Regionalized global budget of the CO₂ exchange at the air‐water interface in continental shelf seas

Abstract

Over the past decade, estimates of the atmospheric CO 2 uptake by continental shelf seas were constrained within the 0.18-0.45 Pg C yr À1 range. However, most of those estimates are based on extrapolations from limited data sets of local flux measurements (n < 100). Here we propose to derive the CO 2 air-sea exchange of the shelf seas by extracting 3 · 10 6 direct surface ocean CO 2 measurements from the global database SOCAT (Surface Ocean CO 2 Atlas), atmospheric CO 2 values from GlobalVIEW and calculating gas transfer rates using readily available global temperature, salinity, and wind speed fields. We then aggregate our results using a global segmentation of the shelf in 45 units and 152 subunits to establish a consistent regionalized CO 2 exchange budget at the global scale. Within each unit, the data density determines the spatial and temporal resolutions at which the air-sea CO 2 fluxes are calculated and range from a 0.5°resolution in the best surveyed regions to a whole unit resolution in areas where data coverage is limited. Our approach also accounts, for the first time, for the partial sea ice cover of polar shelves. Our new regionalized global CO 2 sink estimate of 0.19 ± 0.05 Pg C yr À1 falls in the low end of previous estimates. Reported to an ice-free surface area of 22 · 10 6 km 2 , this value yields a flux density of 0.7 mol C m À2 yr À1 ,~40% more intense than that of the open ocean. Our results also highlight the significant contribution of Arctic shelves to this global CO 2 uptake (0.07 Pg C yr À1 ).

Coastal marine uptake of CO2 around Greenland

“…This estimate, however, is based solely on data from Rysgaard et al (2012) whose measurements are from Godthåbsfjord where the pCO2 levels are much lower than in the higher latitude fjord, Young Sound. Laruelle et al (2014) estimated the uptake by the Greenlandic shelf surface at 16 Tg C yr -1 which is comparable to our estimates, depending on the choice of exchange coefficient and the size of the sea ice cover.…”

“…The coastal marine surface of Greenland is only 0.08% of the total global ocean surface and 1.1% of the global coastal area. However, the uptake of CO2 is around 0.5% of the global ocean uptake (2000 (±600.0) Tg C y -1 (Wanninkhof et al, 2013)) and 6.1% of the global coastal uptake (190 Tg C y -1 (Laruelle et al, 2014)). …”

“…This study estimates a total uptake of 0.3 Pg C yr -1 (= 300 Tg C yr -1 ) by the world's coastal seas but only few measurements from the Greenlandic coastal area were included (Rysgaard et al, 2012;Ruiz-Halpern et al, 2010). A recent compilation of data on CO2 in coastal areas underlines two points: 1) that the Arctic coastal zone is an important area for ocean uptake of CO2 and 2) that the Arctic coastal zone is particularly understudied (Laruelle et al, 2014) An important area for ocean CO2 uptake is the Nordic Seas (Takahashi et al, 2002; and the shelf waters (Chen et al, 2013). The uptake rates of atmospheric CO2 in the Nordic Seas, and particularly the shelf waters around Greenland, are among the highest in the world's oceans (Nordic Seas ~ 2.5 mol m -2 year -1 (Takahashi et al, 2009), and Greenland Shelf ~ 6 mol m -2 year -1 (Chen et al, 2013)).…”

Low &lt;i&gt;p&lt;/i&gt;CO&lt;sub&gt;2&lt;/sub&gt; under sea-ice melt in the Canada Basin of the western Arctic Ocean

“…Manizza et al (2013) argued that increasing SST will enhance biological primary production and drawdown of CO 2 in seawater. Laruelle et al (2014) asserted that larger ice-free areas and longer ice-free periods will provide greater occasion for oceanic CO 2 uptake. In contrast, Cai et al (2010) and Else et al (2013) insisted that the increase in sea-ice melt results in the formation of thin surface mixed layers and limits further uptake of CO 2 from the atmosphere by this layer.…”

Uncertainty of the global oceanic CO&lt;sub&gt;2&lt;/sub&gt; uptake induced by wind forcing: quantification and spatial analysis

“…Sc (dimensionless) is calculated according to the equation reported by . Ice represents the fraction of the ocean covered by sea ice (comprised between 0 for ice free and 1 for entirely covered), which is assumed to inhibit the air-sea CO 2 transfer (Evans 10 et al, 2015;Landschützer et al, 2013;Laruelle et al, 2014).…”