Parts of the world’s oceans appear to be acidifying far faster than scientists have expected.
The culprit: rising levels of carbon dioxide in the atmosphere pumped into the air from cars, power plants, and industries.
The Southern Ocean represents one of the most high-profile examples. There, scientists estimate that the ocean could reach a biologically important tipping point in wintertime by 2030, at least 20 years earlier than scientists projected only three years ago. Among the vulnerable: a tiny form of sea snail that serves as food for a wide range of fish.
Similar trends are appearing in more temperate waters, say researchers.
The studies suggest the CO2-emission targets being considered for a new global warming treaty are likely to be inadequate to prevent significant, long-lasting changes in some ocean basins.
Scientists over the past decade have detected a clear shift toward acidity since preindustrial times. But that “is not really telling you the story” as it unfolds on smaller but ecologically important scales, says David Archer, a researcher at the University of Chicago who studies the global carbon cycle.
The new research draws on long-term data on changes in ocean chemistry and the effect of those changes on marine life. The data are giving scientists their first clear look at the importance of natural swings in sea-water acidification in estimating overall acidification trends and tipping points.
But even these new studies may be conservative. Recent global CO2 emissions have been outstripping so-called business-as-usual emissions scenarios, which assume that no country adopts climate-specific limits on emissions.
From a human perspective, ocean acidification is relative; no one is talking about dissolving surf boards. On the pH scale – which runs from strong acids such as battery acid to strong bases such as laundry bleach – the oceans fall on the base side of the spectrum. The oceans have a pH of 8. Distilled water is considered neutral, with a pH of 7. Battery acid has a pH of 1.
Typically, seawater is heavily saturated with dissolved calcium carbonate from eroded limestone. This neutralizes any acid that forms from CO2 and leaves plenty of carbonate for marine creatures to use for shell- and reef-building. But as oceans absorb increasing amounts of CO2 from fossil fuels, their stores of calcium carbonate dip. Over time, this reduces carbonate available for marine creatures. Shell and coral formation slows.
Once seawater is too deficient in carbonate, these creatures find it hard to form shells or corals at all. In fact, existing shells start to dissolve, notes Ben McNeil, a researcher at the University of New South Wales in Australia.
In a recent study, he and a colleague looked at trends in the Southern Ocean. Oceans at the top and bottom of the world might be expected to lead in acidification because cold water soaks up more CO2 than warm water. But the duo also found large seasonal swings in carbonate levels. They traced increases in the water’s relative acidity to strong wintertime winds off Antarctica that bring to the surface cold water from the deep, which has low levels of carbonate.
The challenge, Dr. McNeil says, is that this seasonal peak in acidification comes just as tiny swimming snails – which some call potato chips of the sea – exist as larvae. The tiny zooplankton, called pteropods, need carbonate to build their shells. They represent a vital source of food for many fish. Some pteropods already show signs of dissolving shells, the team reports.
With a business-as-usual emissions scenario, McNeil and his colleague estimate that the Southern Ocean is likely to reach a wintertime tipping point for these creatures when atmospheric CO2 concentrations reach 450 parts per million, versus today’s level of around 383 ppm. That would occur by 2030 and no later than 2038, they estimate. The results appear in the Dec. 9 issue of the Proceedings of the National Academy of Sciences (PNAS).
On Tatoosh Island, off the northwest tip of Washington State’s Olympic Peninsula, researchers have found acidification trends running some 10 times faster than projected. The University of Chicago’s Timothy Wootton led a team that analyzed more than 24,500 water samples gathered over eight years. They found wide swings in carbonate levels during the year. As acidification increased, they found, larger shell-forming creatures such as mussels and barnacles lost ground to smaller ones and nonshell types of algae. The team’s work also appears in the same issue of PNAS.
As acidification changes the mix of marine life in coastal areas, it could eliminate species important to commercial fisheries, they say.
The picture is more complicated in the Caribbean. Researchers with the National Oceanic and Atmospheric Administration (NOAA) and the University of Miami tracked changes in acidification across the greater Caribbean between 1996 and 2006 using sensors placed on a cruise ship and satellite data.
The region shows a definite trend toward acidification through a reduction in dissolved carbonates, with the highest seasonal swings seen in the waters around the Florida Keys. The results appeared in the Oct. 31 issue of the Journal of Geophysical Research.
The implication for the Keys is unclear, says Dwight Gledhill, a reef expert with NOAA who led the team. Reefs there have clearly adapted to large seasonal swings in minerals.
“If you have a system that shows large variability throughout the year, that could mean that the system may be more resilient to future changes,” he says. “[Or] do they cross a critical threshold sooner?”