Based on atmospheric data and models, the tropical CO2 source anomaly reaches up to 2 GtC yr-1, but the respective contributions of the terrestrial biosphere and the oceans to this flux are difficult to quantify. Here we present a new method for estimating CO2 fluxes from oceanic observations based on the surprisingly good predictive skill of temperature and salinity for surface dissolved inorganic carbon. Using historical temperature and salinity data, we reconstruct the basin scale CO2 flux to the atmosphere in the equatorial Pacific from 1982 to 1993. We find that interannual anomalies do not exceed 0.4¿0.2 GtC yr-1 which suggests that up to 80% of the tropical CO2 source anomaly is due to the terrestrial biosphere. ¿ 2000 American Geophysical Union