The North American Atlantic coast borders a wide, geologically passive-margin shelf that extends from the southern tip of Florida to the continental shelf of the Labrador Sea (Fig. ). The shelf is several hundred kilometers wide in the north (Labrador shelf and Grand Banks) but narrows progressively toward the south in the Mid-Atlantic Bight (MAB), which is between Cape Cod and Cape Hatteras, and the South Atlantic Bight (SAB), which is south of Cape Hatteras. The SAB shelf width measures only several tens of kilometers. Two major semi-enclosed bodies of water are the Gulf of Maine (GOM) and Gulf of St. Lawrence. Important rivers and estuaries north of Cape Hatteras include the St. Lawrence River and Estuary, the Hudson River, Long Island Sound, Delaware Bay, and Chesapeake Bay. South of Cape Hatteras, the coastline is characterized by small rivers and marshes.

The SAB is impacted by the Gulf Stream, which flows northeastward along the shelf edge before detaching at Cape Hatteras and meandering eastward into the open North Atlantic Ocean. North of Cape Hatteras, shelf circulation is influenced by the confluence of the southwestward-flowing fresh and cold shelf-break current (a limb of the Labrador Current) and the warm and salty Gulf Stream . Because shelf waters north of Cape Hatteras are sourced from the Labrador Sea, they are relatively cold, fresh, and carbon rich, while slope waters (those located between the shelf break and the northern wall of the Gulf Stream) are a mixture of Labrador Current and Gulf Stream water. South of Cape Hatteras, exchange between the shelf and open ocean across the shelf break is impeded by the presence of the Gulf Stream and occurs via baroclinic instabilities in its northern wall . In the MAB and on the Scotian Shelf, cross-shelf exchange is hindered by shelf-break jets and fronts .

Air–sea fluxes of CO2 exhibit a large-scale latitudinal gradient along the Atlantic coast (Fig. ) and significant seasonal and interannual variability (Fig. ). Discrepancies in independent estimates of net air–sea flux are largest for the Scotian Shelf and the Gulf of Maine (Fig. ). For the Scotian Shelf, , combining in situ and satellite observations, reported a large source of CO2 to the atmosphere of 8.3±6.6 g C m-2 yr-1. In contrast, estimated a relatively large sink of atmospheric CO2, 14±3.2 g C m-2 yr-1, when using in situ data alone and a much smaller uptake, 5.0±4.3 g C m-2 yr-1, from a combination of in situ and satellite observations. The open GOM (excluding the tidally mixed Georges Bank and Nantucket Shoals) was reported as a weak net source of 4.6±3.1 g C m-2 yr-1 by but with significant interannual variability, while estimated the region to be neutral (Table S1). The shallow, tidally mixed Georges Bank and Nantucket Shoals are thought to be sinks, however (Table S1).

The MAB and SAB are consistently estimated to be net sinks. Observation-based estimates for the MAB sink are 13±8.3 and 13±3.2 g C m-2 yr-1 . Estimates for the SAB sink are 5.8±2.5 and 8.2±2.9 g C m-2 yr-1 . The transition from a neutral or occasional net source on the Scotian Shelf and in the GOM to a net sink in the MAB arises because the properties of shelf water are modified during its southwestward flow by air–sea exchange, inflows of riverine and estuarine waters , and exchange with the open North Atlantic across the shelf break . The cold, carbon-rich water on the Scotian Shelf carries a pronounced signature of its Labrador Sea origin. The GOM, which is deeper than the Scotian Shelf and the MAB and connected to the open North Atlantic through a relatively deep channel, is characterized by a mixture of cold, carbon-rich shelf waters entering from the Scotian Shelf and warmer, saltier slope waters. Shelf water in the MAB is sourced from the GOM and is thus a mixture of Scotian Shelf and slope water.

Shelf water in the SAB is distinct from that in the MAB and has almost no trace of Labrador Current water; instead, its characteristics are similar to those of the Gulf Stream, but its carbon signature is modified by significant organic and inorganic carbon and alkalinity inputs from coastal marshes . estimated that 59 % of the 3.4 Tg C yr-1 of organic carbon export from US East Coast estuaries occurs in the SAB. The subsequent respiration of this organic matter and direct outgassing of marsh-derived carbon make the nearshore regions a significant CO2 source almost year-round. Despite the carbon inputs from marshes, the uptake of CO2 on the mid-shelf and outer-shelf regions during the winter months is large enough to balance CO2 outgassing in the other seasons and on the inner shelf, making the SAB an overall weak sink .

