Introduction

Congress is considering several legislative strategies that would reduce U.S. emissions of greenhouse gases—primarily carbon dioxide (CO₂)—and/or increase uptake and storage of CO₂ 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 large carbon reservoirs: the global oceans and the planet's land surface. A key question is how CO₂ 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 CO₂ buildup in the atmosphere.¹

There is a scientific consensus that human disturbances, or perturbations, to the carbon cycle are a main factor driving climate change over the past 50 years.² For the period 0-1850 CE, the global carbon cycle was roughly in balance, and the concentration of CO₂ in the atmosphere was fairly constant at approximately 280 parts per million (ppm).³ 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 CO₂ are now greater than 400 ppm, having increased by over 46% since the Industrial Revolution.⁴

An understanding of the global carbon cycle has shifted from being of mainly academic interest to being also of policy interest. Policymakers are grappling with, for example, how the United States could achieve a state of net-zero CO₂ emissions, in which emissions to the atmosphere are balanced by removals. This is likely to require the implementation of human methods of carbon dioxide removal (CDR)⁵ and would depend inherently on continued uptake of carbon by the oceans and land surface. Yet how much CO₂ forests or farmland are capable of taking up in the future, and for how long, is not clear. How the oceans and the land surface carbon reservoirs will behave in the future—how much CO₂ they will take up or release and at what rate—are topics of active scientific inquiry.

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.⁶ More than half of the human-caused increase in the earth's radiation balance is due to CO₂, the primary focus of this report.⁷ According to the Intergovernmental Panel on Climate Change (IPCC), CO₂ is the most important greenhouse gas released to the atmosphere from human activities.⁸

Carbon Storage, Sources, and Sinks

The atmosphere, oceans, vegetation, and soils on the land surface all store carbon (Figure 1, panel (a)). Geological reservoirs also store carbon in a variety of forms, including fossil fuels, such as oil, gas, and coal.⁹ A portion of these fossil fuel reservoirs are considered recoverable reserves.¹⁰ 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 soils¹¹ (Figure 1, panel (a); Table 1). The total carbon stored in the live plant biomass of the earth is less¹² than the 870 billion metric tons of carbon¹³ (870 GtC)¹⁴ present in the atmosphere. Carbon contained in the oceans, vegetation, and soils on the land surface is linked to the atmosphere through natural processes such as photosynthesis, respiration, decomposition, and gas exchange. In contrast, carbon in fossil fuels is linked to the atmosphere through the extraction and combustion of fossil fuels.

The nearly constant concentration of CO₂ in the atmosphere has small annual and latitudinal fluctuations (approximately 1%), caused by photosynthesis and respiration.¹⁵ 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 CO₂ in the global atmosphere; emissions in any one region affect the concentration of CO₂ globally.¹⁶

The oceans, vegetation, and soils exchange carbon with the atmosphere constantly on daily and seasonal time cycles (see Figure 1, panel (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 timescale of human history. An understanding of the dynamics of the carbon cycle—and the ways in which human-sourced carbon emissions are apportioned among the atmosphere, oceans, vegetation, and soils—can inform and support deliberations on whether to take action to ameliorate global warming and what type of action that might be.

Figure 1. Panel (a): Storage or Pools (GtC); and Panel (b): Annual Flux or Exchange of Carbon (GtC per year)

Source: IPCC, "Global Carbon and Other Biogeochemical Cycles and Feedbacks" in 2021 IPCC Working Group I Report, 2021, p. 700. Notes: Figure prepared by CRS. One GtC refers to 1 billion metric tons of carbon.

The amount of carbon stored in the atmospheric pool is important because as more CO₂ is added to the atmosphere, its heat-trapping capacity becomes greater.¹⁷ Each storage pool—oceans, soils, and vegetation—is considered a sink for carbon because each pool takes up carbon from the atmosphere. 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 timescales.

