Overview of the Global Carbon Cycle

Concern about the effects of climate change, on the one hand, and the difficulties of reducing emissions of carbon from fossil fuel use, on the other, have led to a target of limiting global average warming to no more than 2°C, with a more conservative target of 1.5°C to reduce the risks of the most serious effects of climate change (USGCRP 2017). The choice of 2°C reflects a balance between a realistic threshold and one that would result in a presumably tolerable amount of climate change. However, as Knutti et al. (2015) points out, no proof exists that this threshold maintains a “safe” level of warming, and the definition of “safe,” as well as the components of the Earth system that the term applies to, are themselves subjective. Several recent studies have suggested that the accumulated carbon in the atmosphere already may have committed the climate system to 2°C or more of global average temperature increase (Mauritsen and Pincus 2017; Raftery et al., 2017).

The relationship of cumulative carbon emissions to global temperature increase depends on the data constraints or model used to simulate the temperature response. Gillett et al. (2013) reports an observationally constrained range of 0.7 to 2.0°C per 1,000 Pg C (5% to 95% confidence interval) and a range of 0.8 to 2.4°C per 1,000 Pg C based on 15 models from the Coupled Model Intercomparison Project Phase 5 (CMIP5). Similarly, IPCC (2013) estimates that limiting the warming with a probability of >33%, >50%, and >67% to less than 2°C since the period 1861 to 1880 will require cumulative emissions from all anthropogenic sources to stay below about 1,570 Pg C, 1,210 Pg C, and 1,000 Pg C since that period, respectively. Cumulative emissions since 1850, including land-use change and forestry, are 572 Pg C (Global Carbon Project 2016; Peters et al., 2015; World Resources Institute 2017). However, this amount includes only the carbon from CO₂ emissions and does not include non-CO₂ emissions (i.e., primarily CH₄ and N₂O), which amount to an additional 210 Pg C equivalent from non-CO₂ sources, bringing the total to 779 Pg C equivalents (Peters et al., 2015). This amount implies that, to achieve a >33%, >50%, and >67% warming probability limited to below 2°C, amounts of no more than 791, 431, or 221 Pg C equivalent, respectively, can be emitted from 2017 forward. Current annual global emissions of CO₂ from fossil fuel combustion and cement production are 10.7 Pg C per year (Le Quéré et al., 2017), so this limit could be reached in less than 80, 40, or 20 years. Although technically achievable (Millar et al., 2017), the most conservative emissions reductions would require immediate and concerted action.

These simple estimates of cumulative emissions and their effect on future global temperature, however, have many uncertainties. Uncertainties in climate models include cloud, aerosol, and carbon cycle feedbacks. Carbon-climate feedbacks, such as the effect on carbon emissions from permafrost thaw, are highly uncertain and may significantly lower the cumulative amount of carbon that can be emitted before exceeding the 2°C global temperature increase.

Attempts to avoid the most severe impacts of climate change through management of the carbon cycle rely on reducing emissions and increasing storage in land and ocean reservoirs. Other means that focus on adaptation are not specifically addressed in this report. Evaluating and predicting the success of these strategies require an understanding of all the natural and anthropogenic components of the global carbon cycle because decreases in emissions or increases in sinks from mitigation activities may be offset partially or wholly by changes in other components. Globally, land and ocean sinks have averaged between 3.9 and 4.7 Pg C per year since 2000 (Le Quéré et al., 2016), growing over time in proportion to emissions (Ballantyne et al., 2012). The sink on land, accounting recently for about 25% of total emissions (Le Quéré et al., 2016), is consistent with the measured increase in carbon stocks of forests (Pan et al., 2011). In North America, the forest sink is currently about 223 Tg C per year (see Ch. 9: Forests), but increases in the frequency of wildfires and insect infestations in the western continent threaten to reduce that sink. The sink in Canadian forests, though much smaller than that in the United States, also is threatened by insects and wildfire and could become a significant source (Kurz et al., 2013), as has happened recently. Mexican forests also are thought to be a small sink based on estimates of regrowth of previously disturbed forests that exceed emissions from deforestation and forest degradation (see Ch. 9: Forests).