The seasonality of CO2 dynamics along the Atlantic coast varies with latitude. North of Cape Hatteras, CO2 dynamics are characterized by strong seasonality with solubility-driven uptake by cooling in winter and biologically driven uptake in spring followed by outgassing in summer and fall due to warming and respiration of organic matter Fig. d;. South of Cape Hatteras, seasonal phytoplankton blooms do not occur regularly and biologically driven CO2 uptake is less pronounced than that further north Fig. e;, although sporadic phytoplankton blooms do occur because of intrusions of high-nutrient subsurface Gulf Stream water . The influence of riverine inputs is small and localized in the SAB . An exception to this north–south importance in riverine input is the GOM, where riverine inputs of carbon and nutrients are relatively small. Nevertheless, even here, these inputs can cause local phytoplankton blooms, CO2 drawdown, and low-pH conditions .

Regional biogeochemical models reproduce the large-scale patterns of air–sea CO2 flux with oceanic uptake increasing from the SAB to the GOM . These model studies elucidate the magnitude and sources of interannual variability as well as long-term trends in air–sea CO2 fluxes. investigated opposite phases of the North Atlantic Oscillation (NAO) and found that the simulated air–sea flux in the MAB and GOM was 25 % lower in a high-NAO year compared to a low-NAO year. In the MAB, the decrease primarily resulted from changes in wind forcing, while in the GOM changes in surface temperature and new production were more important. investigated the impact of future climate-driven warming and trends in atmospheric forcing (primarily wind) on air–sea CO2 flux (without considering the atmospheric increase in CO2). Their results suggest that warming and changes in atmospheric forcing have modest impacts on air–sea CO2 flux in the MAB and GOM compared to that in the SAB where surface warming would turn the region from a net sink into a net source of CO2 to the atmosphere if the increase in atmospheric CO2 were ignored. Model studies also illustrate the effects of interactions between biogeochemical transformations in the sediment and the overlying water column on carbon fluxes. For example, showed that the effective alkalinity flux resulting from denitrification in sediments of the Atlantic coast reduces the simulated ocean uptake of CO2 by 6 % compared to a simulation without sediment denitrification.

The passive-margin sediments along the Atlantic coast have not been considered an area of significant CH4 release until recently . predicted that massive seepage of CH4 from upper-slope sediments is occurring in response to warming of intermediate-depth Gulf Stream waters. and documented widespread CH4 plumes in the water column and attributed them to gas hydrate degradation. Estimated CH4 efflux from the sediment in this region ranges from 1.5×10-5 to 1.8×10-4 Tg yr-1, and the uncertainty range reflects different assumptions underlying the conversion from CH4 plume observations to seepage rates. The fraction of the released CH4 that escapes to the atmosphere remains uncertain .

The North American Pacific coast extends from Panama to the Gulf of Alaska and is an active margin with varying shelf widths (Fig. ). The continental shelf is narrow along the coasts of California, Oregon, and Washington, of the order of 10 km, but widens significantly in the Gulf of Alaska, where shelves extend up to 200 km offshore. In the Gulf of Alaska, freshwater and tidal influences strongly affect cross-shelf exchange, and the shelf is dominated by a downwelling circulation. The region from Vancouver Island to Baja California is a classic eastern boundary current upwelling region – the California Current System . Winds drive a coastal upwelling circulation characterized by equatorward flow in the California Current and by coastal jets and their associated eddies and fronts that extend offshore, particularly off the coasts of Baja California, California, Washington, and Oregon .

The northern California Current System experiences strong freshwater influences and seasonality in wind forcing that diminish toward the south. In addition to the Columbia River and the Fraser River, a variety of small mountainous rivers, with highly variable discharge, supply freshwater. The Central American Isthmus runs from Panama to the southern tip of Baja California and experiences intense and persistent wind events, large eddies, and high waves that combine to produce upwelling and strong nearshore mixing . In addition to alongshore winds, strong seasonal wind jets that pass through the Central American cordillera create upwelling hot spots and drive production during boreal winter months in the gulfs of Tehuantepec, Papagayo, and Panama . The California Current brings water from the North Pacific southward into the southern California and Central American Isthmus regions, while the California Undercurrent transports equatorial waters northward in the subsurface .

