Trends in pCO2 and sea–air CO2 flux over the global open oceans for the last two decades

Abstract

To date, millions of data for partial pressure of carbon dioxide in surface seawater (pCO2s) have been acquired from global oceans. However, pCO2s varies extremely both in space and time, and it is still necessary to fill in its spatiotemporal gaps to evaluate the changes in sea–air CO2 flux from regional to global scales. In the present study, we have analyzed the rates of pCO2s increase for the past decades, ranging from +1.21 µatm year−1 in the western equatorial Pacific to +2.00 µatm year−1 in the Southern Ocean, and developed algorithms to reconstruct global monthly pCO2s fields based on a quality-controlled database, Surface Ocean CO2 Atlas (SOCAT) version 2. The error of pCO2s algorithms is ±16.3 µatm. The mean annual global sea–air CO2 flux for the years 1990–2012 was estimated to be −1.76 PgC year−1 (contemporary flux; a negative value indicates oceanic uptake). The uncertainty in this estimate is calculated to be 0.77 PgC year−1 (44 %), i.e., 0.09 PgC year−1 from our empirical method to interpolate/extrapolate pCO2s, and 0.67 PgC year−1 from determination of the rates of pCO2s increase and the rest from gas transfer processes including wind speed (0.26 PgC year−1) and a scaling factor of piston velocity (0.26 PgC year−1). The decadal mean CO2 flux showed a trend toward increasing uptake from −1.67 PgC year−1 for 1990–1999 to −1.92 PgC year−1 for 2003–2012, due to growing uptake in the Pacific and Atlantic Oceans. However, these estimates of CO2 flux are sensitive to the rates of pCO2s increase used to constrain the long-term pCO2s change for each sub-region.