Options for managing emissions of carbon and other GHGs include 1) reduction or cessation of the use of fossil fuels, replacing them with renewable sources of energy (e.g., solar, wind, and water); 2) climate intervention via carbon dioxide removal (CDR), including carbon capture and storage (CCS), which involves absorption of emissions at point sources; and 3) negative emissions, using approaches to remove previously emitted CO₂ by increasing storage in terrestrial and ocean reservoirs. Climate intervention via albedo modification does not affect the carbon budget directly but is an attempt to counteract climate change by directly influencing the global radiation balance. For example, introducing aerosols into the stratosphere potentially could provide a global cooling effect but would not address other issues such as ocean acidification. Climate intervention will not be discussed here further; rather, the focus of this section is on actions that directly involve the carbon cycle.

The study of MacDonald et al. (2016) estimated that U.S. carbon emissions from the power sector could be reduced by as much as 80% relative to 1990 use without significantly increasing energy costs and using existing technology. Although some studies have argued that a complete transition to decarbonized energy systems is feasible (Jacobson et al., 2015), other authors have pointed out that a transition to a low-carbon energy system is likely to be difficult and expensive without using a range of options (Clack et al., 2017), including some contribution from fossil fuels. This issue is complex, and full discussion of it is beyond the scope of this report.

For the CCS option, there are many unknowns about its implementation and permanence. A special example of CCS involves renewable energy, in this case bioenergy CCS (BECCS), where energy is derived from burning biomass, capturing and storing the resulting CO₂, and then re-growing the biomass. Although BECCS is appealing because it replaces fossil fuels and removes carbon from the atmosphere, there is only one experimental biomass plant of this type and its technology suffers from the same uncertainty as other CCS types (Anderson and Peters 2016; Fuss et al., 2014).

Estimates of the potential for negative emissions are in the range of 1.6 to 4.4 Pg C per year or 34 to 105 Pg C by 2100 (Griscom et al., 2017; Houghton and Nassikas 2018). Achieving the potential of negative emissions, however, has other constraints involving competition for land area, water availability, albedo changes, and nutrient limitations (Smith et al., 2015). Most negative emissions activities on land are useful either as a bridge to a low–carbon emissions energy system for developing and implementing CCS or for assistance with future removals of previously emitted CO₂, but effects are limited in implementing long-term solutions because forests and soils cannot accumulate carbon at high rates indefinitely. The most rapid rates of carbon removal occur in the first 50 to 100 years of forest growth. Soils generally are slow to accumulate carbon, although that process in forests may last for centuries if the forests remain undisturbed (Luyssaert et al., 2008). Thus, negative emissions are a part of the portfolio of mitigation activities, but the timing of impacts needs to be considered. These negative emissions cannot compensate for future emissions that either continue at current rates or increase (Gasser et al., 2015). Furthermore, the effects of climate change on the carbon balance of terrestrial ecosystems are uncertain, as suggested by the increased mortality of U.S. forests from droughts, insects, and fires.

Another unknown is how much of an overshoot is possible—that is, by how much and for how long emissions could exceed the limit imposed by a 2°C ceiling and their effects still be reversible. Moreover, questions include: How would they be reversed with only limited, available negative emissions? What are the tipping points? For example, warming already is thawing permafrost and thereby exposing ­long-frozen organic carbon to oxidation. Estimates are that emissions of carbon from thawing permafrost could be 146 to 160 Pg C by 2100 (Schuur et al., 2015), enough to counter negative emissions. Similarly, disruption of tropical and subtropical ecosystems could lead to substantial releases of carbon into the atmosphere. Avoidance of tipping points is a paramount challenge to civilization. Only by continuing to seek a better understanding of the carbon cycle can the predictability of these events be improved.