The net exchange of CO2 with the atmosphere along the Pacific coast is characterized by strong spatial and temporal variation and reflects complex interactions between the biological uptake of nutrients and degassing of nutrient- and carbon-rich upwelled waters (Fig. b, c). A growing number of coastal air–sea flux studies have used extrapolation techniques to estimate fluxes across the coastal oceans on regional to continental scales (Fig. ). Observation-based studies of air–sea CO2 flux suggest that estimates for the coastal ocean from Baja California to the Gulf of Alaska range from a weak to moderate sink of atmospheric CO2 over this broad longitudinal range. Central California coastal waters have long been understood to have a near-neutral air–sea CO2 exchange because of their large and counterbalancing periods of efflux during upwelling conditions and influx during periods of relaxation and high primary productivity; this pattern is strongly modulated by El Niño–La Niña conditions .

used seasonal data to estimate an uptake of 88 g C m-2 yr-1 by Oregon coastal waters. In a follow-up analysis with greater temporal coverage, showed how large flux events can significantly alter the estimation of net exchanges for the Oregon shelf. After capturing a large and short-lived efflux event, their annual estimate was outgassing of 3.1±82 g C m-2 yr-1 for this same region. The disparity illustrates the importance of basing regional flux estimates on observations that are well resolved in time and space. Capitalizing on the increased and more uniform spatiotemporal coverage of satellite data, estimated an annual mean uptake of 7.9 g C m-2 yr-1 between 22 and 50∘ N within 370 km offshore. The most northern estimates for the Pacific coast by and are influxes of 26 g C m-2 yr-1 for British Columbian coastal waters shoreward of the 500 m isobath and 18 g C m-2 yr-1 for Gulf of Alaska coastal waters shoreward of the 1500 m isobath.

Models for the upwelling region reproduce the pattern of CO2 outgassing nearshore and CO2 uptake further offshore. They also illustrate the intense eddy-driven variability nearshore. simulate a weak source of 0.6±2.4 g C m-2 yr-1 for the region from 30 to 46∘ N extending 800 km of shore. In contrast, reported a sink of 7.9 g C m-2 yr-1 based on observations for the same latitudinal band but only extending 370 km of shore. simulate a source of atmospheric CO2 of 0.6 Tg C yr-1 for the region from 35 to 45∘ N within 600 km of shore, also in contrast to the observation-based estimate of a 14 Tg C yr-1 sink published by . (The estimate of , is not included in Fig.  because the area-normalized flux is not available from that study.) Both models simulate strong outgassing within the first 100 km of shore driven by intense upwelling of nutrient- and carbon-rich water, which is compensated for by biologically driven CO2 uptake from the atmosphere as upwelled nutrients are consumed by photosynthesis during subsequent offshore advection within several hundreds of kilometers of the coast. The disagreement in mean simulated fluxes between the two models may partly result from different choices of averaging region and period and differences in model forcing, such as the climatological forcing in versus realistic variability in . Notably, observations for the Oregon shelf by showed intense summer upwelling that led to strong outgassing with pronounced variability in air–sea fluxes but only weak stimulation of primary production. They hypothesized that nutrient-rich waters might be subducted offshore at convergent surface temperature fronts before nutrients are fully consumed by primary producers.

The cross-shelf exchange of carbon occurs in the California Current System mostly in response to wind-driven circulation and eddies, but river plumes and tides have also been shown to increase offshore transport in the northern part of the system . Uncertainties in published estimates are high, ranging from very small to very high fractions of primary production , again as a result of the region's large spatial and temporal variability. showed that about 30 % of the organic matter produced within 100 km of shore is laterally advected toward the open ocean.

Less is known about the air–sea flux of CH4 along the Pacific margin. Recent studies inventoried sedimentary sources of CH4 hydrates, derived from terrestrial and coastal primary production, and suggested that extensive deposits along the Cascadia margin are beginning to destabilize because of warming .

The Gulf of Mexico is a semi-enclosed marginal sea at the southern coast of the conterminous United States. The passive-margin shelves of its northern portion are relatively wide (up to 250 km west of Florida) but, in contrast to shelf waters of the Atlantic coast, those of the Gulf of Mexico are not separated from open-ocean waters by shelf-break fronts or currents. Ocean water enters the gulf mainly through the Yucatán Channel, where it forms the northeastward meandering Loop Current, which sheds anticyclonic eddies and exits the gulf through the Florida Straits . While shelf circulation is primarily influenced by local wind and buoyancy forcing, outer-shelf regions are at times influenced by Loop Current eddies that impinge on and interact with the shelf . Riverine input is substantial in the northern Gulf of Mexico, where the Mississippi–Atchafalaya river system delivers large loads of freshwater, nutrients, and sediments.

