Much of the carbon dioxide we emit into the atmosphere ends up in the ocean. As CO₂ levels rise, seawater becomes more acidic. This change in chemistry poses a serious threat to marine organisms including snails, corals, such as in the above photo showing a single bleached polyp, and fish. In the April 2011 issue of National Geographic, writer Elizabeth Kolbert tells the story of how scientists are trying to address ocean acidification before it's too late.