Global air-sea flux of CO 2 : An estimate based on measurements of sea–air pCO 2 difference

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Abstract
Approximately 250,000 measurements made for the pCO 2 difference between surface water and the marine atmosphere, ΔpCO 2 , have been assembled for the global oceans. Observations made in the equatorial Pacific during El Nino events have been excluded from the data set. These observations are mapped on the global 4° × 5° grid for a single virtual calendar year (chosen arbitrarily to be 1990) representing a non-El Nino year. Monthly global distributions of ΔpCO 2 have been constructed using an interpolation method based on a lateral advection–diffusion transport equation. The net flux of CO 2 across the sea surface has been computed using ΔpCO 2 distributions and CO 2 gas transfer coefficients across sea surface. The annual net uptake flux of CO 2 by the global oceans thus estimated ranges from 0.60 to 1.34 Gt-C⋅yr −1 depending on different formulations used for wind speed dependence on the gas transfer coefficient. These estimates are subject to an error of up to 75% resulting from the numerical interpolation method used to estimate the distribution of ΔpCO 2 over the global oceans. Temperate and polar oceans of the both hemispheres are the major sinks for atmospheric CO 2 , whereas the equatorial oceans are the major sources for CO 2 . The Atlantic Ocean is the most important CO 2 sink, providing about 60% of the global ocean uptake, while the Pacific Ocean is neutral because of its equatorial source flux being balanced by the sink flux of the temperate oceans. The Indian and Southern Oceans take up about 20% each.