The North American Carbon Budget

Key Finding 2

Fossil fuel emissions were the largest carbon source from North America from 2004 to 2013, averaging 1,774 Tg C per year (±5.5%). Emissions during this time showed a decreasing trend of 23 Tg C per year, a notable shift from the increasing trend over the previous decade. The continental proportion of the global total fossil fuel emissions decreased from 24% in 2004 to 17% in 2013 (very high confidence).

Description of evidence base

Key Finding 2 is supported by fossil fuel inventories collected by the Carbon Dioxide Information and Analysis Center (CDIAC) and made available in the territorial fossil fuel carbon emissions dataset (Boden et al., 2017). Among the various sources of emissions data (see Appendix E: Fossil Fuel Emissions Estimates for North America), the CDIAC dataset was chosen for its consistency and length of record. However, to represent the data uncertainty, the SOCCR2 assessment used the fractional range of estimates from five different inventories, averaged over time.

Major uncertainties

The absolute values of greenhouse gas (GHG) emissions levels from energy consumption and production vary significantly due to differences in system definitions, inclusion of industrial process emissions, emissions factors applied, and other issues (see Ch. 3: Energy Systems). Accuracy of the fossil fuel emissions estimates is less certain at finer spatial and temporal scales, and uncertainty at the scale of individual cities is not well constrained (Gurney et al., 2015; Hutyra et al., 2014; Rayner et al., 2010). Furthermore, the magnitude of methane (CH₄) leakage from fossil fuel production and use has a high degree of uncertainty in the inventories (Brandt et al., 2014).

Assessment of confidence based on evidence and agreement, including short description of nature of evidence and level of agreement

There is very high confidence that fossil fuel emissions are the dominant source of carbon from the North American continent.

Estimated likelihood of impact or consequence, including short description of basis of estimate

Fossil fuel emissions from North America very likely will continue to contribute substantially to the rise in global atmospheric CO₂ concentration.

Summary sentence or paragraph that integrates the above information

Total fossil fuel emissions from the Canadian, U.S., and Mexican energy and transportation systems very likely are and will continue to be substantially greater in magnitude than any other source category, including agriculture and livestock, land-use change, and natural disturbance.

Key Finding 3

Approximately 43% of the continent’s total fossil fuel emissions from 2004 to 2013 were offset by natural carbon sinks on North American land and the adjacent coastal ocean (medium confidence).

Description of evidence base

Key Finding 3 is supported by fossil fuel emissions data (Boden et al., 2015), forest inventories in the United States (Woodall et al., 2015; see Ch. 9: Forests) and Canada (Stinson et al., 2011), atmospheric inverse modeling ensembles (see Ch. 8: Observations of Atmospheric Carbon Dioxide and Methane), terrestrial biosphere model ensembles (Huntzinger et al., 2012), and synthesis studies (Hayes et al., 2012; King et al., 2012, 2015).

Major uncertainties

The land sink is uncertain due to a lack of measurement precision in inventories, along with gaps in spatial coverage and uncertainty in specific components such as the soil carbon pool. The overall land sink is inferred from reconciling a number of estimates from different components, themselves often highly uncertain. In particular, the component with the largest estimate of the inferred ecosystem flux—the lateral transfer to the aquatic system—is also one of the least certain (see Table 2.2).

Assessment of confidence based on evidence and agreement, including short description of nature of evidence and level of agreement

At least some portion of anthropogenic carbon emissions to the atmosphere is very likely offset by vegetation uptake and storage in North American land ecosystems. There is medium confidence in the “best estimate” of 43% as the proportion of total fossil fuel emissions taken up by North American land and coastal ocean areas.

Estimated likelihood of impact or consequence, including short description of basis of estimate

The natural ecosystems of the North American continent likely have represented a net carbon sink over the recent decade, thereby constraining the airborne fraction of anthropogenic emissions from fossil fuel carbon consumption and thus mitigating further climate impacts from rising atmospheric CO₂.

Summary sentence or paragraph that integrates the above information

For Key Finding 3, North America’s natural and managed ecosystems and its adjacent coastal ocean likely will continue to take up some of the total fossil fuel carbon emitted to the atmosphere from anthropogenic activities. However, the fraction of emissions taken up by the ecosystem in the future is uncertain and will depend on energy use, the response of natural ecosystems to climate change and other disturbances, and human management of the land and the coastal ocean.

