An assessment of CO2 storage and sea-air fluxes for the Atlantic Ocean and Mediterranean Sea between 1985 and 2018

Abstract

As part of the second phase of the Regional Carbon Cycle Assessment and Processes project (RECCAP2), we present an assessment of the carbon cycle of the Atlantic Ocean, including the Mediterranean Sea, between 1985 and 2018 using global ocean biogeochemical models (GOBMs) and estimates based on surface ocean carbon dioxide (CO2) partial pressure (pCO(2) products) and ocean interior dissolved inorganic carbon observations. Estimates of the basin-wide long-term mean net annual CO2 uptake based on GOBMs and pCO(2) products are in reasonable agreement (-0.47 +/- 0.15 PgC yr(-1) and -0.36 +/- 0.06 PgC yr(-1), respectively), with the higher uptake in the GOBM-based estimates likely being a consequence of a deficit in the representation of natural outgassing of land derived carbon. In the GOBMs, the CO2 uptake increases with time at rates close to what one would expect from the atmospheric CO2 increase, but pCO(2) products estimate a rate twice as fast. The largest disagreement in the CO2 flux between GOBMs and pCO(2) products is found north of 50 degrees N, coinciding with the largest disagreement in the seasonal cycle and interannual variability. The mean accumulation rate of anthropogenic CO2 (C-ant) over 1994-2007 in the Atlantic Ocean is 0.52 +/- 0.11 PgC yr(-1) according to the GOBMs, 28% +/- 20% lower than that derived from observations. Around 70% of this C-ant is taken up from the atmosphere, while the remainder is imported from the Southern Ocean through lateral transport. This study contributes to the second Regional Carbon Cycle Assessment and Processes project by presenting a carbon cycle evaluation of the Atlantic Ocean including the Mediterranean Sea between 1985 and 2018. The assessment draws on output from global ocean biogeochemical models along with estimates based on observations of surface ocean carbon dioxide (CO2) partial pressure (pCO(2) products) and ocean interior dissolved inorganic carbon. The models suggest that the Atlantic took up -0.47 +/- 0.15 Pg of carbon per year, in reasonable agreement with an uptake of -0.36 +/- 0.06 Pg carbon per year computed from pCO(2) products. In the models, the rate of CO2 uptake is keeping pace with the increase in atmospheric CO2, but it is twice as fast in the pCO(2) products. Most of the uptake of CO2 by the ocean occurs in response to excess CO2 released into the atmosphere from human activities. The so-called anthropogenic carbon accumulates in the Atlantic Ocean at a rate of 0.52 +/- 0.11 Pg carbon per year according to the models. This estimate is 28% +/- 20% lower than that derived from observations. Further investigation reveals that about 70% of the accumulated anthropogenic carbon is taken up from the atmosphere, while the remainder is imported from the Southern Ocean.