Figure 7.8. Estimates (4° × 5°) of sea-to-air flux of CO₂, computed using 940,000 measurements of surface water pCO₂ collected since 1956 and averaged monthly, together with NCEP/NCAR 41-year mean monthly wind speeds and a (10-m wind speed)² dependence on the gas transfer rate (Wanninkhof, 1992). The fluxes were normalised to the year 1995 using techniques described in Takahashi et al. (2002), who used wind speeds taken at the 0.995 standard deviation level (about 40 m above the sea surface). The annual flux of CO₂ for 1995 with 10-m winds is –1.6 GtC yr^–1, with an approximate uncertainty (see Footnote 1) of ±1 GtC yr^–1, mainly due to uncertainty in the gas exchange velocity and limited data coverage. This estimated global flux consists of an uptake of anthropogenic CO₂ of –2.2 GtC yr^–1 (see text) plus an outgassing of 0.6 GtC yr^–1, corresponding primarily to oxidation of organic carbon borne by rivers (Figure 7.3). The monthly flux values with 10-m winds used here are available from T. Takahashi at http://www.ldeo.columbia.edu/res/pi/CO2/carbondioxide/pages/air_sea_flux_rev1.html.