Carbon Flux, or Exchange, with the Atmosphere

Over 77 GtC is exchanged each year between the atmosphere and the oceans, and over 136 GtC is exchanged between the atmosphere and the land surface annually (see Table 1).¹⁸ Human activities—primarily land-use change (e.g., deforestation) and burning of fossil fuels—contribute less than 12 GtC to the atmosphere each year.¹⁹ If the human contribution of CO₂ is subtracted from the global carbon cycle, 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, a balanced state of natural net-zero CO₂ emissions. Research indicates that the average decadal net flux was less than 0.1 GtC per year for about 10,000 years leading up to 1750.²⁰ That small value for net flux is reflected by the relatively stable concentration of CO₂ in the atmosphere—between 260 ppm and 280 ppm—for about the 10,000 years prior to 1750.²¹

How Fast Carbon Is Exchanged

Currently the atmospheric concentration of CO₂ is approximately 130 ppm higher than it was before 1750 (410 ppm versus 280 ppm), primarily 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 so that even though the oceans and the land surface take up more CO₂ per year, on average, than they release, atmospheric concentrations of CO₂ continue to rise (see Table 1). During the 1990s 55% of CO₂ emissions from land-use activities and fossil fuel combustion were taken up by the oceans, vegetation, or soils on the land surface, with 45% persisting in the atmosphere,²² a trend that was also confirmed for the period 2010-2019.²³ CO₂ 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.²⁴ When a pulse²⁵ of CO₂ is added to the atmosphere, in 30 years about half will be withdrawn; thereafter, removal proceeds more slowly, with centuries required to remove an additional 30% and millennia before the final 20% is removed.²⁶ If CO₂ emissions continue or increase, however, adding to the cumulative CO₂ emissions since the start of the Industrial Revolution, atmospheric concentrations of CO₂ will also continue to rise. This will increase radiative forcing, the degree to which the atmosphere traps incoming radiation from the sun. There is a scientific consensus that there is an almost linear relationship between global temperature increases and such cumulative human-caused emissions of CO₂,²⁷ and a likely result is a continued warming of the planet.

At present the oceans and land surface are acting as sinks for CO₂ emitted from fossil fuel combustion, deforestation, and other human sources, 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)²⁸ may alter the balance between sources and sinks due to changes in the complicated feedback mechanisms among the atmosphere, oceans, and land surface.²⁹ How carbon sinks will behave in the future is a prominent question for both scientists and policymakers.

Land Surface-Atmosphere Flux

The IPCC estimates of the carbon cycle indicate that the land surface (vegetation plus soils) accumulates approximately 5.3 GtC more carbon per year than it emits to the atmosphere³⁰ (see Figure 1, panel (b); and Table 1). Recent research indicates that during the past 60 years, the land surface sink for CO₂ has increased,³¹ as the growth rate of atmospheric CO₂ has increased, among other factors.³² That means that the land surface acts as a net sink for CO₂ at present. Some policymakers advocate strategies for increasing the amount of CO₂ taken up and stored, or sequestered, by soils and plants, typically through land-use change, such as agricultural or forestry practices.³³ How effective these land-use practices will be for large-scale and long-term carbon sequestration is not clear.

There are large uncertainties associated with the land-use component of the global carbon cycle, although these uncertainties have been reduced.³⁴ There is uncertainty regarding which environmental factors are foremost in influencing the behavior of the land surface sink.³⁵ Studies combining satellite land cover change with biomass data suggest that tropical deforestation has been responsible for the largest share of CO₂ released to the atmosphere from land-use changes.³⁶ Tropical deforestation and other land-use changes released approximately 3.6 GtC per year to the atmosphere in the 1990s and approximately 3.8 GtC per year for the period 2011-2020.³⁷ Even though deforestation emits more carbon than is taken up by forest growth within some regions,³⁸ net forest regrowth takes up sufficient carbon such that the land surface acts as a global net sink.

