Changing Ocean Chemistry
For the ocean, climate change is more than a matter of temperature: about 30 percent of the CO2 released by human activities over the past 200 years has already been absorbed by the ocean, and much more will ultimately end up there. And all that CO2 is having an unfortunate effect: the ocean is "acidifying," or becoming less basic.
While scientists aren't sure how different marine organisms will respond to changing ocean waters, many argue that ocean acidification will make it harder for shell-forming organisms, from corals to tiny plankton, to grow their shells. Eventually, shell-forming organisms could disappear from some parts of the ocean, and coral reefs could even begin to die.
The pHuture Ocean
Ocean acidity is measured on the "pH" scale: liquids with a pH less than 7 are acidic and those with a pH above 7 are basic. So far, the ocean's pH has dropped from about 8.2 in preindustrial times to around 8.05. This seemingly small change may already be affecting ocean organisms—and future CO2 emissions could lower ocean pH even further.
What's Ocean Acidification?
The reaction of CO2 and ocean water produces chemicals such as carbonic acid, which lowers ocean pH, and bicarbonate. On the one hand, increased amounts of carbonic acid should make it more difficult for many organisms, like snails or corals, to build their calcium carbonate shells or skeletons. On the other hand, increased amounts of bicarbonate may stimulate growth in some marine organisms, so the overall effect of rising CO2 is uncertain and may affect different organisms in varied ways. Most scientists, however, view changing ocean composition with alarm.
In normal water, the common periwinkle snail (Littorina littorea) responds to the presence of crab predators both by thickening its shell and by crawling away. But experiments suggest that in the waters expected under climate change, periwinkles will not be able to thicken their shells to ward off crabs. Other ocean organisms will likely also be affected in unanticipated ways.
Coral reefs are already under siege: destructive fishing practices and runoff from overdevelopment all take a toll, weakening the reefs and making them more susceptible to storms and diseases. Some 20 percent of all reefs have been destroyed in the past few decades and another 50 percent are on the verge of collapse.
But corals will likely suffer even more as oceans warm: spikes in water temperature stress coral polyps--the tiny animals that make up coral colonies—and cause them to drive out algae that live within their tissues. These algae use sunlight to provide polyps with nutrients and lend reefs their brilliant colors. When polyps expel the algae, reefs turn pale and begin to starve, a phenomenon known as coral bleaching.
Coral reefs cover only about 0.1 percent of the ocean bottom but are vital to ocean life: 9 million marine species, including 4,000 kinds of fish, rely on coral reefs for food or shelter. Further, reefs form a central pillar of many countries' economies, supporting fishing and tourism industries and protecting coastlines from storm surges.
Human activity puts the world's coral reefs at risk. Here, reefs are colored according to the level of risk from human-threat factors.
In the summer of 1998, unusually warm water in the western Pacific and Indian Oceans bleached many of the corals living there. Most eventually recovered, but the event still killed about 16 percent of all the corals on Earth. As ocean waters warm under climate change, coral bleaching events like this one will become more frequent.