Corals in Peril

In the latest issue of Science, Hoegh-Guldberg et al. 2007 (subscription required) reports on the effects on coral reefs due to the strong influences of both the increase in global temperatures, and the acidification of ocean because of high atmospheric CO2 levels, of which are now higher than any time in at least 800,000 years, and probably longer.

Hoegh-Guldberg et al. use Vostok ice core and look at {CO2}atm concentrations, temperature, and assess carbonate-ion concentration in oceans today relative to the last 420,000 years. Cooler periods are associated with high carbonate concentrations, and warmer interglacial periods with lower ones. High {CO2}atm concentrations have the effect of enhancing oceanic CO2 levels, and once CO2 is taken up by the ocean, it reacts with water creating carbonic acid (CO2 + H2O –> H2CO3 ) which releases hydrogen (H+) ions and bicarbonate. Higher concentrations of hydrogen ions are associated with lower pH levels in the oceans, or making the water more acidic.  Although a small pH difference so far, this can correspond to some 30% change in the hydrogen ion concentration, as shown with

pH = -log [H+]

Hydrogen ions combine with carbonate ions to produce bicarbonate, which reduced the carbonate concentrations, thereby making carbonate less useful to calcifying organisms. From the study, seawater carbonate concentrations have been reduced by approximately 30 µmol kg–1 over the 20th century, with a drop of about 0.1 on the pH scale (which in itself is not terrible, but the rate of change is greater than ever as far as paleo records can tell us)

From Turley et al., 2006

Carbonite-ion concentations are now at about 210 µmol kg–1 which is lower than anytime on the ice core record. Ocean Acidification has become another issue associated with higher {CO2}atm concentrations, due to enhanced uptake by the oceans. Expected impacts on calcifying organisms are expected to severe. It has been suggested that southern oceans and pacific will become toxic to organisms with calcium carbonate shells after about 2050. This will have a drastic effect on ecosystems and food chains (including baleen whales who scoop them up in large amounts), as carbonate-ion concentratios are lowered and pH of oceans may drop by about 0.5 by 2100 (Orr et al., 2005). The following image, in Hoegh-Guldberg et al. 2007, is useful for reference (click for expansion).

This shows linkages between the buildup of CO2{atm} and the slowing of coral calcification due to ocean acidification.

As a response to a more acidic ocean, corals have displayed reductions of skeletal density. Such impacts are harmful to ecosystems, and decrease resistance to things like storm damage. Another impact is less energy invested in, say, reproduction in order to maintain their physical extent.

Coral bleaching occurs when the thermal tolerance of corals and their symbiont zooxanthellae is exceeded. The symbiotic relationship between reef-building corals and zooxanthellae allows them to thrive tropical oceans, and reefs are associated with at least a quarter of the marine diversity (Buddemeier et al., 2004). Many studies unequiocally link coral bleaching to warmer SST’s (ex. McWilliams et al., 2005) and it is well known that increases in the frequency, severity, and magnitude of coral reef bleaching will be related to higher ocean temperatures. Both the biological consequences and temperature trends are well documented.

Hoegh-Guldberg et al., 2007 use three projection scenarios for putting into perspective what may happen to corals

1) If CO2 is stablilized today, at 380 ppm-like conditions, corals will change a bit but areas will remain coral dominated. Hoegh-Guldberg et al. emphasize the importance of solving regional problems such as fishing pressure, and air/water quality which are human-induced but not directly linked to climate change/ocean acidification.

2) Increases of CO2 at 450 to 500 ppmv at current >1 ppmv/yr scenario will cause significant declines in coral populations. Natural adaptive shifts to symbionts with a +2 C resistance may delay the demise of some reefs,and this will differ by area. Carbonate-ion concentrations will drop below the 200 µmol kg–1 threshold and coral erosion will outweigh calcification, with significant impacts on marine biodiversity.

3) In the words of the study, a scenario of >500 ppmv and + 2 C SST’s “will reduce coral reef ecosystems to crumbling frameworks with few calcareous corals”. Due to latitudinally decreasing aragonite concentrations and projected atmospheric CO2 increases adaptation to higher latitudes with areas of more thermal tolerance is unlikely. Coral reefs exist within a narrow band of temperature, light, and aragonite saturation states, and expected rises in SST’s will produce many changes on timescales of decades to centuries (Hoegh-Guldberg 2005). Rising sea levels may also harm reefs which necessitate shallow water conditions. Under Business-as-usual to higher range scenarios used by the IPCC corals will become rare in the tropics, and have huge impacts on biodiversity and the ecosystem services they provide.

It is sad to know that scenario 2, which uses a low-end IPCC scenario (and to be quite honest, rather conservative the way we are going), is very alarming. The fact there are scenarios is not due to scientific uncertainty, but due to the fact that we ourselves have a choice in emission scenarios. If we take strong action very soon, or continue under “normal” conditions will have noticeable impacts on many thing, corals and associated biodiversity being one of them. Reef builsing corals have low rates of adaptation, and are currently in a world changing at a very fast rate.


Buddemeier, R.W., J.A. Kleypas and R.B.Aronson, 2004: Coral Reefs and Global Climate Change: Potential Contributions of Climate Change to Stresses on Coral Reef Ecosystems. Pew Center on Global Climatic Change. URL at

McWilliams, J.P., etl, 2005: Accelerating impacts of temperature-induced coral bleaching in the Caribbean. Ecology, 86, 2055-2060.

O. Hoegh-Guldberg (2005) Low coral cover in a high-CO2 World, Journal of Geophysical Research, 110, C09S06, doi: 10.1029/2004JC002528.

O. Hoegh-Guldberg et al. 2007. Science Vol. 318. no. 5857, pp. 1737 – 1742 DOI: 10.1126/science.1152509

Orr, James C.; Fabry, Victoria J.; Aumont, Olivier; Bopp, Laurent; Doney, Scott C.; Feely, Richard A. et al. (2005). “Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms”. Nature 437 (7059): 681-686.

Turley, C., et al. (2006), Reviewing the Impact of Increased Atmospheric CO2 on Oceanic pH and the Marine Ecosystem, in Avoiding Dangerous Climate Change, 65-70, Cambridge University Press.


2 responses to “Corals in Peril

  1. great summary. Keep up the good work.

  2. Pingback: Dying Coral Reefs « Azimuth

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