	Introduction

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. 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. Carbon Storage, Sources, and Sinks

The atmosphere, oceans, vegetation, and soils on the land surface all store carbon. 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. 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. 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. The "Missing Sink"

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). 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. 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. 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. 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. 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. 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.
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.