Many estimates of the atmospheric carbon budget suggest that most of the sink for CO2 produced by fossil fuel burning and cement production must be in the Northern Hemisphere. Keeling et al.  hypothesized that this asymmetry could be explained instead by a northward preindustrial transport of ∼1 Pg C y−1 in the atmosphere balanced by an equal and opposite southward transport in the ocean. We explore this hypothesis by examining the processes that determine the magnitude of the preindustrial interhemispheric flux of carbon in three ocean carbon models. This study is part of the first stage of the Ocean Carbon Model Intercomparison Project organized by International Geosphere Biosphere Programme Global Analysis, Interpretation, and Modelling Task Force. We find that the combination of interhemispheric heat transport (with its associated carbon transport), a finite gas exchange, and the biological pump, yield a carbon flux of only −0.12 to +0.04 Pg C y−1across the equator (positive to the north). An important reason for the low carbon transport is the decoupling of the carbon flux from the interhemispheric heat transport due to the long sea-air equilibration time for surface CO2. A possible additional influence on the interhemispheric exchange is oceanic transport of carbon from rivers.
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