2011-01-23 19:39:52Intermediate Rebuttal No.135 - more CO2 increases coral calcification
Rob Painting


Carbonates are "building blocks" of the coral skeleton. More CO2 in the atmosphere increases ocean acidity and lowers carbonate levels in seawater, leading to decreased coral growth. Recent studies show falling coral growth rates around the world. Given the current path of carbon dioxide emissions, ocean acidification during the 21st Century is likely to promote the gradual deterioration of coral reefs and eventually result in seawater which is corrosive to coral.


One of the nasty side effects of burning fossil fuels is increasing ocean acidity. Seawater pH (a measure of the hydrogen ion concentration) is very important to chemical processes in marine organisms, but it's actually the effect that pH has on the level of carbonates dissolved in the oceans that is key to the growth of coral. Ocean acidification is encapsulated in the following graphic:

Figure 1.

Increased atmospheric carbon dioxide leads to more dissolving into the oceans according to Henry's Law. Although a small amount of carbon dioxide stays dissolved in the oceans, most of it reacts with seawater to form carbonic acid. From there a further reaction takes place, much of the carbonic acid breaks down releasing hydrogen ions which lower ocean pH, and some of the hydrogen ions combine with carbonates already in the water to form bicarbonates. The overall effect therefore is; a) a drop in pH as more hydrogen ions are disassociated from water molecules, b) a rise in bicarbonate concentrations and c) a fall in carbonates (Orr 2005)

All forms of carbon in seawater are known as dissolved inorganic carbon and the relative proportions in the surface ocean should be noted; 91 % bicarbonate and 8 % carbonate, 


Coral make their skeletons of aragonite, a relatively soluble (easily dissolved) form of calcium carbonate. The building blocks of which are calcium and carbonate ions extracted from the surrounding seawater. As calcium, the other "building block", has stable levels in seawater, it is carbonate that dictates the ability of coral to form their skeletons. The reduction in carbonates lowers the aragonite saturation state, which is a measure of the relative abundance of calcium and carbonate dissolved in seawater.

That coral growth is heavily dependent on carbonates has been shown in a number of experiments (Langdon 2000, Leclercq 2000 , Marubini 2003, Silvmerman 2007). A recent study Cohen 2009 gives a graphic example of the effect of carbonates on calcification. In this study they grew juvenile coral polyps in seawater at various saturation states and found that as the level of carbonate ions fell (i.e lower aragonite saturation), so did the rates of skeleton formation. 


 Figure 2. A-D show falling coral skeleton growth as aragonite saturation declines (photo with polyp removed). From Cohen 2009  


The experimental evidence linking carbonate levels with calcification is compelling, but what is the actual mechanism by which coral build their skeletons?. On this issue scientific opinion has differed, however emerging research (Cohen & Holcomb 2009) helps to shed some light on this issue.

Coral carry out calcification in compartments isolated from the surrounding ocean. The coral polyp sits on top of its skeleton and connects with adjacent polyps to form a sheet of living tissue over the calcium carbonate framework. Because aragonite doesn't spontaneously form, at the saturation levels typically found in the oceans, the polyp has to manipulate the chemistry of the seawater in the compartment. One method it employs is to pump hydrogen ions out of the chamber, raising pH. As seen in the Ocean Acidification primer, changing the levels of various carbon forms dissolved in seawater changes pH, but this process can work in reverse. Changing pH also changes the concentration of dissolved carbon forms too. The upshot of the polyp raising pH is to convert the abundant bicarbonate ions into carbonate ions. But it also raises the aragonite saturation state much higher than the surrounding seawater, allowing aragonite crystals to form.

So it appears the main obstacle for coral is that ocean acidification means coral have to expend more energy in the calcification process, de-acidifying the increasingly acidified seawater. So why don't they allocate more energy to calcification?. Experiments and worldwide observations indicate that coral simply are not able to, and doing so would mean other vital processes like reproduction would be negatively affected.


Several studies are often put forth by skeptics to counter the evidence that carbonates are critical for calcification, but neither of these two studies stand up to scrutiny. Herfort 2008 used sodium bicarbonate to alter the bicarbonate level of the seawater in their experiment while acidity was kept constant. This raises carbonate too. In other words the experiment did not replicate conditions of ocean acidification, which would require that pH was reduced. Jury 2009 simulated bicarbonate levels which are unrealistic (being far above conditions likely in the next century) and notably; low calcification and bicarbonate in the experiment also coincided with low carbonate conditions.


Since the beginning of the Industrial Revolution humans have increased the acidity of the oceans by about 30% through the burning of fossil fuels a href="http://url">Description of page you're linking to(study) This has in turn lowered the saturation state of aragonite by some 16% over pre-industrial values a href="http://url">Description of page you're linking to(study). We might expect to see this reflected in lower rates of coral growth and this is precisely what the latest research is finding.

One recent study (De'ath 2009) looked at coral growth rates on the Great Barrier Reef in Australia. Using cores taken from coral, and counting annual coral growth bands, they were able to construct a record of calcification on the reef dating back over 400 years. What they found is that calcification steadily increased for hundreds of years as ocean warming aided growth then, around 1990, began to decline once warming and acidification exceeded coral tolerance thresholds . From 1990 to 2005 the calcification rate of coral of the Great Barrier Reef declined 14.2%. Wei  2009 confirm that waters of the Great Barrier Reef have acidified during the 20th century.


Figure 3 - From De'ath 2009. Partial effects plots showing the variation of calcification (g cm-2 yr-1), linear extension (cm yr-1) and density (g cm-3) in Porites over time. Plots A - C are based on 1900 – 2005 data for all colonies, and plot D on data for the 10 long cores. Light blue bands indicate 95% confidence intervals for comparison between years, and gray bands indicate 95% confidence intervals for the predicted value for any given year  

Using other techniques for measuring coral growth, similar declines are reported in both the South-East Asian (Tanzil 2009) and Caribbean reef-coral communities. (Bak 2009). Another study on the coral reefs of Bermuda (Bates 2010) found that the decline in skeleton growth from pre-industrial times was over 50%,(because of cooler waters & lower pH) and noted large swings in coral growth related to seasonal changes in pH, and therefore aragonite saturation. Furthermore the reefs are expected to have seasonal periods of zero calcification within the next decade, and may be the first coral reef to reach a period of zero growth because of acidification.




14% falls in coral growth probably don't sound like much, but are large enough to endanger the functioning and integrity of coral reefs. A reason coral have been so successful is because of their immense ability to build aragonite skeletons at a very high rate. This enables them to keep steps ahead of natural erosion and competitive species. Declines in growth rates of 20% are enough to halt the skeleton-building process and tip them into a state of gradual deterioration (Hoegh-Guldberg 2007)


The growth of coral is dependant on the level of carbonate "building blocks" in the oceans. That further ocean acidification will lead to a continued fall in coral growth rates is strongly supported by a large number of experiments, and now real-world observations.

Looking ahead, the current course of fossil fuel emissions points to a future where the oceans are actually corrosive to coral. Silverman 2009 show that by the time CO2 in the atmosphere reaches 560 ppm (later this century) all coral reefs will have completely stopped growing and begin to dissolve. Add to the mix coral beaching and ocean warming, which amplifies deterioration of coral growth (Anlauf 2010), and the future for coral is very shakey. 

2011-01-23 19:41:20
Rob Painting
Grrrrrr, ignore. Will finish off later.