Fertilizing the ocean to suck up CO2 failed in a recent experiment. The idea was to dump iron in the ocean, fertilize plankton growth, and sequester CO2 to lower atmospheric CO2.
The experiment showed that fertilizing ocean algae growth did a great job of feeding things that eat ocean algae--tiny animals called zooplankton. It was a nice job of creating an all-you-can-eat ocean buffet.
I'm not a big fan of geoengineering, but climate fix experiments are probably necessary in this world of rising CO2. The fact that the experiment failed allows smarter debates about future geoengineering schemes. Instead of only waving our hands, we can actually look at scientific experiments on the question of using iron to lower CO2.