The document discusses ocean acidification, which is the ongoing decrease in ocean pH caused by absorbing CO2 from the atmosphere. This absorption has lowered ocean pH by 0.1 units since the pre-industrial period. Ocean acidification affects organisms that rely on calcium carbonate to build shells and skeletons, as acidity decreases availability of carbonate ions. It also impacts metabolism, photosynthesis, nutrient absorption and more. Effects vary by ecosystem but tropical coral reefs, polar regions, and deep sea corals are threatened by slowed growth and structural damage if acidification continues unchecked. Mitigation requires reducing CO2 emissions and improving ocean health.