Key Finding 1
Observing networks and high-resolution models are now available to construct coastal carbon budgets. Efforts have focused primarily on quantifying the net air-sea exchange of carbon dioxide (CO2), but some studies have estimated other key fluxes, such as the exchange between shelves and the open ocean.
Description of evidence base
Observing networks are in place along the Atlantic, Pacific, and Arctic coasts of North America and the U.S. Gulf Coast (Alin et al., 2015; Bates et al., 2006, 2011; Cai et al., 2010a; Chen et al., 2013; Cross et al., 2014a; Dai et al., 2013; DeGrandpre et al., 2002; Evans et al., 2011, 2012, 2015b; Hales et al., 2005, 2012; Jiang et al., 2008; Mucci et al., 2010; Najjar et al., 2018; Robbins et al., 2009, 2014; Salisbury et al., 2008b, 2009; Shadwick et al., 2010, 2011; Vandemark et al., 2011; Wang et al., 2013, 2017).
Regional models are in place for the same regions (Cahill et al., 2016; Fennel et al., 2008; Fiechter et al., 2014; Pilcher et al., 2018; Previdi et al., 2009; Turi et al., 2014; Xue et al., 2016).
The emphasis on quantifying air-sea exchange is illustrated by the fact that the references listed in Table 16.1 all provide an estimate of this flux, but few provide estimates of other fluxes. Few studies exist that do provide estimates of carbon exchange between shelves and open ocean; they include Fennel and Wilkin (2009), Barth et al. (2002), Hales et al. (2006), Xue et al. (2016), and Najjar et al. (2018).
Major uncertainties
This key message essentially contains statements of fact. Hence, this statement is not considered uncertain.
Key Finding 2
Available estimates of air-sea carbon fluxes, based on more than a decade of observations, indicate that the North American margins act as a net sink for atmospheric CO2. This net uptake is driven primarily by fluxes in the high-latitude regions. The estimated magnitude of the net flux is 160 ± 80 teragrams of carbon per year (medium confidence) for the North American Exclusive Economic Zone, a number that is not well constrained.
Description of evidence base
This statement is supported by the numbers summarized in Tables 16.1 and 16.2. Consistent reports of outgassing exist only for the Gulf of Maine (GOM), where the net flux is almost neutral, and the West Florida Shelf. Contradictory reports exist for the Scotian Shelf. Everywhere else the net flux is reported as net uptake (i.e., sink), although with large uncertainties. Three independent studies also provide estimates of net air-sea CO2 exchange in North American coastal waters. Two are global data syntheses (Chen et al., 2013; Laruelle et al., 2014), and one is from a process-based global model (Bourgeois et al., 2016; see Table 16.2). The model of Bourgeois et al. (2016) estimates a net air-sea CO2 flux of 160 teragrams of carbon (Tg C) per year for the North American Exclusive Economic Zone (EEZ). The estimate is that the uncertainty is 50%.
These individual estimates cannot be combined because of discrepancies in numbers and gaps in coverage.
Major uncertainties
The consistency among studies pointing at North American coastal waters as a sink provides confidence, although each individual estimate is uncertain.
Assessment of confidence based on evidence and agreement, including short description of nature of evidence and level of agreement
The statement that North American coastal waters act as a sink overall can be made with high confidence and reflects the fact that studies are consistent in supporting this conclusion, even though each number itself comes with a large uncertainty. The overall uptake estimate is uncertain; hence, there is high confidence in stating that this flux estimate is poorly constrained.
Summary sentence or paragraph that integrates the above information
The consistency of many independent estimates reporting coastal uptake of atmospheric CO2 builds confidence that these waters indeed act as a sink.
Key Finding 3
The increasing concentration of CO2 in coastal and open-ocean waters leads to ocean acidification. Corrosive conditions in the subsurface occur regularly in Arctic coastal waters, which are naturally prone to low pH, and North Pacific coastal waters, where upwelling of deep, carbon-rich waters has intensified and, in combination with the uptake of anthropogenic carbon, leads to low seawater pH and aragonite saturation states in spring, summer, and early fall (very high confidence, very likely).
