Ocean Acidification Threatening Marine Ecosystems

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Since the beginning of the Industrial Revolution, the world's oceans have absorbed approximately one quarter of all carbon dioxide emitted through human activities, a process that, while moderating the pace of atmospheric warming, has triggered a profound and accelerating chemical transformation of seawater that scientists describe as ocean acidification. When carbon dioxide dissolves in seawater it reacts with water molecules to form carbonic acid, which subsequently dissociates and releases hydrogen ions, lowering the pH of the ocean and simultaneously reducing the concentration of carbonate ions that countless marine organisms depend upon to construct their shells and skeletal structures. Coral reefs, which support an estimated quarter of all marine species despite covering less than one percent of the ocean floor, are particularly vulnerable to this process because the calcium carbonate structures that constitute reef architecture dissolve more readily as seawater becomes more acidic, undermining the physical integrity of entire ecosystems that took thousands of years to develop. Pteropods, small free-swimming molluscs that form a critical component of polar food webs, have already been observed with visibly dissolving shells in regions where acidification is most advanced. Oysters, mussels, and other commercially harvested shellfish are experiencing reproducible declines in larval survival rates at aquaculture facilities located in particularly affected coastal zones. Scientists warn that without dramatic reductions in global carbon emissions, ocean pH will continue declining throughout this century at a rate unprecedented in the geological record, pushing marine ecosystems toward thresholds beyond which recovery may prove effectively impossible.