Ocean acidification
All the earth’s systems, including soil, rocks, plants, animals and even the ocean, cycle and recycle carbon. This is referred to as the ‘carbon cycle’. The ocean is very important to this cycle and acts as a sink, or store, for excess carbon dioxide in the atmosphere. This carbon is dissolved into the water at its surface and is used by marine species such as plankton and corals. One way carbon is used by organisms is to form calcium carbonate shells.
The ocean has been absorbing between 40 and 50 per cent of the excess carbon dioxide put into the atmosphere by human activity since the Industrial Revolution. The issue is that the carbon dioxide dissolves in sea water to form carbonic acid – that’s right, an acid – which lowers the pH of the seawater and can make it difficult for organisms to form shells. This increase in acidity is the origin of the term 'ocean acidification' referring to the trend or direction of change in ocean chemistry rather than the level it is likely to reach. That is, it will remain a slightly basic solution.
What is pH?
The pH of a solution describes how acidic or alkaline a solution may be and is measured against a pH scale of 0 to 14. On this scale 7 is neutral, with points higher on the scale being ‘alkaline’ and points lower being ‘acidic’. So something with a low pH is referred to as acidic, such as lemon juice and vinegar for example. The ocean is typically a slightly alkaline (pH around 8.0 to 8.1) and reasonably well-buffered solution which tends to remain slightly basic due to a number of buffering mechanisms.
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pH of some household items to put into perspective what high and low pH actually means
The ocean is slightly basic now, at just a bit over 8, though it has dropped about 0.1 pH units since the industrial era began due to the input of anthropogenic (manmade) carbon dioxide. The pH scale is a ‘log’ scale so an apparently small change in pH represents a big change in concentration of hydrogen ions (or ‘acidity’). Therefore the 0.1 drop in pH represents an approximate 30 per cent increase in acidity. It is not clear exactly how far the ocean’s pH will drop with ongoing carbon dioxide emissions, but best estimates suggest a pH drop of a few tenths more to about 7.7 – 7.9 over the next 100 to 200 years.
The Research
Dr Will Howard and Dr Donna Roberts
Meet Dr William Howard and Dr Donna Roberts. They form part of the ‘Ocean Acidification Team’ at the Antarctic Climate and Ecosystems Cooperative Research Centre (ACE CRC). Will and Donna compare the weights of modern-day plankton shells with the weights of fossilised shells (very old shells that have not been affected by modern-day acidification) that were trapped in the ocean floor sediments between Tasmania and Antarctica.
Will, a Tasmanian scientist, recently released field observations of ocean acidification. Will collects tiny zooplankton (microscopic organisms living near the surface of the ocean) shells from the ocean to determine the effect of pH on the ability of the zooplankton to produce calcium carbonate, which is essential for zooplankton to grow strong shells or skeletons. He found signs that modern shells have been weakened by ocean acidification.
The shell of a foraminifer, a tiny (about the size of a sand grain one-celled
organism that lives in the ocean. This is a Southern Ocean foraminifer
Globigerina bulloides (Photo: Will Howard, ACECRC).
Donna is currently conducting experiments to determine the effect of acidification on pteropods, tiny marine snails with thin shells especially at risk of changing water chemistry in the cold waters south of Tasmania. Donna collects shells from the ocean and analyses the changes in shell size and weight over time to see if the effects of ocean acidification are already detectable. She is seeing small changes that may signal the start of a worrying trend in shell weight loss for these beautiful animals, often called sea butterflies because of their double “winged” swimming action (the “wings” they use to swim are the same organs as the “feet” land or bottom-dwelling snails use to move on the surface).
Southern Ocean pteropod Limacina helicina antarctica (photo Donna Roberts, ACECRC)
Want to know more about the impact of acidification on our oceans – check out these links:
You too can conduct an experiment like Dr Will and determine the effects of an acid environment on calcium carbonate shells by comparing the weights of shells before and after exposure to acid. Just follow this link to the experiment instructions and worksheets.
Great explanation of acidification by kids for kids http://www.youtube.com/watch?v=kvUsSMa0nQU
Watch (or read the transcript of) the ABC’s Catalyst story ‘Ocean acidification – The BIG global warming story’
Watch NBC (USA) Nightly News story about ocean acidification featuring tasmanian scientists or read a related article 'For the world's oceans - a disturbing early warning ' .
Sources and acknowledgements:
Special thanks to Dr. William Howard and Dr Donna Roberts for their substantial contribution to the Acidification section of Redmap.
Andrew D. Moy, William R. Howard, Stephen G. Bray and Thomas W. Trull Reduced calcification in modern Southern Ocean planktonic foraminifera, NATURE GEOSCIENCE DOI: 10.1038/NGEO460
IPCC (2007) Summary for policymakers. In: Climate change 2007: The physical science basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, S Solomon, D Qin, M Manning, Z Chen, M Marquis, KB Averyt, M Tignor and HL Miller (eds.). Cambridge University Press,Cambridge, United Kingdom and New York, NY, USA.
Poloczanska ES, Babcock RC, Butler A, Hobday AJ, Hoegh-Guldberg O, Kunz TJ, Matear R, Milton D, Okey TA, Richardson AJ (2007) Climate change and Australian marine life.Oceanography and Marine Biology Annual Review, 45, 409–480.
