The availability of iron exerts a significant control on primary production and the export of organic matter over large areas of the remote ocean. We explore the regulation of the global soft tissue pump of carbon and atmospheric carbon dioxide by the atmospheric delivery of iron in a three-dimensional ocean circulation and biogeochemistry model. There is little change in atmospheric carbon dioxide when the aeolian iron source is increased several fold but a significant increase in response to a reduction in the aeolian iron source. This strong asymmetry suggests a positive feedback, amplifying the increase of atmospheric carbon dioxide in a warming, less dusty world.