Riverine‐driven interhemispheric transport of carbon

Controversy surrounds the role of the ocean in interhemispheric transport of carbon. On one hand, observations in the atmosphere and in the ocean both seem to imply that the preindustrial ocean transported up to 1 Pg C yr−1 from the Northern to the Southern Hemisphere. On the other hand, three dimensional (3‐D) ocean models suggest that global interhemispheric transport of carbon is near zero. However, in this debate, there has been a general neglect of the river carbon loop. The river carbon loop includes (1) uptake of atmospheric carbon due to inorganic and organic erosion on land, (2) transport of carbon by rivers, (3) subsequent transport of riverine carbon by the ocean, and (4) loss of riverine carbon back to the atmosphere by air‐sea gas exchange. Although carbon fluxes from rivers are small compared to natural fluxes, they have the potential to contribute substantially to the net air‐sea fluxes of CO2. For insight into this dilemma, we coupled carbon fluxes from a global model of continental erosion to a 3‐D global carbon‐cycle model of the ocean. With rivers, total southward interhemispheric transport by the ocean increases from 0.1 to 0.35±0.08 Pg C yr−1, in agreement with oceanographic observations. Resulting air‐sea fluxes of riverine carbon and uptake of CO2 by land erosion were installed as boundary conditions in a 3‐D atmospheric model. The assymetry in these fluxes drives a preindustrial atmospheric gradient of CO2 at the surface of −0.6±0.1 μatm for the North Pole minus the South Pole and longitudinal variations that exceed 0.5 μatm. Conversely, the gradient for Mauna Loa minus South Pole is only −0.2±0.1 μatm, much less than the −0.8 μatm gradient extrapolated linearly from historical atmospheric CO2 measurements from the same two sites. The difference may be explained by the role of the terrestrial biosphere. Regardless, the river loop produces large gradients both meridionally and zonally. Accounting for the river carbon loop changes current estimates of the regional distribution of sources and sinks of CO2, particularly concerning partitioning between natural and anthropogenic processes.

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