Scientists & Engineers for America

Carbon Cycle

Introduction

Congress is considering several legislative strategies that would reduce U.S. emissions of greenhouse gases — primarily carbon dioxide (CO2) — or increase uptake and storage of CO2 from the atmosphere, or both. Both approaches are viewed by many as critical to forestalling global climate changed 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 back and forth each year between the atmosphere and two huge carbon reservoirs: (1) the global oceans, and (2) the land surface of the entire planet. The exchange, or flux, of carbon between the atmosphere, oceans, and land surface is called the global carbon cycle.

An understanding of the details of the global carbon cycle has shifted from being of mainly academic interest to being of political interest. Policy makers are grappling with, for example, the importance of recognizing carbon sequestration by forests; or determining under a greenhouse gas cap what level of carbon emissions would limit the concentration of atmospheric CO2 to a specific value; or understanding the magnitude and timing of ocean acidification and its impact on marine life. In sheer magnitude, human activities contribute a relatively small amount of carbon, primarily as CO2, to the global carbon cycle. Fossil fuel combustion, for example, adds less than 5% to the total amount of carbon released from the oceans and land surface to the atmosphere each year. If humans add only a small amount of CO2 to the atmosphere, why is that contribution important enough to influence global climate change?

This report explores the answer to that question and attempts to put the human contribution of carbon to the atmosphere into the larger context of the global carbon cycle. The report focuses almost entirely on CO2, although methane (CH4), black carbon, and organic carbon pollution are also part of the carbon cycle and have roles in human-induced climate change. Carbon dioxide, alone, is responsible for over half of the change in Earth’s radiation balance, and methane for about an additional 20%.

Carbon Storage, Sources, and SInks

Carbon is stored in the atmosphere, in the oceans, in vegetation, and in soils on the land surface. Carbon is actively exchanged (fluxes) between the atmosphere and the other storage pools, or stocks, of carbon. The atmosphere is also linked to fossil carbon in geological reservoirs — for example, oil, gas, and coal — via their extraction and combustion as fossil fuels. Dissolved inorganic carbon in the ocean is the largest storage pool, followed in size by fossil carbon in geological reservoirs, and by the total amount of carbon contained in soils. The atmosphere itself contains nearly 800 billion metric tons of carbon (or gigatonnes, GtC), which is more carbon than all of the Earth’s living vegetation contains. Table 1 and Figure 1 show the global amount of carbon held in storage in each pool that is linked to the atmosphere.

Carbon dioxide in the atmosphere has an average concentration around the globe of approximately 380 parts per million (ppm). The atmosphere has a fairly uniform concentration of CO2, although it shows minor variations (about 1%) by season — due to photosynthesis and respiration — and by latitude. Carbon dioxide released from fossil fuel combustion mixes readily 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 concentration of CO2 in the atmosphere; emissions in any one region affect the concentration of CO2 everywhere else in the atmosphere.

The oceans, vegetation, and soils truly exchange carbon with the atmosphere constantly on daily and seasonal time cycles. 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 at the human time scale. Some of the CO2 currently in the atmosphere may become fossil fuel someday, after it is captured by vegetation, buried under heat and pressure, and converted into coal, for example, but the process takes millions of years. 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 today’s global warming debate.

How much carbon is stored in each pool — especially the atmospheric pool — is important in the global warming debate because as more CO2 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. The pool of fossil carbon is only a source, not a sink, except over geologic time scales, as described above. How much carbon is transferred between the atmosphere and the sources and sinks is a topic of scientific scrutiny because the mechanisms are still not understood completely. Whether a storage pool is a net sink or a net source for carbon in the future depends very much on the balance of mechanisms assumed to drive its behavior, and how those mechanisms may change.

Carbon Flux, or Exchange, with the Atmosphere

Over 90 billion tonnes (or 90 Gigatonnes of carbon, GtC) of carbon 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. Human activities — primarily land-use change and fossil fuel combustion — contribute less than 9 GtC to the atmosphere each year. If the human contribution of CO2 is removed from the equation, then the average net flux — the amount of CO2 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 CO2 in the atmosphere — between 260 and 280 ppm — for the past 10,000 years prior to 1750.

Currently the atmospheric concentration of CO2 is almost 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 can remove it. The relatively rapid addition of CO2 to the atmosphere has tipped the balance between sources and sinks. Why is that occurring?

The short answer is timing. First, the oceans and land surface are taking up carbon released from human activities, but not as quickly as CO2 is accumulating in the atmosphere. About 45% of the CO2 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 CO2 will be removed within 30 years, a further 30% removed in within a few centuries, and the remaining 20% may persist in the atmosphere for thousands of years. As the CO2 concentration grows it increases the radiative forcing of the atmosphere, warming the planet. Second, the oceans and land surface are acting at present as sinks for CO2 emitted from fossil fuel combustion and deforestation, but as they accumulate more carbon the nature of the sinks may change. It is also likely that climate change itself — for example, higher temperatures, 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 currently a prominent question for both scientists and policy makers.

Land Surface-Atmosphere Flux. 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 . . . The land surface thus acts as a net sink for CO2 at present. Some policy makers advocate strategies for increasing the amount of CO2 taken up and stored, or sequestered, by soils and plants, typically through agricultural or forestry practices. How effective those strategies are likely to be depends, in part, on our understanding of the carbon cycle and the land-atmosphere flux.

The land use change component has the largest uncertainty of any component in the overall carbon cycle. Most scientists agree, however, that in the past two decades tropical deforestation has been responsible for the largest share of CO2 released to the atmosphere from land use changes. Tropical deforestation and other land use changes may be responsible for releasing 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 in some regions, net forest regrowth in other regions uptakes 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; tropical systems uptake substantial carbon to offset what is lost through deforestation.

