively little difference to the average concentration of CO 2 in the global atmosphere; emissions in any one region affect the concentration of CO 2 globally. The oceans, vegetation, and soils exchange carbon with the atmosphere constantly on daily and seasonal time cycles. (See Figure 1 b . ) In contrast, carbon from fossil fuels is not exchanged with the atmosphere, but is transferred in a one-way direction from geologic storage, at least within the time scale of human history. How much of the fossil fuel carbon ends up in the atmosphere, instead of the oceans, vegetation, and soils, and over what time scale, is driving much of the debate over what type of action to take to ameliorate global warming. How much carbon is stored in the atmospheric pool is important because as more CO 2 is added to the atmosphere, its heat-trapping capacity becomes greater. Each storage pool—oceans, soils, and vegetation—is considered a sink for carbon because each pool takes up carbon from the atmosphere. Conversely, each storage pool is also a source of carbon for the atmosphere, because of the constant exchange or flux between the atmosphere and the storage pools. For example, vegetation in the northern hemisphere is a sink for atmospheric carbon during the spring and summer months due to the process of photosynthesis. In the fall and winter it is a source for atmospheric carbon because the process of respiration returns carbon to the atmosphere from the vegetation pool. In contrast to the oceans, soils, and vegetation, the pool of fossil carbon is only a source, not a sink, except over geologic time scales. Over 90 GtC is exchanged each year between the atmosphere and the oceans, and close to 60 GtC is exchanged between the atmosphere and the land surface annually. (See Table 1 .) Human activities—primarily land-use change and fossil fuel combustion—contribute slightly less than 9 GtC to the atmosphere each year. If the human contribution of CO 2 is subtracted from the global carbon cycle, then the average net flux —the amount of CO 2 released to the atmosphere versus the amount taken up by the oceans, soils, and vegetation—is close to zero. Most scientists conclude that for 10,000 years prior to 1750, the net flux was less than 0.1 GtC per year when averaged over decades. That small value for net flux is reflected by the relatively stable concentration of CO 2 in the atmosphere—between 260 and 280 ppm—for the 10,000 years prior to 1750. Currently the atmospheric concentration of CO 2 is approximately 100 ppm higher than it was before 1750 because human activities are adding carbon to the atmosphere faster than the oceans, land vegetation, and soils remove it. The relatively rapid addition of CO 2 to the atmosphere has tipped the balance so that even though the oceans and the land surface take up more CO 2 per year on average than they release, atmospheric concentrations of CO 2 continue to rise. (See Table 1 .) About 45% of the CO 2 released from fossil fuel combustion and land use activities during the 1990s has remained in the atmosphere, while the remainder has been taken up by the oceans, vegetation, or soils on the land surface. Carbon dioxide is nonreactive in the atmosphere and has a relatively long residence time, although eventually most of it will return to the ocean and land sinks. About 50% of a single pulse of CO 2 will be removed within 30 years, a further 30% will be removed within a few centuries, and the remaining 20% may persist in the atmosphere for thousands of years. If CO 2 emissions continue or grow, however, atmospheric concentrations of CO 2 will likely also continue to rise, increasing radiative forcing —the degree to which the atmosphere traps incoming radiation from the sun. A likely result is continued warming of the planet. At present, the oceans and land surface are acting as sinks for CO 2 emitted from fossil fuel combustion and deforestation, but as they accumulate more carbon the capacity of the sinks—and the rate at which they accumulate carbon—may change. It is also likely that climate change itself (e.g., higher temperatures, a more intense hydrologic cycle) may alter the balance between sources and sinks, due to changes in the complicated feedback mechanisms between the atmosphere, oceans, and land surface. How carbon sinks will behave in the future is a prominent question for both scientists and policy makers. Most estimates of the carbon cycle indicate that the land surface (vegetation plus soils) accumulates more carbon per year than it emits to the atmosphere. (See Figure 1 b and Table 1 . ) That means the land surface acts as a net sink for CO 2 at present. Some policy makers advocate strategies for increasing the amount of CO 2 taken up and stored, or sequestered , by soils and plants, typically through land use changes, such as agricultural or forestry practices. How effective these land use practices will be for large-scale and long-term carbon sequestration is not clear. Land use overall has the largest uncertainty of any component in the global carbon cycle. Most scientists agree, however, that in the past two decades tropical deforestation has been responsible for the largest share of CO 2 released to the atmosphere from land use changes. Tropical deforestation and other land use changes released approximately 1.6 GtC per year to the atmosphere in the 1990s, and may be contributing similar amounts of carbon to the atmosphere today. Even though deforestation releases more carbon than is captured by forest regrowth within some regions, net forest regrowth in other regions takes up sufficient carbon so the land surface acts as a global net sink of approximately 1 GtC per year. By some estimates, even tropical lands, despite widespread deforestation, may be carbon-neutral or even net carbon sinks