The availability of iron exerts a significant control on primary production and the export of organic matter over large areas of the ocean, especially those far from land sources. We explore the regulation of the global soft tissue pump of carbon and atmospheric CO2 by the atmospheric delivery of iron in a three-dimensional ocean circulation and biogeochemistry model. There is only a small change in atmospheric CO2 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 an increase in atmospheric CO2 if a warmer world is also less dusty.