The equatorial oceans are the dominant oceanic source of CO2 to the atmosphere, annually amounting to a net flux of 0.7-1.5 Pg (10(15) g) of carbon, up to 72% of which emanates from the equatorial Pacific Ocean(1-3). Limited observations indicate that the size of the equatorial Pacific source is significantly influenced by El Nino events(4-10), but the effect has not been well quantified. Here we report spring and autumn multiannual measurements of the partial pressure of CO2 in the surface ocean and atmosphere in the equatorial Pacific region. During the 1991-94 El Nino period, the derived net annual sea-to-air flux of CO2 was 0.3 Pg C from autumn 1991 to autumn 1.992., 0.6 Pg C in 1993, and 0.7 Pg C in 1994. These annual fluxes are 30-80% of that of 1996, a non-EG Nino year. The total reduction of the regional sea-to-air CO2 nux during the 1991-94 El Nino period is estimated to account for up to one-third of the atmospheric anomaly (the difference between the annual and long-term-average increases in global atmospheric CO2 content) observed over the same period.
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