Estimates of air–sea CO2 flux are available from observations and model simulations (Fig. ). Observational estimates indicate that the Gulf of Mexico, as a whole, is a weak net sink of atmospheric CO2 with an annual average of 2.3±1.0 g C m-2 yr-1 . Smaller shelf regions within the gulf differ markedly from this mean flux. The West Florida shelf and western gulf shelf act as sources to the atmosphere, with estimated annual average fluxes of 4.4±1.3 and 2.2±0.6 g C m-2 yr-1, respectively; the northern gulf acts as a sink, with an estimated flux of 5.3±4.4 g C m-2 yr-1, and the Mexican shelf is almost neutral, with an estimated uptake flux of 1.1±0.6 g C m-2 yr-1 . A more recent estimate for the west Florida shelf is 4.4±18.0 g C m-2 yr-1 . estimated a larger uptake on the northern gulf shelf of 11±44 g C m-2 yr-1 (i.e., about twice the estimate of ) and reported a much larger uncertainty. In an analysis that combines satellite and in situ observations, estimated a similar uptake for the northern Gulf of Mexico of 13±3.6 g C m-2 yr-1. The overall air–sea carbon exchanges in the gulf vary significantly from year to year because of interannual variability in wind, temperature, and precipitation .

Model-simulated air–sea CO2 fluxes by agree relatively well with the estimates of , reproducing the same spatial pattern though their simulated gulf-wide uptake of 8.5±6.5 g C m-2 yr-1 is larger. This discrepancy results largely from a greater simulated sink in the open gulf. Also, the uncertainty estimates of the model-simulated fluxes by are much larger than those of ; the latter might be too optimistic in reporting uncertainties of the flux estimates. Overall, the various observation- and model-derived estimates for gulf regions agree in terms of their broad patterns, but existing discrepancies and, at times, large uncertainties indicate that current estimates need further refinement.

The quantitative understanding of CH4 dynamics in coastal and oceanic environments of the Gulf of Mexico is limited. speculated that deep CH4 hydrate seeps in the gulf are a potentially significant CH4 source to the atmosphere. They estimated ocean–atmosphere fluxes from seep plumes of 1150±790 to 38000±21000 g CH4 m-2 d-1 compared with 2.2±2.0 to 41±8.2 g CH4 m-2 d-1 for background sites. Subsequent acoustic analyses of bubble plume characteristics question the finding that CH4 bubbles make their way to the surface , and the fate of CH4 emissions from seeps and their overall contribution to atmospheric CH4 remain uncertain.

The North American Arctic coastal ocean comprises broad (∼300 km) shallow shelves in the Bering and Chukchi seas, the narrower (<100 km) Beaufort Sea shelf, Hudson Bay, and the extensive Canadian Arctic shelf (Fig. ). Shelf water enters these regions from the North Pacific through the Bering Strait and follows a large-scale pathway via the Chukchi and Beaufort seas onto the Canadian Arctic shelf and, ultimately, the North Atlantic . Hudson Bay receives significant inputs of freshwater . Except for the southernmost Bering Sea, most of the coastal Arctic is covered with sea ice from about October to June. Areas of persistent multiyear sea ice at the northernmost extent of the Canadian Arctic shelf are rapidly declining .

Coastal waters in the Arctic have been consistently described as a net sink for atmospheric CO2 (Fig. ). An observation-based estimate for uptake in the Bering Sea is 9.6 g C m-2 yr-1 by . Estimates for the Chuckchi Sea range from 15 g C m-2 yr-1 to 175±44 g C m-2 yr-1. For the Beaufort Sea, estimates range from 4.4 g C m-2 yr-1 to 44±28 g C m-2 yr-1 . In Hudson Bay, estimated an uptake of 3.2±1.8 g C m-2 yr-1. The consistently observed uptake is thought to be caused by low surface water pCO2 relative to the atmosphere during ice-free months (see, e.g., Fig. a). These low levels are set by a combination of low water temperatures and seasonally high rates of both ice-associated and open-water primary production , as well as by limited gas exchange through sea ice relative to open water during winter .

