Ocean acidification

No doubt everyone will have read articles and seen TV documentaries on the subject of global climate change. I’m equally sure that there will be many who are sceptical about the effects of man-made increases in Carbon Dioxide levels in the atmosphere being the cause of these changes, or indeed if changes are happening at all.

However, there are few (although amazingly there are still some) who dispute that CO2 levels have increased in the Earth’s atmosphere since the start of the Industrial Revolution when we began burning fossil fuels in earnest. Indeed science can clearly demonstrate that CO2 concentrations have increased from about 200 parts per million (ppm) pre-Industrial Revolution to about 380ppm currently. This concentration is increasing at a rate of between 1 and 2 ppm per year. The Earth hasn’t seen CO2 concentrations at these levels in about 720,000 years and the rate of increase has not been this high in a million years.

But I don’t want to talk about global warming or climate change here; rather another, much less discussed, impact of this increase in CO2 levels. As Carbon Dioxide levels in the atmosphere rise the world’s oceans are becoming more acidic.

The changes may appear small (a decline in pH of about 0.1 in a few hundred years) and yet the effects are profound. The oceans are in fact alkaline and even with pH levels falling they are almost certain to stay so for many millennia yet. In reality we should be talking about decreasing alkalinity rather than increasing acidity. But this is pure semantics as the results are the same.

The oceans absorb between 30 and 50% of all the CO2 in the atmosphere and as they do so a chemical reaction occurs which decreases the number of carbonate ions available in the water. This is of critical importance since many organisms require these carbonate ions to secrete Calcium Carbonate; the building block of so much in the marine environment. In particular, corals need to secrete Calcium Carbonate to build their skeletons and create the structures we know as coral reefs. With the reduction in carbonate ions the corals will find it a harder and harder job to keep up with the natural erosion of the reefs by storm action, predation and sea water dissolution.

It is a reasonably straightforward chemical calculation to determine the level of carbonate ions that would result in reefs being unable to regenerate faster than they are eroded and recent studies have shown that level to be 200 micromoles/kg. The current level is 270 micromoles/kg. Since the availability of carbonate ions is determined by the concentration of CO2 dissolved in the sea, it can be determined that a level of approximately 500 ppm CO2 would result in this level of carbonate ions. Disturbingly a whole raft of scientific papers predict these kind of CO2 concentrations occurring sometime between 2050 and 2100 if serious measures are not taken to curtail the production of CO2 in the atmosphere.

A recent report by GBRMPA identified ocean acidification as the number one threat to the reef in the coming 50 to 100 years; so expect to see and hear much more about this issue in the future!

Pete Faulkner

Pete is a PADI Master Instructor and has been involved with reef conservation for almost 20 years. He is Chairman of Reef Check Australia, Vice-Chair of Coral Cay Conservation, owns and runs Conus Dive Training and is a member of the Cassowary Coast Marine Advisory Committee.


Pete Faulkner, Mission Beach Business & Tourism, 12 March 2012. Article



1 Response to “Ocean acidification”

  1. 1 John 13 March 2012 at 23:24

    A serious question: at the end of the Eocene, about 34.5 million years ago, CO2 levels were approximately 1,100 ppm, give or take about 150 ppm, for about four hundred thousand years (Fig. 3a, Pearson et al., “Atmospheric carbon dioxide through the Eocene–Oligocene climate transition”, 2009, Nature 461, pp. 1110-1113).

    Coral reefs continued to grow throughout this period. I don’t know if research had determined how many coral species, if any, died out (I assume there were some, but I don’t know if this research has been done).

    If coral reefs and most corals seemed to survive this period with 1,100 ppm CO2, why will 500 ppm be such a hard limit to survival?

    Recent research showed that cold water corals, given six months to adapt to lower pH, with aragonite saturation below 1.0 (0.8) still managed to have slightly enhanced rates of calcification at these low pH, low aragonite saturation conditions (Form and Riebesell, “Acclimation to ocean acidification during long-term CO2 exposure in the cold-water coral Lophelia pertusa,” 2011, Global Change Biology, in press). Also, no “appreciable” increase in metabilic rates occurred.

    The key sentence in the Abstract: “This is the first evidence of successful acclimation in a coral species to ocean acidification, emphasizing the general need for long-term incubations in ocean acidification research.”

    Ries et al. (“A nonlinear calcification response to CO2-induced ocean
    acidification by the coral Oculina arbuscula,” Coral Reefs, 2010) examined a warm water coral at four different CO2/pH/aragonite saturation states — the CO2 roughly 400, 600, 900, and 2850 ppm CO2. “Skeletal accretion was only minimally impaired…” when CO2 increased from 400 to 900 ppm, and aragonite saturation decreased from 2.6 to 1.6. There were major reductions in the growth rate at the highest CO2 level, when aragonite saturation was 0.8, but corals still continued to accrete new material at this rate.

    Don’t these studies suggest the need for greater research? Don’t they suggest that the 500 ppm figure does not represent a bright line beyond which corals will disappear?

    If this analysis is incorrect — it could well be — please respond with another comment below.

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