Here’s the basic idea: Tiny photosynthesizing plankton in the ocean use carbon dioxide from the air to make food. When they die, they sink down to the ocean floor, taking the carbon with them. Because iron stimulates phytoplankton growth, some people have suggesting fertilizing parts of the ocean with iron to create huge plankton blooms to suck up some of the excess carbon dioxide we’ve emitted into the atmosphere. Several private companies have attempted ventures to dump iron into the ocean to sell carbon credits, but many scientists question just how effective the massive blooms are at trapping and storing carbon. Environmental groups have also warned that iron dumps may harm the local marine ecosystems.