on a regional basis. What used to be known as "the missing sink" component in the global carbon cycle is now understood to be that part of the terrestrial ecosystem responsible for the net uptake of carbon from the atmosphere to the land surface (especially high-latitude, or boreal, forests). Scientists now prefer the term "residual land sink" to "missing sink" as it portrays the residual—or left over—part of the global carbon cycle calculation once the other components are accounted for (fossil fuel emissions, land-use emissions, atmospheric increase, and ocean uptake). Precisely which mechanisms are responsible for the residual land sink is not well understood. One mechanism postulated for many years has been the fertilizing effect of increased atmospheric CO 2 concentrations on plant growth. Most models predict enhanced growth and carbon sequestration by plants in response to rising CO 2 levels; however, results of experiments have been mixed. Many experiments show enhanced growth from increased CO 2 concentrations—at least initially—but nutrient and water availability and other limitations to growth are common. Long-term observations of biomass change and growth rates suggest that fertilization effects are too small to account for the residual land sink, at least in the United States. In North America, particularly the United States, the land-atmosphere flux is strongly tilted towards the land, with approximately 0.5 GtC per year accumulating in terrestrial sinks. That amount constitutes a large fraction—possibly 40%—of the global terrestrial carbon sink. According to some estimates, approximately half of the North American terrestrial carbon sink stems from regrowth of forests on abandoned U.S. farmland. Woody encroachment—the increase in woody biomass occurring mainly on former grazing lands—is thought to be another potentially large terrestrial sink, possibly accounting for more than 20% of the net North American sink (although the actual number is highly uncertain). Wood products (e.g., furniture, house frames, etc.), wetlands, and other smaller, poorly understood carbon sinks are likely responsible for accumulating the remaining carbon in North America. If the land surface and oceans were not acting as net sinks, the CO 2 concentration in the atmosphere would be increasing at a faster rate than observed. Like the land surface, the oceans today accumulate more carbon than they emit to the atmosphere each year, acting as a net sink for about 1.7 GtC per year. (See Figure 1 b and Table 1 .) The oceans have a much larger capacity to store carbon than the land surface. Ultimately, the oceans could store more than 90% of all the carbon released to the atmosphere by human activities, but the process takes thousands of years. Policy makers are likely more concerned about CO 2 accumulating in the oceans now and its behavior as a net sink over the next few decades. Also, the additional CO 2 is increasing the acidity of the ocean's surface waters, which may affect marine life. Carbon dioxide enters the oceans by dissolving into seawater at the ocean surface, at a rate controlled by the difference in CO 2 concentration between the atmosphere and the sea surface. Because the surface waters of the ocean have a relatively small volume—and thus a limited capacity to store CO 2 —how much CO 2 is stored in the oceans over the time scale of decades depends on ocean mixing and the transport of CO 2 from the surface to intermediate and deep waters. Vertical mixing between surface waters and deeper portions of the ocean is a sluggish process; for example, the oldest ocean water in the world—found in the North Pacific—has not flowed to the ocean surface for about 1,000 years. The slow rate of ocean mixing and slow transport of CO 2 from the surface to the ocean depths influence the effectiveness of the ocean sink for CO 2 . In addition to the vertical mixing of the ocean, large-scale circulation of the oceans around the globe is a critical component for determining the effectiveness of the ocean sink. Surface waters carrying anthropogenic CO 2 descend into the ocean depths primarily in the North Atlantic and the Southern Ocean, part of the so-called oceanic "conveyor belt." Some model simulations suggest that the Southern Ocean around Antarctica accounts for nearly half of the net air-sea flux of anthropogenic carbon. From that region, a large portion of dissolved CO 2 is transported north towards the subtropics. Despite its importance as a CO 2 sink, the Southern Ocean is poorly understood, and studies suggest that its capacity for absorbing carbon may be weakening. Congress is exploring legislative strategies that would alter the human component of the global carbon cycle. Of possible concern is how the ocean and land surface sinks will behave over the coming decades and longer, and whether they will continue to take up more carbon than they release. This is an area of considerable scientific uncertainty. Whether the oceans and land surface will continue to be net sinks for carbon if atmospheric CO 2 concentrations continue to rise is unclear. Alternatively, they may continue to be net sinks, but their capacity for taking up carbon may weaken or slow down. Very little carbon is sequestered by deliberate action. Most of the North American terrestrial carbon sink, such as the forest regrowth component, is sometimes referred to as the unmanaged , or background, carbon cycle. The future behavior of the unmanaged terrestrial carbon sink is another consideration for lawmakers. Whether the United States will continue its trajectory as a major terrestrial carbon sink is highly uncertain, and little evidence suggests that the terrestrial ecosystem sinks will increase in the future; some current terrestrial sinks may even become sources for