Surface-ocean CO2 variability and vulnerability

Improved sampling technologies, international observing networks, and data synthesis efforts are providing an unprecedented view of the global patterns and decadal variability of surface-ocean partial pressure of carbon dioxide (pCO2) and air–sea CO2 flux. A new observational synthesis for surface-ocean pCO2 leads to a global ocean net CO2 sink of −1.8 Pg C yr−1 (±0.7); continental margins and estuaries together act as a small net source (∼0.15 Pg C yr−1). New results on decadal trends indicate recent decreases in efficiency of the ocean to absorb CO2 in the Southern Ocean, North Atlantic, and Equatorial Pacific. Although we do not yet have an estimate of the trend in the global oceanic sink of CO2, the existing observations may help explain temporal variations in the fraction of anthropogenic CO2 emissions that remain in the atmosphere (airborne fraction). Major vulnerabilities in the future behavior of the ocean-carbon–climate system include the effects of ocean warming, enhanced vertical stratification, strengthening and poleward contraction of westerly winds in the Southern Ocean, and shifts in the biological pump and ecosystem functioning. To address issues of both ocean-carbon variability and vulnerability, a sustained surface-ocean-carbon-observing system needs to be established with improved global spatial coverage and internationally coordinated data synthesis activities.