Global sea-air CO2 flux based on climatological surface ocean pCO2, and seasonal biological and temperature effects

Abstract

Based on about 940,000 measurements of surface-water pCO2 obtained since the International Geophysical Year of 1956-59, the climatological, monthly distribution of pCO2 in the global surface waters representing mean non-El Niño conditions has been obtained with a spatial resolution of 4° × 5° for a reference year 1995. The monthly and annual net sea-air CO2 flux has been computed using the NCEP/NCAR 41-year mean monthly wind speeds. An annual net uptake flux of CO2 by the global oceans has been estimated to be 2.2 (+22% or -19%) Pg Cyr-1 using the (wind speed)2 dependence of the CO2 gas transfer velocity of Wanninkhof (J. Geophys. Res. 97 (1992) 7373). The errors associated with the wind-speed variation have been estimated using one standard deviation (about±2m s-1) from the mean monthly wind speed observed over each 4° × 5° pixel area of the global oceans. The new global uptake flux obtained with the Wanninkhof (wind speed)2 dependence is compared with those obtained previously using a smaller number of measurements, about 250,000 and 550,000, respectively, and are found to be consistent within ±0.2 Pg Cyr-1. This estimate for the global ocean uptake flux is consistent with the values of 2.0±0.6 Pg Cyr-1 estimated on the basis of the observed changes in the atmospheric CO2 and oxygen concentrations during the 1990s (Nature 381 (1996) 218; Science 287 (2000) 2467). However, if the (wind speed)3 dependence of Wanninkhof and McGillis (Res. Lett. 26 (1999) 1889) is used instead, the annual ocean uptake as well as the sensitivity to wind-speed variability is increased by about 70%. A zone between 40° and 60° latitudes in both the northern and southern hemispheres is found to be a major sink for atmospheric CO2. In these areas, poleward-flowing warm waters meet and mix with the cold subpolar waters rich in nutrients. The pCO2 in the surface water is decreased by the cooling effect on warm waters and by the biological drawdown of pCO2 in subpolar waters. High wind speeds over these low pCO2 waters increase the CO2 uptake rate by the ocean waters.