WHOI biogeochemist Anne Cohen investigates how changing climate and ocean conditions may affect the health of coral reef ecosystems, which provide an estimated $375 billion to the global economy annually.
Corals combine calcium and carbonate ions in seawater to make calcium carbonate skeletons. Coral polyps produce a special kind of calcium carbonate called aragonite, fashioning aragonite crystals into structures that create coral reefs. Lower ocean pH means fewer carbonate ions available for corals to build skeletons. (Glenn Gaetani, Woods Hole Oceanographic Institution)
Left to right, skeletons of baby corals grown in lab tanks with seawater containing diminishing carbonate-ion concentrations, ranging from today's (far left) to lower levels predicted for the year 2200 (far right). (Anne Cohen, Woods Hole Oceanographic Institution)
Since the Industrial Revolution, increased burning of fossil fuels has sent more carbon dioxide into our atmosphere. Almost 30 percent of that carbon dioxide has been absorbed by the ocean, causing a fundamental change in the seawater chemistry. The carbon dioxide reacts with water molecules, creating more free hydrogen ions that drive down the pH of seawater and make it more acidic. Some of the hydrogen ions bind with existing carbonate ions in seawater to form bicarbonate. The net result is that the concentration of carbonate ions declines, so that corals have less of it to build their skeletons. Scientists predict that by the end of this century, the concentration of carbonate ions that corals need to grow will drop to less than half of preindustrial levels.