Ocean acidification: how carbon dioxide reduction will affect ocean pH

Even if (contrary to evidence) the global warming deniers were right that increased green house gasses have nothing to do with global temperatures, they still have an extremely dangerous impact on the planet. That is because increased carbon dioxide in the atmosphere increases the acidification (or more precisely decreases the pH) of the oceans (and other water bodies).

This fact does not depend on complex computer modeling, taking into account jet streams, ocean current, climate feedbacks, or accelerators. This fact can be shown experimentally in your own home. All you need is a carbon dioxide source and a pH reagent test kit or pH meter. The carbon dioxide source could be compressed carbon dioxide from a welding supply store or simply a block of dry ice (which is frozen carbon dioxide).* The test kit is easily obtained from any aquarium supply store or pool supply store. (A pH meter is harder to find and requires calibration with distilled water so unless you plan to use it repeatedly it’s much cheaper to stick with the reagent kits.) You simply measure the pH of a container of water. Then either bubble the gas into the water or drop the dry ice in. Then measure the pH of the water again. You will discover that the pH drops after injection of carbon dioxide. This fact is known and depended on by many aquarium enthusiasts who either need to lower the pH of water they have on hand for the metabolic needs of the fish they possess or wish to increase the dissolved carbon dioxide to promote aquatic plant growth.

The reason for the drop in pH is that when carbon dioxide dissolves in water it creates an equilibrium between the carbon dioxide and carbonic acid (resulting from a combination of carbon dioxide and water):

CO2 + H2O ⇌ H2CO3

Carbon dioxide naturally dissolves in water from the atmosphere above it. This is a specific result from a general law first stated by William Henry in 1803, known as Henry’s Law, which holds that the solubility of a gas in a liquid depends on temperature, the partial pressure of the gas over the liquid, the nature of the solvent and the nature of the gas. In other words at any given temperature more carbon dioxide will dissolve in water or sea water (the particular “solvent”) when there is more carbon dioxide above that water.

So why does anyone care?

First organisms that use calcium to build body parts such as the exoskeletons of crustaceans and corals or the shells of mollusk and other invertebrates are adversely affected by lower pH. This is because calcium reacts with acid, which you can see in this video:

So without calcium carbonate these animals will be unable to construct vital body parts. The food chain that depends on them will therefore collapse.

Moreover, hobbyists know that having fish in water with pH lower than their specific tolerance can impair their breathing, affect their skin, make them more susceptible to diseases and render certain chemicals more toxic to them.

In addition, lowering pH of the habitat affects the behavior of fish. A recent study by Philip L. Munday of James Cook University in Australia (and others) entitled “Replenishment of fish populations is threatened by ocean acidification,” Proc. Nat’l Acad. of Sciences, July 6, 2010 (online before print) (abstract; article behind pay wall) showed that acidification of water affected the behavior of larval fish:

“[A]t 700 ppm CO2, … many individuals becom[e] attracted to the smell of predators. At 850 ppm CO2, the ability to sense predators was completely impaired. Larvae exposed to elevated CO2 were more active and exhibited riskier behavior in natural coral-reef habitat. As a result, they had 5–9 times higher mortality from predation than current-day controls, with mortality increasing with CO2 concentration.”

According to the National Oceanographic Center of the British Natural Environmental Research Council the concentration of carbon dioxide in the oceans is higher than it has ever been at any time in the last 800,000 years.  This is principally from burning fossil fuels and large scale deforestation (which eleminates carbon sinks).  The mean ocean pH has decreased from 8.2 in 1750 to 8.1 today. (Note that the pH scale is logarithmic. One unit pH decrease is an increase of “acidity” by ten fold.) So what is necessary to stop this trend?

An answer is provided by Dan Bernie (Met Office Hadley Centre, Exeter), Jason Lowe (Met Office Hadley Centre, University of Reading), and Toby Tyrrell and Oliver Legge (University of Southampton) in an article entitled “Influence of mitigation policy on ocean acidification,” published online by Geophysical Research Letters (August 7, 2010) (abstract; article behind pay wall). They calculate that if there is no mitigation in carbon emissions ocean pH in 2100 will be 7.67 to 7.81. If, however, emissions reach a peak in 2016 and thereafter are reduced by 5% per year, then mean ocean pH will be 8.02 in 2100. The projected change without mitigation seems disastrous. “As far as we know, such a rate of change would be without precedent for millions of years, and a concern must be whether and how quickly organisms could adapt to such a rate of change after such a long period of relative stability in ocean pH,” said Tyrrell. Even the smaller projected change assuming aggressive yearly emission reductions is substantial. Concludes the National Oceanography Center:

“A clear message from the study is that substantial emission reductions need to occur as soon as possible and that further reductions after atmospheric carbon dioxide concentration peaks will be needed if ocean pH is to be stabilized.

“’Over the longer term, out to say 2500, the minimum pH will depend on just how far the annual rate of carbon dioxide emissions can be reduced to,’ said Tyrrell.”

* You can make your own source of carbon dioxide by filling a platic bottle (such as a soda bottle) with a cup of sugar and wat to about the top. Then add baker’s yeast to the mix. The yeast will metabolize the sugar respiring carbon dioxide. You have jerry-rig a system to get that gas into water by gluing (or plastic-cementing) an air tube through a hole in the plastic cap. This system was used about in the 1990s (and perhaps earlier) as a means of dissolving carbon dioxide into water for the benefit of aquarium plants. You can probably still find a variety of schematics on the internet. With the advent of inexpensive regulators designed for aquarium hobbyists, most serious aquatic plant enthusiasts converted to compressed carbon dioxide.


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