What used to be known as “the missing sink” component in the overall 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 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 are a topic of scientific controversy. One mechanism postulated for many years has been the fertilizing effect of increased atmospheric CO2 concentrations on plant growth. Most models predict enhanced growth and carbon sequestration by plants in response to rising CO2 levels; however, results of experiments have been mixed. Experiments show enhanced growth from increased CO2 concentrations — at least initially — but nutrient 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 surface, where approximately 0.5 GtC per year is accumulating in terrestrial sinks. That amount constitutes a large fraction — possibly 25% — of the global terrestrial carbon sink. According to some estimates, approximately 50% 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 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 responsible for accumulating the remaining carbon in North America.

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. Very little carbon is sequestered by deliberate action. 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 some evidence suggests that the terrestrial ecosystem sinks may not increase in size. Some current sinks may even become sources for carbon.

Policy makers may also need to evaluate 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. How forests, rangelands, and croplands are managed in the future for carbon sequestration may become an important factor in the overall land-atmosphere flux.

Ocean-Atmosphere Flux

Similar to the land surface, the oceans today accumulate more carbon than they emit to the atmosphere each year, acting as a net sink of about 1.7 GtC per year . . . If the land surface and oceans were not acting as net sinks, the CO2 concentration in the atmosphere would be increasing at a faster rate than observed. More than the land surface, the oceans have a huge capacity to store carbon. 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 may be more concerned about how CO2 is accumulating in the oceans now, what its impact is on ocean chemistry and marine life (e.g. ocean acidification), and how its behavior as a net sink may change over the next few decades.

Carbon dioxide enters the oceans by dissolving into seawater at the ocean surface, at a rate controlled by the difference in CO2 concentration between the atmosphere and the sea surface.(36) Because the surface waters of the ocean have a relatively small volume — and thus a limited capacity to store CO2 — how much CO2 is stored in the oceans over the time scale of decades depends on ocean mixing and the transport of CO2 from the surface to intermediate and deep waters. 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 been out of contact with the ocean surface for about 1,000 years. Thus the slow rate of ocean mixing, and slow transport of CO2 from the surface to the ocean depths, is of possible concern to policymakers because it influences the effectiveness of the ocean sink for CO2, and because CO2 added to the surface waters of the ocean increases its acidity.

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 CO2 descend into the ocean depths primarily in the North Atlantic and the Southern Oceans, 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 CO2 is transported north towards the subtropics. Despite its importance as a CO2 sink, the Southern Ocean is poorly understood, and at least one study suggests that its capacity for absorbing carbon may be weakening.

As CO2 is added to the surface of the ocean from the atmosphere, it increases the acidity of the sea surface waters, with possible impacts to the biological production of organisms, such as corals. Corals, and calcifying phytoplankton and zooplankton, are susceptible to increased acidity as their ability to make shells in the water column is inhibited or possibly reversed, leading to dissolution. Some reports indicate that sea surface pH has dropped by 0.1 pH units since the beginning of the industrial revolution. One report suggests that pH levels could drop by 0.5 pH units by 2100, and suggests further that the magnitude of ocean acidification can be predicted with a high level of confidence. The same report states, however, that research on the impacts of high concentrations of CO2 on marine organisms is in its infancy.

The oceans appear to be a larger net sink for carbon than the land surface at present. As with the land surface, however, a consideration for policy makers is the future behavior of the ocean sink, particularly the Southern Ocean, given its importance to the net ocean-atmosphere CO2 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 proposed techniques for increasing ocean sequestration of carbon, such as iron fertilization and deep ocean injection of CO2, are in an experimental phase and have unknown long-term environmental consequences.

Conclusions

Huge amounts of carbon are exchanged between the atmosphere, the land surface, and the oceans each year. Humans are responsible for only a small fraction of the total exchange that, nonetheless, affects the global system by adding a large net flux of CO2 to the atmosphere. Before the industrial revolution — and the large-scale combustion of fossil fuels, land-clearing and deforestation activities — the average net flux of CO2 to the atmosphere hovered around zero for nearly 10,000 years. Because of the human contribution to the net flux, the amount of CO2 in the atmosphere is now 100 ppm higher today than is has been for the past 10,000 years.

Congress is exploring legislative strategies that would alter the human component of the global carbon cycle. Strategies that limit emissions from fossil fuel combustion would reduce the current one-way transfer of fossil carbon to the atmosphere. What took millions of years to accumulate geologically is being released in only a few hundred years. Capturing CO2 before it is released to the atmosphere and injecting it back into geological reservoirs — direct carbon sequestration — is one possible strategy to “short circuit” the geologic process and return the carbon underground over a human time scale. CO2 injection into the subsurface has been used for decades to enhance recovery of oil; however, large-scale geologic sequestration of CO2 for storage is currently in a pilot testing stage.

Less than half of the total amount of CO2 released from burning fossil fuels over the past 250 years remains in the atmosphere, because two huge sinks for carbon — the global oceans and the land surface — take up more carbon than they release at present. Congress is exploring if and how management practices, such as afforestation, conservation tillage, and other techniques, might increase the net flux of carbon from the atmosphere to land surface. 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.

Also of possible concern to Congress is how the ocean and land surface sinks will behave over the coming decades and longer, and whether they will continue to uptake more carbon than they release. For example, carbon emissions may be capped so as to keep atmospheric CO2 concentrations below a prescribed level at some future date, but changes in the magnitude, or even the direction, of the ocean or land-surface sinks may affect whether those target concentrations can be achieved. Congress may wish to incorporate what is known about the carbon cycle into its legislative strategies. Congress may also wish to evaluate whether the global carbon cycle is sufficiently well understood that the consequences of long-term policies aimed at mitigating global climate change are fully appreciated.

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