The Iron Hypothesis
The iron hypothesis posits that adding iron to the oceans will trigger blooms of plankton and algae, which pull CO2 out of the air. When plankton and algae eventually die, they simply sink to the ocean floor and become buried.
Scientists who have studied past phases of climate change have found that iron dust may have caused similar effects during ice ages and are hoping to replicate those without causing unwanted fallout.
Research shows that the strategy might work in the Atlantic Ocean but would probably be less successful in the equatorial part of the Pacific Ocean because of its circulation. In other words, the equatorial Pacific gets nutrient-deficient water from Antarctic Ocean currents, and experiments have shown that the supplemental iron alone isn't enough to trigger the plankton blooms. Scientists aren't even yet sure about other oceans.
Scientists from the Carnegie Institution have also considered pumping cold water from the ocean's floor up to the surface, which could have a slightly cooling effect on the atmosphere. However, this process would cool for about 50 years, and could create numerous other problems by disturbing marine ecosystems.