In recent years, sea ice growth and decay has been shown to significantly affect the air–sea CO2 flux . During sea ice formation, brine rejection forms dense high-saline water that is enriched in DIC relative to alkalinity . During sea ice decay, an excess of alkalinity relative to DIC is released in meltwater. The sinking of dense, carbon-rich brine provides a pathway for carbon sequestration as suggested by , although modeling studies suggest that carbon export through this process is relatively small .

With regard to Arctic CH4 fluxes, much more is known about the emission potential, distribution, and functioning of terrestrial sources ; knowledge of marine CH4 sources is developing slowly due to sparse observations and the logistical challenges of Arctic marine research. The largest marine CH4 source in the Arctic is the dissociation of gas hydrates stored in continental margin sediments . As sea ice cover continues to retreat and ocean waters warm, CH4 hydrate stability is expected to decrease with potentially large and long-term implications. An additional potential marine CH4 source, unique to polar settings, is release from subsea permafrost layers, with fluxes from thawed sediments reported to be orders of magnitude higher than fluxes from adjacent frozen sediments .

Despite the variability in regional estimates discussed above and summarized in Fig.  and Table S1, North American coastal waters clearly act as a net sink of atmospheric carbon. However, because of the local footprint of some studies, discrepancies in temporal and spatial coverage among studies, and gaps in space and time, it is difficult to combine these various regional estimates into one summary estimate of CO2 uptake for the North American EEZ with any confidence. In order to arrive at such a summary estimate, we draw on the regional information compiled in the previous sections in combination with two global approaches: the observation-based synthesis of and the process-based global model of .

First, we compare estimates from the two global approaches, which are available for a global segmentation of the coastal zone and associated watersheds known as MARCATS MARgins and CATchments Segmentation;. At a resolution of 0.5∘, MARCATS delineates a total of 45 coastal segments, eight of which surround North American. analyzed the Surface Ocean CO2 Atlas 2.0 database and estimated an uptake in the North American MARCATS, excluding Hudson Bay, of 44.5 Tg C yr-1 (Table ). The process-based model of simulated an uptake of 48.8 Tg C yr-1, or 45.0 Tg C yr-1 when excluding Hudson Bay (Table ). Although there are significant regional discrepancies between the two estimates for the eastern tropical Pacific Ocean, the Gulf of Mexico, the Florida upwelling region (actually covering the eastern United States including the SAB, MAB, and GOM), the Labrador Sea, and the Canadian Arctic shelf, the total flux estimates for North America are in close agreement. This builds some confidence in the global model estimates.

Next, we use the global model to obtain CO2 flux estimates for the North American EEZ in a finer-grained decomposition (Fig. , Table ). First, we compare the regional area-specific flux estimates from Fig.  and Table  with the global model (see Fig. a, b, d). For the Atlantic coast there is excellent agreement for the SAB, but the global model simulates a higher CO2 uptake than the regional studies in the MAB, GOM, and the Scotian Shelf. This discrepancy is likely due to the fact that the global model is too coarse to accurately simulate the shelf-break current systems and thus significantly underestimates the water residence times on the shelf . No regional estimates are available for the Gulf of St. Lawrence. For the Pacific coast and the Gulf of Mexico, the global model's estimates are within the range of available regional estimates for the Gulf of Alaska, the northern California Current System, and the Gulf of Mexico. In the central and southern California Current System, the model favors outgassing more than in the regional estimates. In the sub-Arctic and Arctic regions, the global model's estimates are smaller than the regional ones for the Beaufort and Chukchi seas and Hudson Bay, but they are similar for the Bering Sea. No regional estimates are available for the Canadian Arctic shelf and Labrador shelf.

