Such short-term variability has often been underappreciated and may prove an important consideration when predicting the long-term impacts of ocean acidification to coral reefs, according to the authors.

A second paper, by scientists from the University of New South Wales (Australia) and the Centre for Australian Weather and Climate Research and Antarctic Climate & Ecosystems Cooperative Research Centre, concludes that the Southern Ocean will acidify past a critical tipping point when atmospheric CO₂ levels pass 450 ppm, projected to occur within 30 years at most.

At this point of aragonite undersaturation, the shells of sea creatures will start to dissolve. Previous estimates found that this dissolution point for shells in the Southern Ocean would occur after atmospheric carbon dioxide concentrations reached 550 ppm, which is projected to occur in the latter part of the century. This paper is published in the journal Proceedings of National Academy of Sciences.

Since the beginning of the industrial era, the oceans have absorbed about a third of all anthropogenic carbon dioxide emissions released into the air—this has served as a carbon sink, and retarded some of the warming that otherwise would have occurred.

Atmospheric CO₂ reacts with seawater to form carbonic acid (H₂CO₃). Increasing the amount of CO₂ dissolved in the ocean lowers the pH, decreases the availability of carbonate (CO₃^2-) ions, and lowers the saturation state of the major shell-forming carbonate minerals such as aragonite. Carbonate ions are building blocks for the calcium carbonate (e.g., aragonite, calcite) that many marine organisms use to grow their skeletons and create coral reef structures.

With increasing carbon dioxide in seawater, shellfish and corals cannot absorb enough calcium carbonate to build strong skeletons and shells. The greater acidity slows the growth and even dissolves ocean plant and animal shells.

Ocean acidification has been called the ‘silent climate change issue’ because it gets far less press than global warming, sea level rise and rainfall changes. Yet for marine systems, it could end up being the most important consequence of industrial pollution of all because it risks dissolving coral reefs and undermining the food chain upon which the world’s fisheries and marine mammals depend.

—Professor Barry Brook, Sir Hubert Wilkins Chair of Climate Change and Director of the Research Institute for Climate Change and Sustainability at Adelaide University.

The Caribbean. A number of recent studies demonstrate that ocean acidification is likely to harm coral reefs by slowing coral growth and making reefs more vulnerable to erosion and storms.

In the new study, Dwight Gledhill, Rik Wanninkhof, Frank Millero and Mark Eakin used four years of ocean chemistry measurements taken aboard the Royal Caribbean Cruise Line ship Explorer of the Seas together with daily satellite observations to estimate changes in ocean chemistry over the past two decades in the Caribbean region.

(The resulting new ocean acidification tracking products are available online along with animations of the changes since 1988.)

The study supports other findings that ocean acidification is likely to reduce coral reef growth to critical levels before the end of this century unless humans significantly reduce carbon dioxide emissions. While ocean chemistry across the region is currently deemed adequate to support coral reefs, it is rapidly changing as atmospheric carbon dioxide levels rise.

The study demonstrates a strong natural seasonal variability in ocean chemistry in waters around the Florida Keys that could have important consequences for how these reefs respond to future ocean acidification.

—NOAA’s Dwight Gledhill, lead author

The Southern Ocean. By using a new technique that better quantifies natural variations of carbon dioxide in the Southern Ocean, Ben McNeil and Richard Matear found that natural seasonal variations in carbonate ion concentrations could either hasten or dampen the future onset of the dissolution point.

Their large-scale observational analysis of the Southern Ocean shows an intense wintertime minimum in CO₃^2- south of the Antarctic Polar Front. When combined with anthropogenic CO₂ uptake, it “is likely to induce aragonite undersaturation when atmospheric CO₂ levels reach ˜450 ppm.”

Under the IPCC IS92a scenario, Southern Ocean wintertime aragonite undersaturation is projected to occur by the year 2030 and no later than 2038.

—McNeil and Matear 2008

Oceanic acidification is a direct consequence of increasing atmospheric carbon dioxide concentrations. Our new results point to irreversible and detrimental impacts to Southern Ocean marine calcifying organisms if atmospheric CO₂ exceeds 450 ppm. It provides additional and direct scientific evidence for the world to do everything in its power to limit CO₂ concentrations to 450 ppm in order to avoid the irreversible consequences associated with ocean acidification.

—Dr. McNeil

In July, a team of researchers led by Richard Zeebe of the University of Hawai’i at Manoa published a paper in the journal Science warning that the ecological and economic consequences of ocean acidification are difficult to predict but possibly calamitous, and that halting the changes already underway will likely require even steeper cuts in carbon emissions than those currently proposed to curb climate change. (Earlier post.)

Resources

• Dwight K. Gledhill, Rik Wanninkhof, Frank J. Millero, Mark Eakin (2008) Ocean acidification of the Greater Caribbean Region 1996–2006. J. Geophys. Res., 113, C10031, doi: 10.1029/2007JC004629

• Ben I. McNeil and Richard J. Matear (2008) Southern Ocean acidification: A tipping point at 450-ppm atmospheric CO₂. PNAS doi: 10.1073/pnas.0806318105