carbon. Sinks may convert to sources, for example, if melting permafrost under warming conditions releases large amounts of methane currently trapped in frozen tundra, or if severe and extended droughts cause more frequent and severe wildfires that release large amounts of CO 2 . Policy makers may also need to evaluate and quantify how management practices, such as afforestation, conservation tillage, and other techniques, would increase the net flux of carbon from the atmosphere to the land surface. For example, some land management practices may be eligible to receive carbon offsets in cap-and-trade legislation that is under consideration. A cap-and-trade system designed to include carbon offsets would likely need an accurate and precise accounting for the tons of carbon sequestered deliberately by land management practices. Another consideration for policy makers is the future behavior of the ocean sink, particularly the Southern Ocean, given its importance to the net ocean-atmosphere CO 2 flux. In contrast to the terrestrial carbon sink, where management practices such as afforestation and conservation tillage may increase the amount of carbon uptake, it is unclear how the ocean carbon sink can be managed in a similar fashion. Some techniques proposed for increasing ocean sequestration of carbon, such as iron fertilization and deep ocean injection of CO 2 , are in an experimental phase and have unknown long-term environmental consequences. The uncertainty in the future behavior of carbon sinks implies that accurately calculating the concentration of atmospheric CO 2 in the future is also uncertain, even if the amount of CO 2 emitted to the atmosphere could be controlled precisely. For example, if global carbon emissions are capped at some agreed-upon level by 2050, such as 80% of the current amount, it would still be difficult to accurately predict atmospheric CO 2 concentrations by that date. A change in the rate of uptake, or release, of carbon from the oceans, soils, or vegetation between now and 2050 would affect any such predictions. Consequently, if Congress seeks policies aimed at keeping atmospheric CO 2 concentrations below a specific threshold, such as 450 or 550 ppm, then Congress may also wish to understand if, how, and why the oceans and land surface are speeding up or slowing down the rate at which they take up or release carbon. 

Summary: Huge quantities of carbon are actively exchanged between the atmosphere and other storage pools, including the oceans, vegetation, and soils on the land surface. The exchange, or flux, of carbon among the atmosphere, oceans, and land surface is called the global carbon cycle. Comparatively, human activities contribute a relatively small amount of carbon, primarily as carbon dioxide (CO2), to the global carbon cycle. Despite the addition of a relatively small amount of carbon to the atmosphere, compared to natural fluxes from the oceans and land surface, the human perturbation to the carbon cycle is increasingly recognized as a main factor driving climate change over the past 50 years. If humans add only a small amount of CO2 to the atmosphere each year, why is that contribution important to global climate change? The answer is that the oceans, vegetation, and soils do not take up carbon released from human activities quickly enough to prevent CO2 concentrations in the atmosphere from increasing. Humans tap the huge pool of fossil carbon for energy, and affect the global carbon cycle by transferring fossil carbon—which took millions of years to accumulate underground—into the atmosphere over a relatively short time span. As a result, the atmosphere contains approximately 35% more CO2 today than prior to the beginning of the industrial revolution. As the CO2 concentration grows it increases the degree to which the atmosphere traps incoming radiation from the sun, which further warms the planet. The increase in atmospheric CO2 concentration is mitigated to some extent by two huge reservoirs for carbon—the global oceans and the land surface—which currently take up more carbon than they release. They are net sinks for carbon. Currently, most of the total global carbon sink is referred to as the unmanaged, or background, carbon cycle. Very little carbon is removed from the atmosphere and stored, or sequestered, by deliberate action. If the oceans, vegetation, and soils did not act as sinks, then the concentration of CO2 in the atmosphere would increase even more rapidly. Congress is considering legislative strategies to reduce U.S. emissions of CO2 and/or increase the uptake of CO2 from the atmosphere. Congress may also opt to consider how land management practices, such as afforestation, conservation tillage, and other techniques, might increase the net flux of carbon from the atmosphere to the land surface. Some land management practices may be eligible to receive carbon offsets in cap-and-trade legislation that is under consideration. A cap-and-trade system designed to include carbon offsets would likely need an accurate and precise accounting for the tons of carbon sequestered deliberately by land management practices. How the ocean sink could be managed to store more carbon is unclear. Iron fertilization and deep ocean injection of CO2 are in an experimental stage, and their promise for long-term enhancement of carbon uptake by the oceans is not well understood. Of additional concern is how the ocean and land surface sinks will behave over the coming decades and longer, and whether they will continue to take up more carbon than they release. The uncertainty in the future behavior of carbon sinks implies that accurately predicting the concentration of atmospheric CO2 in the future is also uncertain, even if the amount of CO2 emitted to the atmosphere could be controlled precisely.