Conversion of the area-specific fluxes to total fluxes (see Fig. c, e) shows that the Labrador shelf, Gulf of Alaska, and Bering Sea are the biggest contributors to the total flux in the North American EEZ in the global model. For the Labrador shelf, the region with the largest flux estimate, there are no regional estimates available. For the two next largest, the Gulf of Alaska and the Bering Sea, the global model agrees well with the regional estimates. Thus, despite some regional discrepancies, it seems justifiable to use the global model estimates for a North American coastal carbon budget. The model simulates a net uptake of CO2 in the North American EEZ (excluding the EEZ of the Hawaiian and other islands) of 160 Tg C yr-1 with an anthropogenic flux contribution of 59 Tg C yr-1. The three biggest contributors (LS, GAK, and BS; see Fig. ) account for 93 Tg C yr-1, 58 % of the total uptake, with an anthropogenic flux contribution of 26 Tg C yr-1, while making up only 29 % of the combined EEZ area. These three regions have a large contribution because they are characterized by large area-specific fluxes and they are vast. The global model also estimates large fluxes for the MAB, GOM, and SS, which are probably overestimates, but these regions are significantly smaller and thus contribute much less (28 Tg C yr-1) to the overall flux estimate. When dividing the North American EEZ into the Arctic and sub-Arctic (GAK, BS, BCS, CAS, HB, LS), mid-latitude Atlantic (GMx, SAB, MAB, GOM, SS, GStL), and mid-latitude Pacific (CCN, CCC, CCS, Isthmus), their relative flux contributions are 104, 62, and -3.7 Tg C yr-1, respectively, with area contributions of 51 %, 25 %, and 24 % to the total area of the North American EEZ, respectively.

Next, we construct a carbon budget for the North American EEZ by combining the atmospheric CO2 uptake estimate with estimates of carbon transport from land and carbon burial in ocean sediments. We assume 160 Tg C yr-1 as the best estimate of net uptake by coastal waters of North America, excluding tidal wetlands and estuaries. Unfortunately, there are no formal error estimates for this uptake. Instead, we estimate an error by first noting that the model is in good agreement with the observation-based estimates for the MARCATS regions of North America; furthermore, the error estimate for the uptake by continental shelves globally is about 25 %, with the North American MARCATS regions having mainly “fair” data quality . Hence, assuming an error of ±50 % for the uptake by North American EEZ waters seems reasonable.

Carbon delivery to the coastal ocean from land via rivers and from tidal wetlands after estuarine processing (i.e., CO2 outgassing and carbon burial in estuaries) is estimated to be 106±30 Tg C yr-1 . Estimates of carbon burial, based on the method of for the regional decomposition of the North American EEZ, are reported in Table , with a total flux of 120 Tg C yr-1. We consider these fluxes to be an upper bound because they are substantially larger than other estimates. The global estimates of organic carbon burial in waters shallower than 2000 m are 19±9 g C m-2 yr-1, much larger than the estimates of 6 and 1 g C m-2 yr-1 by and , respectively, although the areas are slightly different in the three studies. The organic carbon burial estimates of for the GOM, MAB, and SAB (Table ) are larger by factors of 8, 17, and 3, respectively, than the best estimates of the empirical model of . However, due to different definitions of the boundary between coastal waters and the open ocean, the combined area of the GOM, MAB, and SAB in is about a third of that of . Finally, estimated the organic carbon burial in Hudson Bay to be 19 g C m-2 yr-1 compared to a mean estimate of 1.5±0.7 g C m-2 yr-1 of burial from sediment cores . Given these results, we consider the estimates of to be an upper bound and assume that a reasonable lower bound is about an order of magnitude smaller, thus placing the organic carbon burial estimate at 65±55 Tg C yr-1.

If these estimates of net air–sea flux, carbon burial, and carbon input from land are accurate, then the residual must be balanced by an increase in carbon inventory in coastal waters and a net transfer of carbon from coastal to open-ocean waters (Fig. ). The rate of carbon accumulation in the North American EEZ from the model of is 50 Tg C yr-1 (Fig. ). Here again, we assume an uncertainty of ±50 %. The residual of 151±105 Tg C yr-1 is the inferred export of carbon to the open ocean (Fig. ). The fact that the error in this residual is large in absolute and relative terms emphasizes the need for more accurate quantification of the terms in the coastal carbon budget. The challenge, however, is that many of these terms are small compared to internal carbon cycling in coastal waters, which is dominated by primary production and decomposition. Two separate estimates of primary production (Table ) are in broad agreement and reveal that the fluxes in Fig.  (of the order of 10 to 100 Tg C yr-1) are just a few percent of primary production (of the order of 1000 Tg C yr-1; see total in Table ). This also indicates that small changes in carbon cycling in coastal waters can result in large changes in atmospheric uptake and transport to the open ocean.

