Phytoplankton - how to reverse global warming?

This item on the science pages of the BBC is certainly intriguing, suggesting an entirely natural way to absorb CO2.

I have just about enough knowledge to understand the theory of the process, and empirically it seems to work; but can anyone shed light on the scientific validity of the process as a holistic cycle? Has urea production - as a manufacturing process - got side effects or by-products that we should be worried about.

I'm on record as having concerns about carbon sequestration in the old oil wells, given the potential chemical reaction with the rock strata, but I've really not got enough knowledge to be confident either way (so I err on the side of caution). This is a very similar situation, and it appears to be one that could benefit from urgent, informed, debate.