Air–sea exchanges of CO₂ in the world's coastal seas

Abstract. The air–sea exchanges of CO₂ in the world's 165 estuaries and 87 continental shelves are evaluated. Generally and in all seasons, upper estuaries with salinities of less than two are strong sources of CO₂ (39 ± 56 mol C m⁻² yr⁻¹, positive flux indicates that the water is losing CO₂ to the atmosphere); mid-estuaries with salinities of between 2 and 25 are moderate sources (17.5 ± 34 mol C m⁻² yr⁻¹) and lower estuaries with salinities of more than 25 are weak sources (8.4 ± 14 mol C m⁻² yr⁻¹). With respect to latitude, estuaries between 23.5 and 50° N have the largest flux per unit area (63 ± 101 mmol C m⁻² d⁻¹); these are followed by lower-latitude estuaries (23.5–0° S: 44 ± 29 mmol C m⁻² d⁻¹; 0–23.5° N: 39 ± 55 mmol C m⁻² d⁻¹), and then regions north of 50° N (36 ± 91 mmol C m⁻² d⁻¹). Estuaries south of 50° S have the smallest flux per unit area (9.5 ± 12 mmol C m⁻² d⁻¹). Mixing with low-pCO₂ shelf waters, water temperature, residence time and the complexity of the biogeochemistry are major factors that govern the pCO₂ in estuaries, but wind speed, seldom discussed, is critical to controlling the air–water exchanges of CO₂. The total annual release of CO₂ from the world's estuaries is now estimated to be 0.10 Pg C yr⁻¹, which is much lower than published values mainly because of the contribution of a considerable amount of heretofore unpublished or new data from Asia and the Arctic. The Asian data, although indicating high pCO₂, are low in sea-to-air fluxes because of low wind speeds. Previously determined flux values rely heavily on data from Europe and North America, where pCO₂ is lower but wind speeds are much higher, such that the CO₂ fluxes are higher than in Asia. Newly emerged CO₂ flux data in the Arctic reveal that estuaries there mostly absorb rather than release CO₂.

Most continental shelves, and especially those at high latitude, are undersaturated in terms of CO₂ and absorb CO₂ from the atmosphere in all seasons. Shelves between 0 and 23.5° S are on average a weak source and have a small flux per unit area of CO₂ to the atmosphere. Water temperature, the spreading of river plumes, upwelling, and biological production seem to be the main factors in determining pCO₂ in the shelves. Wind speed, again, is critical because at high latitudes, the winds tend to be strong. Since the surface water pCO₂ values are low, the air-to-sea fluxes are high in regions above 50° N and below 50° S. At low latitudes, the winds tend to be weak, so the sea-to-air CO₂ flux is small. Overall, the world's continental shelves absorb 0.4 Pg C yr⁻¹ from the atmosphere.