Description of evidence base
In Arctic coastal waters, pH and carbonate saturation state are naturally low (Cai et al., 2010b; Mathis et al., 2011; Steiner et al., 2013). The pace of ocean acidification is faster in the Arctic than in other coastal and open-ocean regions (Fabry et al., 2009; Feely et al., 2009; Mathis et al., 2015a). The Beaufort Sea upper halocline and deep waters now regularly show aragonite undersaturation (Mathis et al., 2015a; Miller et al., 2014). These chemical seawater signatures are propagated via M’Clure Strait and Amundsen Gulf into the Canadian Archipelago and beyond (Azetsu-Scott et al., 2010; Turk et al., 2016; Yamamoto-Kawai et al., 2013). Variability in the carbon content of freshwater end members also has been shown to contribute to undersaturation events in coastal waters of the Gulf of Alaska region (Siedlecki et al., 2017; Evans et al., 2014)
In the North America Pacific Coast (NAPC) region, anthropogenic CO2 uptake combined with climate-driven changes in upwelling circulation result in coastal acidification events. Upwelling-favorable winds along the NAPC have intensified over recent years, especially in the northern parts of the upwelling regimes (García-Reyes et al., 2015; McClatchie et al., 2016; Rykaczewski and Checkley 2008; Rykaczewski et al., 2015; Sydeman et al., 2014). In the northern California Current System, pCO2 (partial pressure of CO2), pH, and aragonite saturation reach levels known to be harmful to ecologically and economically important species during the summer upwelling season (see Ch. 17: Biogeochemical Effects of Rising Atmospheric Carbon Dioxide; Barton et al., 2012, 2015; Bednaršek et al., 2014, 2016, 2017; Feely et al., 2008, 2016, 2018; Harris et al., 2013; Siedlecki et al., 2016).
Major uncertainties
Statement is well supported by the literature. No major uncertainties.
Assessment of confidence based on evidence and agreement, including short description of nature of evidence and level of agreement
Statement is well supported by the literature. No major uncertainties.
Estimated likelihood of impact or consequence, including short description of basis of estimate
Corrosive waters have been observed in the Arctic and North Pacific coastal regions (Feely et al., 2008, 2016; Mathis et al., 2015a; Miller et al., 2014). A more comprehensive list of references is given in the description above and in the chapter body.
Summary sentence or paragraph that integrates the above information
Statement that corrosive waters regularly occur is well supported by the literature because these conditions have been directly observed. There are no major uncertainties.
Key Finding 4
Expanded monitoring, more complete syntheses of available observations, and extension of existing model capabilities are required to provide more reliable coastal carbon budgets, projections of future states of the coastal ocean, and quantification of anthropogenic carbon contributions.
Description of evidence base
The underlying motivation for constructing complete carbon budgets for coastal waters is that permanent burial of carbon in coastal sediments and export of carbon from coastal waters to the deep ocean both remove anthropogenic carbon from the atmospheric reservoir. The relevant carbon flux in this context is the burial or export of anthropogenic carbon, not total burial or export. Only total fluxes can be observed directly. Distinction between anthropogenic fluxes and the natural background has not been attempted in regional observational or modeling studies, because more comprehensive accounting than is available for carbon fluxes and improved modeling capabilities would be needed. The study by Bourgeois et al. (2016) is the first to estimate coastal anthropogenic carbon uptake in a global model. The estimated net air-sea exchange of CO2 from this global model is reported for a regional decomposition of the EEZs of the United States, Canada, and Mexico in Table 16.3. The model simulates a net uptake of CO2 in North American coastal waters that is of similar magnitude to estimates of organic carbon burial and riverine carbon input, but the latter two numbers are uncertain because they are each taken from one individual study and not corroborated by multiple references. However, the similar magnitudes of these numbers illustrate that current coastal carbon budgets are uncertain and that constraining just the air-sea gas exchange will not be sufficient to quantify the export of anthropogenic carbon by coastal processes.
Major uncertainties
This report’s synthesis of the current literature shows that the magnitudes of several significant components of coastal carbon budgets are currently uncertain.
Summary sentence or paragraph that integrates the above information
The synthesis in this chapter shows that coastal carbon budgets and anthropogenic contributions to the underlying fluxes are currently uncertain. Thus, more observations and modeling efforts could reduce these uncertainties.
