The effect of hurricanes on the thermal and physical structure of the upper ocean has been described(1) but their influence on the ocean carbon cycle and the exchange of carbon between ocean and atmosphere is not well understood. Here we present observations from the Sargasso Sea, before and after hurricane Felix in summer 1995, that show a short-lived (2-3 weeks) surface seawater cooling of about 4 degrees C, and a decrease in seawater partial pressure of CO2 by about 60 mu atm. Despite the localized decrease in seawater partial pressure of CO2, strong winds during the passage of hurricane Felix increased the efflux of CO2 from ocean to atmosphere. We estimate that hurricane Felix and two other hurricanes increased the summertime efflux of CO2 into the atmosphere over this part of the Sargasso Sea by nearly 55%. We estimate that hurricanes contribute to the global ocean-to-atmosphere flux of CO2 by between +0.04 to +0.51 Pg C (10(15) g C) per year. Such hurricane-forced effluxes are quantitatively significant compared to regional (14 degrees to 50 degrees N zone)(2) and global effluxes(2,3). Hurricanes therefore exert an important influence on ocean-atmosphere CO2 exchange and the inferred(4) year-to-year variability of CO2 fluxes over the subtropical oceans.