June 17, 2010. Copyright 2010, Graphic News. All rights reserved

Acidifying oceans add urgency to carbon cuts

LONDON, June 17, Graphic News: Climate change could be affecting marine ecosystems on a scale that is similar to those of past events, such as volcanic episodes or meteorite strikes that occurred 55-65 million years ago.

In a study appearing in Friday’s edition of the journal Science, Ove Hoegh-Guldberg of the University of Queensland in Australia and John Bruno of the University of North Carolina say that the world’s oceans are absorbing increasing amounts of carbon dioxide, causing rapid acidification of surface waters.

Oceans absorb approximately a third of the carbon dioxide that our cars, factories and other technologies are releasing into the atmosphere from burning fossil fuels. However, as CO2 dissolves in seawater, the pH of the water decreases. The pH scale, “potential of hydrogen”, is a 14 step measure in which strong, caustic alkalis such as oven cleaner have a pH of around 13, pure water is neutral at 7, and any substance with a pH between 7 and 1 is considered acidic. The pH scale is logarithmic; for example, pH 4 is ten times more acidic than pH 5 and 100 times more acidic than pH 6. 

When CO2 dissolves in seawater, it forms carbonic acid, which releases hydrogen ions into solution. As the concentration of these positively-charged hydrogen ions rises it leads to a decrease in pH, making the water acidic.

Hydrogen ions react with carbonate ions, which are the basic building blocks of the calcium carbonate shells and skeletons of many marine organisms, locking them in the form of bicarbonate. The formation of bicarbonate through this chemical reaction removes carbonate ions from the water, making them less available for use by organisms such as tropical corals, echinoderms (starfish), molluscs, microscopic plankton and certain algae. When the hydrogen ion concentration of seawater gets high enough, the calcium carbonate in these organisms begins to dissolve.

Overall, researchers estimate there has been a 0.1-pH-unit decline since the beginning of the industrial revolution a couple of centuries ago. In logarithmic pH units, the change may seem tiny, but in absolute terms, that translates into a 30% increase in surface-ocean acidity.

“Ocean pH is now lower than it’s been for 20 million years, and it’s going to get lower”, says marine chemist Richard Feely of the National Oceanic and Atmospheric Administration’s (NOAA) Marine Environmental Laboratory in Seattle, Washington.

Feely and his colleagues have modelled future pH based on a business-as-usual growth in carbon dioxide emissions. The model predicts a drop from a pre-industrial pH of 8.2 to about 7.8 by the end of this century -- an increase in surface ocean’s acidity by about 150% on average.

Colder waters of the Southern Ocean, South and North Atlantic, and parts of the North Pacific, which have a greater capacity to absorb carbon dioxide, will be affected first. By the end of the century they will become corrosive to pteropods (sea snails) and other organisms which depend on aragonite -- the most vulnerable crystal form of calcium carbonate -- to build their shells.

Though most of the scientific and public focus has been on the climate impacts of human carbon emissions, ocean acidification is now seen by many as just as imminent and potentially as severe a crisis./Ends