Ocean-Atmosphere Flux

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. Like the land surface, the oceans today accumulate more carbon than they emit to the atmosphere each year, acting as a net sink of about 1.9 GtC per year.³⁹ (See Figure 1, panel (b); and Table 1.) The oceans have a much larger capacity to store carbon than the land surface. Ultimately, over the course of millennia, the oceans could take up all but about 7% of carbon emitted to the atmosphere from fossil fuel combustion and land-use change.⁴⁰ Policymakers are likely more concerned about CO₂ accumulating in the oceans now and its behavior as a net sink over the next few decades. Also, the additional CO₂ is increasing the acidity of the ocean's surface water, which may affect marine life.⁴¹

Uptake of CO₂ by the ocean occurs in stages. First, CO₂ dissolves in ocean water at the surface.⁴² Higher CO₂ concentrations increase the rate of this reaction. Once dissolved, the CO₂ may be taken up biologically through phytoplanktonic photosynthesis, or chemically through the formation of calcium carbonate.

The quantity of uptake of CO₂ by the ocean is determined by a number of factors, some of which are affected by increases or decreases in the concentration of atmospheric CO₂, and by global temperature. CO₂, which diffuses into the ocean, reacts with water to form carbonic acid, which then reacts with calcium to form calcium carbonate.⁴³ This allows the ocean to absorb more CO₂ than is simply dissolved in the water. This chemical ability of the ocean to absorb CO₂ is known as the buffering capacity.

These uptake processes take place in the surface layers of the ocean, a small component of the ocean's total volume.⁴⁴ This surface water is moved into the ocean's depths by a slow process of mixing whereby it may be a millennium before the water returns to the surface.⁴⁵ The vertical mixing of the ocean, by the ocean waters' extensive movement around the globe, is a critical component of the ocean sink.⁴⁶

This extensive mixing of the ocean, due to overturning circulation by the large ocean "conveyor belt" currents such as the Atlantic Meridional Overturning Current (AMOC), brings absorbed atmospheric carbon into the deep ocean, where it is sequestered from the atmosphere for long periods of time.⁴⁷ The scale and speed of this circulation contribute to the effectiveness of the ocean sink. The North Atlantic and near Antarctica are sites of the downward movement of frigid, dense, carbon-rich surface water into the deep ocean.⁴⁸

Recent research has shown that increases in atmospheric CO₂ due to human activity have started to affect the buffering capacity of the ocean, although there is no scientific consensus that this has changed the uptake of CO₂ by the ocean.

Some researchers have raised concerns that continued increases in atmospheric CO₂ and global temperatures could decrease the ocean's capacity to absorb CO₂ in the later part of this century. For example, increasing concentrations of atmospheric CO₂ reduce the buffering capacity of the ocean, which could lead to a decrease in CO₂ absorption by the ocean. In addition, CO₂ is less soluble in water at higher temperatures, and increased temperatures could also contribute to decreased ocean CO₂ uptake. If overturning circulation or mixing of ocean water were to decrease during this time,⁴⁹ a smaller quantity of carbon initially absorbed by surface water would be transported to the deep ocean. Ocean uptake of CO₂ could decrease for these reasons; however, models indicate that these changes are more likely to occur under future scenarios of higher CO₂ emissions.

The Carbon Budget Imbalance

The global carbon budget is the accounting of the carbon added to the global carbon cycle by human activity, often measured since the start of the Industrial Revolution (defined as the year 1750 CE). The global carbon budget includes CO₂ emissions from fossil fuel use (E_FOS); CO₂ emissions from land use change (E_LUC); the increased CO₂ concentration in the atmosphere (G_ATM); the removal of CO₂ from the atmosphere by the ocean sink (S_OCEAN); and removal of CO₂ from the atmosphere by the land-use sink (S_LAND). In the global carbon cycle, these quantities balance and sum to zero. However, our knowledge of the pools and fluxes of carbon in the global carbon cycle is imperfect, and when the estimates of these quantities do not sum to zero, there is a carbon budget imbalance (B_IM) with the following equation:⁵⁰