Key Finding 4

Using bottom-up, inventory-based calculations, the Second State of the Carbon Cycle Report (SOCCR2) estimates that the average annual strength of the land-based carbon sink in North America was 606 Tg C per year (±75%) during the 2004 to 2013 time period, compared with the estimated 505 Tg C per year (±50%) in ca. 2003, as reported in the First State of the Carbon Cycle Report (CCSP 2007). There is apparent consistency in the two estimates, given their ranges of uncertainty, with SOCCR2 calculations including additional information on the continental carbon budget. However, large uncertainties remain in some components (very high confidence).

Description of evidence base

Key Finding 4 is supported by observational evidence from forest inventories in the United States (Woodall et al., 2015) and Canada (Stinson et al., 2011), atmospheric inverse modeling ensembles (see Ch. 8: Observations of Atmospheric Carbon Dioxide and Methane), terrestrial biosphere model ensembles (Huntzinger et al., 2012), and synthesis studies (Hayes et al., 2012; King et al., 2012, 2015). The U.S. forest sink is maintained because of the net accretion of forest land use in combination with continued forest growth (i.e., forests remaining forests; Woodall et al., 2015, 2016).

Major uncertainties

Components of the North American carbon cycle measured as part of formal inventory programs, such as the forest and agricultural sectors, are estimated with a high level of certainty. However, other components potentially contribute significantly to the magnitude of the continental carbon sink (see Table 2.2). The largest of these comprises the net emissions from inland waters, which at the continental scale are poorly constrained (i.e., uncertainty is effectively 100% of the estimate). Also contributing substantially to the overall uncertainty are other important components of the land base in regions where measurement gaps exist over large areas, such as in Mexico and the remote northern areas of Canada and Alaska.

Assessment of confidence based on evidence and agreement, including short description of nature of evidence and level of agreement

There is very high confidence that the North American land base has maintained an overall carbon sink over the past decade, with net carbon uptake and storage in the vegetation and soils of natural and managed ecosystems.

Estimated likelihood of impact or consequence, including short description of basis of estimate

North America’s natural ecosystems likely have maintained a net carbon sink over recent decades, thereby constraining the airborne fraction of fossil fuel carbon and mitigating further climate impacts from rising atmospheric CO₂.

Summary sentence or paragraph that integrates the above information

For Key Finding 4, the sink is likely to maintain its approximate current magnitude because of carbon uptake and storage in the forest sector (i.e., the land base and wood products).

Key Finding 5

The magnitude of the continental carbon sink over the last decade is estimated at 699 Tg C per year (±12%) using a top-down approach and 606 Tg C per year (±75%) using a bottom-up approach, indicating an apparent agreement between the two estimates considering their uncertainty ranges.

Description of evidence base

The integrated, continental-scale estimates of the overall carbon sink comprise compilations from 1) recent top-down, atmospheric approaches (see Ch. 8: Observations of Atmospheric Carbon Dioxide and Methane); 2) comparisons of bottom-up, inventory-, and model-based estimates from the various sector-focused chapters in this report; and 3) data and estimates synthesized in Table 2.2 and Figure 2.3 and discussed in the context of the results from previous continental carbon cycle synthesis efforts (e.g., CCSP 2007; Hayes et al., 2012; King et al., 2015).

Major uncertainties

The bottom-up estimate of the overall continental-scale carbon sink presented here is inferred from reconciling a number of estimates from different components, themselves often highly uncertain. Even components estimated in formal inventories (e.g., the forest sector) have pools and fluxes that are less well quantified (e.g., forest soils) and regional and temporal gaps in measurements. A large component of the uncertainty stems from limited information about the magnitude, spatial distribution, and temporal variability of carbon sources and sinks in inland, tidal, and coastal waters. Uncertainty in the top-down, atmospheric-based estimates is primarily from sparse observational networks and often poorly constrained models of atmospheric transport.

Summary sentence or paragraph that integrates the above information

In previous studies over the past decade, the larger bottom-up sink estimates have approached the lower end of the uncertainty in atmospheric model estimates (King et al., 2012). For Key Finding 5, the results presented here show further convergence between the top-down, ­continental-scale carbon sink estimate from atmospheric modeling and the synthesis of estimates from bottom-up approaches across the major components of North America (see Figure 2.5).