Two important open questions remain: how will the coastal ocean CO2 sink and its anthropogenic flux component change in the future? And how will changing climate and other forcings affect the total and anthropogenic flux proportions? As stated in Sect. , when considering the ocean's role in sequestering anthropogenic carbon, the most relevant component is anthropogenic flux, not the total uptake flux. Neither quantifying the anthropogenic carbon flux component nor predicting its future trend is straightforward. Here we only describe the factors likely to result in trends in total carbon fluxes, but changes in total carbon fluxes suggest changes in anthropogenic fluxes as well.

A direct effect of increasing atmospheric CO2 will be an increase in net uptake by the coastal ocean, but this is tempered by a decrease in the ocean's buffer capacity as DIC increases. More indirect effects include changes in climate forcings (i.e., surface heat fluxes, winds, and freshwater input).

Ocean warming reduces the solubility of gases and thus directly affects gas concentrations near the surface; this will likely decrease the net air–sea flux of CO2 by reducing the undersaturation of CO2 seefor the North American Atlantic coast. Surface warming may also strengthen vertical stratification and thus impede vertical mixing, which will affect the upward diffusion of nutrients and DIC. Enhanced stratification could therefore lead to decreases in both biologically driven carbon uptake and CO2 outgassing. However, model projections for the northern Gulf of Mexico show that the direct effect of increasing atmospheric CO2 overwhelms the other more secondary effects . Along the Pacific coast, surface warming will increase the horizontal gradient between cold, freshly upwelled source waters and warm, offshore surface water, leading to a greater tendency for the subduction of upwelled water at offshore surface temperature fronts during periods of persistent and strong upwelling-favorable winds. The cumulative effect of these processes for the Pacific coast may be greater and more persistent CO2 outgassing nearshore and lower productivity offshore as upwelled nitrate is exported before it can be used by the phytoplankton community . In the Arctic, warming leads to reductions in ice cover, which increases air–sea gas exchange , and the melting of permafrost, which leads to the release of large quantities of CH4 to the atmosphere, from both the land surface and the coastal ocean .

Changes in wind stress directly affect air–sea CO2 fluxes via the gas transfer velocity, which is a function of wind speed, but also indirectly through changes in ocean circulation . For the North American Atlantic coast, changes in wind stress were shown to significantly modify air–sea fluxes . Along the North American Pacific coast, upwelling-favorable winds have intensified in recent years, especially in the northern parts of the upwelling regimes . This has led to a shoaling of nutrient-rich subsurface waters , increased productivity , higher DIC delivery to the surface , and declining oxygen levels . In the coastal Arctic, late-season air–sea CO2 fluxes may become increasingly directed toward the atmosphere as Arctic low-pressure systems with storm-force winds occur more often over open water, thus ventilating CO2 respired from the high organic carbon loading of the shallow shelf .

In a study that directly assesses changes in coastal carbon uptake, investigated trends in the air–sea pCO2 gradient (ΔpCO2 = atmospheric pCO2 - ocean pCO2), which imply a strengthening or weakening of the net CO2 uptake by shelf systems. An increasing ΔpCO2 implies that ocean pCO2 rises more slowly than atmospheric pCO2 and corresponds to increased net uptake and potentially increased cross-shelf export. In their observation-based analysis of decadal trends in shelf pCO2, found that coastal waters lag compared to the rise in atmospheric CO2 in most regions. In the MAB, the Labrador shelf, the Vancouver shelf, and the SAB, ΔpCO2 has increased by 1.9±3.1, 0.68±0.61, 0.83±1.7, and 0.51±0.74 µatm yr-1, respectively, implying that surface ocean pCO2 does not increase or increases at a slower rate than atmospheric CO2. The only North American coastal region that exhibits a negative trend is the Bering Sea, with -1.1±0.74 µatm yr-1, meaning that surface ocean pCO2 increases at a faster rate than in the atmosphere. concluded that the lag in coastal ocean pCO2 increase compared to that in the atmosphere in most regions indicates an enhancement in the coastal uptake and export of atmospheric CO2, although they did not investigate alternative explanations.