B_IM = E_FOS + E_LUC – (G_ATM + S_OCEAN + S_LAND)

In the past, this imbalance was referred to as the "missing carbon" and was thought to be largely due to the difficulty of measuring the uptake of carbon by terrestrial ecosystems—for example, in the Northern Hemisphere.⁵¹ However, the average imbalance from recent calculations is now about 3% of emissions. This small percentage of imbalance (individual years sometimes differ) in the global carbon cycle suggests the partitioning of emissions among the components of the carbon cycle almost completely accounts for carbon in the global carbon budget.⁵² These low values for the carbon budget imbalance support increased confidence in the scientific understanding of the global carbon cycle. As a recent scientific journal article on the global carbon budget noted, "Therefore, the near-zero mean and trend in the budget imbalance is seen as evidence of a coherent community understanding of the emissions and their partitioning on those timescales."⁵³

It is not currently possible to attribute, with certainty, the source of the discrepancy producing these imbalances. Uncertainties in the measurements of the ocean and land surface sinks have been suggested, but a scientific consensus on the source of the imbalance has not been reached.

Policy Implications

The behavior of the global carbon cycle can be affected by the policy decisions and legislative actions of Congress. Assessing policy options in the context of the broader carbon cycle summarized above may be useful to project the potential effects of, and interactions among, various options. The vast array of policy choices that can affect GHG emissions and uptake may be direct and intentional, or indirect and collateral in their impact on other policy objectives.

There is a scientific consensus that in order to reduce the risk of increasing impacts of climate change, it is necessary to stabilize global temperatures, and that reaching a state of net-zero CO₂ emissions is necessary to achieve this stabilization.⁵⁴ Net zero means that emissions of CO₂ are balanced by removals of CO₂ and there is no net addition of CO₂ to the atmosphere, a situation that existed before the start of the Industrial Revolution. There have been legislative proposals introduced during the 117^th Congress aimed at achieving net-zero CO₂ emissions domestically.⁵⁵ Additionally, having the United States achieve net-zero CO₂ emissions by 2050 is a stated climate policy goal of the Biden Administration.⁵⁶

Any of the policy options to achieve net-zero CO₂ will require changes to the carbon cycle. These changes may include the following:

• Reducing emissions of CO₂ from fossil fuels and land-use change.
• Techniques for reducing CO₂ emissions from fossil fuel use and land-use change are well understood and span a range of approaches, including energy efficiency and use of renewable energy sources to reduce emissions from fossil fuels, and maintaining indigenous forests and reducing deforestation to mitigate emissions from land-use change.
• Increasing the uptake of CO₂ by the land surface through management.
• Techniques for doing so are well understood and include increasing forested land and changing cultivation practices to increase soil carbon.
• Increasing the uptake of CO₂ by the oceans.⁵⁷
• Techniques for doing so are not developed, are the subject of ongoing research, and have given rise to concerns about their feasibility and potential environmental impacts.
• Increasing the capacity for removal and long-term storage of CO₂ from the atmosphere.
• Techniques to develop additional carbon dioxide removal (CDR) capacity through technologies such as direct air capture (DAC)—essentially the development of an additional human-created carbon sink that removes CO₂ from the atmosphere for long-term sequestration—are under development and have not been demonstrated at scale.

The success of policies in achieving net zero is dependent, in part, on the behavior of the carbon cycle, specifically on the behavior of the land and ocean sinks. If the sink capacity of the ocean or the land carbon pools were to decrease, then achieving net zero would potentially require greater levels of emissions reductions, greater levels of CDR capacity, or a combination of the two. Such levels might not be necessary if the sink capacity of the ocean and the land continue to increase. The ongoing scientific effort to understand and anticipate the behavior of the global carbon cycle, now and in the future, can help to inform congressional deliberations on these and other climate policies.