Ocean acidification, like climate change, is caused by excess carbon dioxide (CO2) released when we burn fossil fuels and clear forests. But unlike changes to our climate, which are physical phenomena driven by rising global temperatures, ocean acidification is a chemical reaction between CO2 and seawater. About a quarter of total human CO2 emissions–roughly 150 billion metric tons–have already been absorbed by the ocean. On the one hand, this is a good thing–the planet would be even warmer if this heat-trapping CO2 had remained in the atmosphere–but it has come at a cost. The reaction between dissolved CO2 and seawater generates carbonic acid, which eats away at the minerals needed by oysters, crabs, pteropods, deep sea corals and other marine life to build their shells and skeletons. Ocean acidification has been nicknamed "osteoporosis of the sea" because it affects so many shell and skeleton-forming species that support the marine food web.