Adding iron to the water stimulates growth in phytoplankton. These phytoplankton eat up carbon dioxide through photosynthesis, which a reduction should reduce temperatures in the environment. Carbon dioxide is one the main problems that causes the greenhouse effect, as it traps heat on the earth's surface. With a large scale use of iron fertilization, in theory, i tshoudl significantly combat global warming.
Although it could certainly combat global warming, we do have to stop skirting around the idea (the fact, even) that numerous actions we take are affecting global warming and no one solution is likely to "significantly" combat it. I agree because if it can be shown to reduce CO2 in the atmosphere, then so be it.