Here are a few excerpts from this excellent publication (PDF), published by the European Project on Ocean Acidification and the Ocean Carbon Biogeochemistry project.
The ocean is not acidic, and model projections say the oceans won’t ever become acidic. So why call it ocean acidification?
“Acidification” simply means a lowering of the pH from a starting point to an end point. Even though seawater’s pH is greater than 7.0 and therefore considered basic in terms of the pH scale, increasing CO2 in the atmosphere is being absorbed by the ocean, lowering pH and thus acidifying the water. Consider an air temperature change of, say, -40 to -25—it’s still cold, but we call it “warming.”
Is ocean acidification just another name for climate change?
No. Climate change has to do with changes in Earth’s heat budget due to the “greenhouse effect” of CO2 and other gases, which is causing global warming and changes in weather patterns. OA refers only to changes in ocean pH and does not include warming of the ocean.
Why should we care about a more acidic ocean?
As ocean pH decreases, it reduces the availability of carbonate ions, which help corals, marine plankton, and shellfish create tough, protective shells. Smaller calcifying organisms such as Pteropods and other planktonic calcifiers may be particularly prone to increases in acidity, and if they can’t survive, creatures that feed on them can’t survive either. As a result, crabs and oysters—as well as coraline marine habitats—may all decline as a result of increased ocean acidification (OA). Ultimately, OA is a major threat to Alaska’s economically vital, world-class commercial fisheries.
How do we know what ocean pH was in the past even though the pH scale was not introduced until 1909?
Scientists have analyzed the CO2 concentration of air in bubbles trapped in glaciers to develop a record of past atmospheric CO2 levels. Because much of the ocean’s CO2 concentration remains roughly equal to the concentration of atmospheric CO2, the ocean CO2 content—and thus the pH—can be calculated from the bubbles.
How do we know what ocean pH was millions of years ago?
Scientists look to related parameters called “proxies.” For example, ancient marine animals incorporated elements such as boron, as well as calcium carbonate, into their hard shells and skeletons. We can calculate historical ocean pH values and changes by measuring the concentration of boron and the ratio of its stable isotopes in marine carbonates.
What evidence suggests that ocean acidification is happening and that it results from human activity?
Semi-continuous records of seawater CO2 and pH for the last 20–30 years increases in marine C02 mirroring increases in atmospheric CO2.
Shellfish can survive in fresh water where pH can drop as low as 5, so what’s the problem?
Creatures that live in freshwater or salt water with lower pH are adapted to such conditions, but marine creatures that have evolved in seawater with a higher and less variable pH are more susceptible to changes in pH. A good example of this is in an estuary, where the river meets the sea: Shells of Thais gadata, a marine shellfish species found along estuaries tend to dissolve more frequently at the freshwater end of the estuary (where pH is lower and varies widely) than at the seawater end, with its higher and less variable pH.
Will ocean acidification kill all ocean life?
No. But geological history tells us that ocean acidification will lead to important changes in marine ecosystems, such as species extinctions of some tropical corals and swimming snails. These are key ecosystem components because they provide habitat, protection, and/or food for other marine species.
Will capping atmospheric CO2 at 350 or 400 ppm stop ocean acidification?
Atmospheric CO2 is at 390 ppm and increasing by 2 ppm a year. Without dramatic reductions in CO2 emissions, atmospheric CO2 is expected to continue rising before leveling off and decreasing, possibly via natural and artificial CO2 uptake mechanisms. The chemistry of seawater is reversible, and returning to 350–400 ppm would return ocean pH and carbonate saturation levels to approximately current conditions.