Seven tons of iron may sequester up to 1,800 tons of carbon. In contrast, the mass of "bad" anthropogenic carbon released annually that is not already sequestered naturally is about 16 billion tons (45% of 34 billion in 2010 emissions, but still increasing each year except in developed nations where it is decreasing due to the recession and cheap natural gas).
Unless my math is very much off, we would need to add about 62 million tons of iron annually, less than that if we managed to get on an emissions reductions trajectory. Phasing out coal worldwide, for instance, would reduce the need for iron by, very roughly, forty to fifty percent.
The jury is still out, but this is starting to look like a serious option. Emissions reduction is still necessary in the long run, but we could buy some time this way. The ecological effects need to be carefully studied.
Our Hobson's Choice may, however work out to be something like, ruin the southern ocean to save agriculture in the lower mid-latitudes.
What a pretty pass poor old humanity has come to!