Spreading iron dust in the Southern Polar Ocean has been suggested as a means of increasing ocean phytoplankton populations, which in turn consume large amounts of atmospheric carbon dioxide and hopefully suppress the rise in global atmospheric greenhouse gases. Boyd (2004) summarizes current research results that suggest artificially stimulated phytoplankton blooms last longer than naturally occurring blooms, which may mean the process might be more efficient than previously thought.
Reference
Boyd, P., 2004: Ironing out algal issues in the Southern Ocean. Science, 304, 396-397.