Abstract
Abstract. Minimising the uncertainties in estimates of air–sea CO2 exchange is an important step toward increasing the confidence in assessments of the CO2 cycle. Using an atmospheric transport model makes it possible to investigate the direct impact of atmospheric parameters on the air–sea CO2 flux along with its sensitivity to, for example, short-term temporal variability in wind speed, atmospheric mixing height and atmospheric CO2 concentration. With this study, the importance of high spatiotemporal resolution of atmospheric parameters for the air–sea CO2 flux is assessed for six sub-basins within the Baltic Sea and Danish inner waters. A new climatology of surface water partial pressure of CO2 (pCO2w) has been developed for this coastal area based on available data from monitoring stations and on-board pCO2w measuring systems. Parameterisations depending on wind speed were applied for the transfer velocity to calculate the air–sea CO2 flux. Two model simulations were conducted – one including short-term variability in atmospheric CO2 (VAT), and one where it was not included (CAT). A seasonal cycle in the air–sea CO2 flux was found for both simulations for all sub-basins with uptake of CO2 in summer and release of CO2 to the atmosphere in winter. During the simulated period 2005–2010, the average annual net uptake of atmospheric CO2 for the Baltic Sea, Danish straits and Kattegat was 287 and 471 Gg C yr−1 for the VAT and CAT simulations, respectively. The obtained difference of 184 Gg C yr−1 was found to be significant, and thus ignoring short-term variability in atmospheric CO2 does have a sizeable effect on the air–sea CO2 exchange. The combination of the atmospheric model and the new pCO2w fields has also made it possible to make an estimate of the marine part of the Danish CO2 budget for the first time. A net annual uptake of 2613 Gg C yr−1 was found for the Danish waters. A large uncertainty is connected to the air–sea CO2 flux in particular caused by the transfer velocity parameterisation and the applied pCO2w climatology. However, as a significant difference of 184 Gg C yr−1 is obtained between the VAT and CAT simulations, the present study underlines the importance of including short-term variability in atmospheric CO2 concentration in future model studies of the air–sea exchange in order to minimise the uncertainty.
Highlights
The capacity of ocean and land to take up and re-emit atmospheric CO2 has a dominating effect on the greenhouse gas balance, and changes in climate
The present study aims to determine the importance of the short-term variability in atmospheric CO2 concentrations on the net air–sea CO2 flux of the Baltic Sea and Danish inner waters
The performance of the model for this period is evaluated by comparing simulated atmospheric CO2 concentrations against observed
Summary
The capacity of ocean and land to take up and re-emit atmospheric CO2 has a dominating effect on the greenhouse gas balance, and changes in climate. Land areas and global oceans are estimated to take up about 27 and 28 %, respectively, of the CO2 emitted by anthropogenic sources (Le Quéré et al, 2013). Biogeochemically active coastal seas have been given increased attention (Borges et al, 2006; Chen et al, 2013; Mørk et al, 2014). Such coastal waters only amount to 7 % of global oceans, high inputs, production, degradation and export of organic matter might result. Lansø et al.: Sensitivity of the air–sea CO2 exchange in the Baltic Sea and Danish inner waters in coastal air–sea CO2 fluxes contributing a great deal more than 7 % to the global air–sea flux (Gattuso et al, 1998)
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