From May 1959 to June 1981 the concentration of atmospheric carbon dioxide was measured in 2419 samples of air collected on a weather ship situated at 50¿N and 145¿W in the North Pacific Ocean. Three principal characteristics of the variation in concentration of atmospheric CO2 are revealed by these data: an annual variation that repeats with nearly the same pattern each year, an interannual variation that correlates with the large-scale circulation of the atmosphere, and a long-term increase that is nearly proportional to the global input of CO2 from the combustion of fossil fuels. The peak-to-trough amplitude of the smoothed annual signal increased from 13.3 ppm in 1969 to 14.5 ppm in 1981. The phasing of the annual CO2 cycle suggests a close relation to the activity of land plants in the broad region of the northern hemisphere where plants grow mainly during the summer. The increasing amplitude suggests a heightening plant activity. The interannual variation and its first derivative correlate with the Southern Oscillation. A lag of 6 months in the derivative suggests a distant oceanic or terrestrial source-sink in the tropics or southern hemisphere. The seasonally adjusted CO2 concentration increased from 324.9 ppm in May 1969 to 340.8 ppm in June 1981. This increase is 60% of the increase that would have occurred if all the CO2 from fossil fuel combustion had remained in the atmosphere and had been uniformly distributed there. The seasonally adjusted concentration, when averaged from 1975 to 1981, is 0.8 ppm lower than that found at Point Barrow, Alaska, at 71¿N and 0.9 ppm higher than that found at Mauna Loa Observatory, Hawaii, at 19¿N, suggesting a steadily decreasing concentration in CO2 from north to south in the broad band from 70¿N to 20¿N.