Interhemispheric transport of carbon dioxide by ocean circulation

ALTHOUGH anthropogenic emissions of carbon dioxide have today created a greater atmospheric CO2concentration in the Northern than in the Southern Hemisphere, a comparison of interhemispheric CO2 profiles from 1980 and 1962 led Keeling and Heimann1,2 to conclude that, before the Industrial Revolution, natural CO2 sources and sinks acted to set up a reverse (south to north) gradient which drove about one gigatonne of carbon each year through the atmosphere from the Southern to the Northern Hemisphere. At steady state, this flux must have been balanced by a counter flow of carbon from north to south through the ocean. Here we present a means to estimate this natural flux by a separation of oceanic carbon anomalies into those created by biogenic processes and those created by CO2 exchange between the ocean and atmosphere. We find that before the Industrial Revolution, deep water formed in the northern Atlantic Ocean carried about 0.6 gigatonnes of carbon annually to the Southern Hemisphere, providing support for Keeling and Heimann's proposal. The existence of this oceanic carbon pump also raises questions about the need for a large terrestrial carbon sink in the Northern Hemisphere, as postulated by Tans et al.3, to balance the present global carbon budget