Doomed Planet

The Myth of ‘Acidification’ of Oceans

To demonise CO2 yet again, a false claim is that human production of CO2 will cause the oceans to become acid.

‘Acid’ is an emotive word to the general public, which is why it is seized upon by the alarmists in their search for yet another scare. In reality increasing CO2 makes the ocean become ‘less alkaline’, but never ‘acid’.

pH is a measurement of the amount of hydrogen ion concentration in a solution, the log of the hydrogen ion concentration with the sign changed.  Because it is a log scale it is very hard to move a pH of 8.2 to 7.0, which is neutral.

The pH needs to be less than 7 to be ‘acid’, and this has not happened through at least the past 600 million years because it would dissolve limestones, and limestone have been deposited in the sea and not re-dissolved in the sea through all that time.

Many marine organisms need CO2 to make their coral skeletons, carbonate shells and so on. Corals also have symbiotic plants within their flesh that use CO2 in photosynthesis.

Marine life flourishes where CO2 is abundant. Professor Walter Stark wrote about a favourite place for scuba divers, the ‘Bubble Bath’ near Dobu Island, Papua New Guinea. Here CO2 of volcanic origin is bubbling visibly through the water so that the water is saturated with CO2. Abundant life flourishes to make the spot a spectacular diver’s delight. He reported many accurate measurements of pH in the area and concluded “It seems that coral reefs are thriving at pH levels well below the most alarming projections for 2100.”

The pH of sea water can be very variable and makes temperature measurement look like child’s play. Ocean pH varies regionally by 0.3, and seasonally in a particular location by 0,3. But nobody has ever measured ocean water below 7, which is what “acid” means. Rhodes Fairbridge told me that he found the day-night variation in a coral pool was 9.4 to 7.5.

There is another factor called Henry’s Law. Cold water can hold more CO2 than warm, so if you warm saturated water it gives off CO2. You can see the effect if you warm a glass of fizzy drink: it goes flat. The ocean-air interface is usually rough so interchange is rapid. Actually if the aim of the AGW activists is to keep the world cooler by reducing atmospheric CO2 they are going in the direction of increasing ‘acidification’ of the oceans.

One of the factors affecting ocean pH is photosynthesis by plants. Experimental results show that plants grow better if CO2 is increased, and greenhouse managers commonly increase the CO2 artificially to increase crops, often by 30% or more. There is every reason to suppose that marine plants also thrive if CO2 is increased.  There is also experimental evidence that carbonate secreting animals thrive in higher CO2. Herfort and colleagues concluded that the likely result of human emissions of CO2 would be an increase in oceanic CO2 that could stimulate photosynthesis and calcification in a wide variety of corals.

Marine life, including that part that fixes CO2 as the carbonate in limestones such as coral reefs, evolved on an Earth with CO2 levels many times higher than those of today, as reported by Berner and Kothaval. It may be true to say that today’s marine life is getting by in a CO2-deprived environment.

Tuvalu has long been ‘hot news’ as the favourite island to be doomed by sea level rise driven by global warming, allegedly caused in turn by anthropogenic carbon dioxide. But if a coral island is sinking slowly (or relative sea level rising slowly) the growth of coral can keep up with it. In the right circumstances some corals can grow over 2 cm in a year, but growth rate depends on many factors. Coral islands, made of living things, are not static dip-sticks against which sea level can be measured. We have to consider coral growth, erosion, transport and deposition of sediment and many other aspects of coral island evolution – not just the pH of seawater. Webb and Kench studied the changes in plan of 27 atoll islands located in the central Pacific, and found that most had remained stable or grown in area over about the past twenty years (despite measured rises in atmospheric CO2 over the same period), and only 15% underwent net reduction in area. One of the largest increases was the 28.3% on one of the islands of Tuvalu. This destroys the argument that the islands are drowning, and coral growth is not reduced by ‘acidity’.

Marine life depends on CO2, and some plants and animals fix it as limestone, which is not generally re-dissolved. Over geological time enormous amount of CO2 have been sequestered by living things, so that today there is far more CO2 in limestones than in the atmosphere or ocean. This sequestration of CO2 by living things is far more important than trivial additions to the atmosphere caused by human activity. 

Emeritus Professor Cliff Ollier is a geologist and geomorphologist.


Berner., R.A. and Kothavala, Z. 2001. A revised model of atmospheric CO2 over Phanerozoic time. American Journal of Science, 301, 182-204.

Herfort, L, Thake, B. and Taubner, I. 2008. Bicarbonate stimulation of calcification and photosynthesis in two hermatypic corals. Journal of Phycology, 44, 91-8.

Webb, A. P. and Kench, P. S. 2010. The dynamic response of reef islands to sea level rise: evidence from multi-decadal analysis of island change in the central pacific, Global and Planetary Change, 72, 234-246.

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