Increasing atmospheric CO2 emissions lead to rising atmospheric CO2 levels (Fig. ) and a net ocean uptake of CO2. Since about 1750, the ocean has absorbed 27 % of anthropogenic CO2 emissions to the atmosphere from fossil fuel burning, cement production, and land-use changes . As a result of this uptake, the surface ocean pCO2 has increased (Fig. ) and oceanic pH, carbonate ion concentration, and carbonate saturation state have decreased . Commonly called “ocean acidification”, this suite of chemical changes is defined more precisely as “any reduction in the pH of the ocean over an extended period, typically decades or longer, which is caused primarily by uptake of CO2 from the atmosphere but can also be caused by other chemical additions or subtractions from the ocean” p. 37. In addition to the uptake of CO2 from the atmosphere, variations in DIC concentrations and thus pH can be caused by ocean transport processes and biological production and respiration. Ocean acidification can significantly affect the growth, metabolism, and life cycles of marine organisms and most directly affects marine calcifiers, organisms that precipitate CaCO3 to form internal or external body structures. When the carbonate saturation state decreases below the equilibrium point for carbonate precipitation–dissolution, conditions are said to be corrosive, or damaging, to marine calcifiers. Ocean acidification makes it more difficult for calcifying organisms to form shells or skeletons, perform metabolic functions, and survive. Early life stages are particularly vulnerable as shown by recent large-scale die-offs of oyster larvae in the coastal Pacific where increased energetic expenses under low pH have led to compromised development of essential functions and insufficient initial shell formation .

Acidification trends in open-ocean surface waters tend to occur at a rate that is commensurate with the rate of the increase in atmospheric CO2 (see trends of atmospheric CO2 in comparison to surface ocean pCO2 from the Hawaii Ocean Time Series in Fig. ). Acidification in coastal waters is more variable and often event-driven because coastal waters have greater seasonality (Fig. ) and are more susceptible to changes in circulation, such as upwelling. Along the Pacific coast, climate-driven changes in upwelling circulation result in coastal acidification events . As mentioned in Sect. , upwelling-favorable winds along this coast have intensified over recent years, especially in the northern parts of the upwelling regime . Intensified upwelling supplies deep water to the shelf that is rich in DIC and nutrients but poor in oxygen. Ocean acidification and hypoxia are thus strongly linked ecosystem stressors because low-oxygen, high-CO2 conditions derive from the microbial respiration of organic matter . In the northern California Current System, pCO2, pH, and aragonite saturation reach levels known to be harmful to ecologically and economically important species during the summer upwelling season . In the Gulf of Alaska, aragonite saturation drops to near saturation values during the winter months when deep mixing occurs and surface ocean pCO2 exceeds atmospheric pCO2 . Along the Pacific coast, 50 % of shelf waters are projected to experience year-long undersaturation by 2050 .

Polar regions are naturally prone to low pH values and thus closer to critical acidification thresholds than lower-latitude waters. The low pH levels result from the naturally high ratio of DIC to alkalinity in polar waters due to their low temperatures, multiple sources of freshwater (e.g., riverine, glacial melt, and sea ice melt), and high respiratory DIC content. In addition to the naturally low pH, the rate of acidification is relatively high in polar waters because retreating sea ice adds meltwater from multiyear ice and increases the surface area of open water, thereby enhancing the uptake of atmospheric CO2 . The Beaufort Sea upper halocline and deep waters already show aragonite undersaturation, i.e., aragonite saturation states below 1, favoring dissolution . These chemical seawater signatures are propagated via M'Clure Strait and Amundsen Gulf into the Canadian Arctic shelf and beyond . Model projections based on the IPCC high-CO2 emissions scenario RCP8.5 suggest the Beaufort Sea surface water will become undersaturated with respect to aragonite around 2025 . As these conditions intensify, negative impacts for calcifying marine organisms are expected to become a critical issue reshaping ecosystems and fisheries across the Arctic .

In contrast, surface aragonite saturation states typically range from 3.6 to 4.5 and are thus well above the dissolution threshold in the northern Gulf of Mexico . Here excessive nutrient inputs from the Mississippi River result in hypoxia and the eutrophication-induced acidification of near-bottom waters . Similar to the California Current System, low-oxygen and high-CO2 conditions coincide and derive from the microbial respiration of organic matter . Currently, aragonite saturation states are around 2 in hypoxic bottom waters and thus well above the saturation threshold. Projections suggest that the aragonite saturation states of these near-bottom waters will drop below the saturation threshold near the end of this century .

Along the Atlantic coast, the northern regions (the Mid-Atlantic Bight and Gulf of Maine) have, on average, lower pH and lower aragonite saturation states than more southern coastal regions (i.e., the South Atlantic Bight) . These properties are primarily explained by a decrease in alkalinity from the SAB toward the GOM. The seasonal undersaturation of aragonite in subsurface water is already occurring in the GOM, which supports a significant shellfish industry .