The Great Carbon Dioxide Sink

In 2005 The Royal Society published a report titled “Ocean acidification due to increasing atmospheric carbon dioxide”. This bleak analysis described how increasing levels of carbon dioxide in the atmosphere are altering the makeup of the oceans. Driving this change is the process of solubility – the mechanism by which one substance dissolves in another.

The solubility of solids is well known: a spoonful of sugar will dissolve in a cup of hot tea, so the sugar is said to be soluble in the solvent, in this case, the tea. But gases too can dissolve, and the solubility of carbon dioxide in the oceans is a case in point. So what determines how much carbon dioxide dissolves in the ocean?

The main factor influencing the solubility of carbon dioxide in seawater is the partial pressure of the gas. This relationship was first observed by the nineteenth century chemist William Henry, and is set out as Henry’s Law. In essence, this says that the more gas that is present, the more that will dissolve in a solvent or liquid. Thus as the concentration of carbon dioxide in the atmosphere rises, more will dissolve in the oceans.

Henry’s Law doesn’t expressly reference temperature: it is assumed to be a constant, but in practice temperature has a significant effect on solubility. In the case of solids, such as the sugar going in to the tea, as the solvent (the tea) gets warmer, the solubility of the sugar increases, meaning that more can dissolve. However, with gases the opposite is true: seawater can hold less carbon dioxide as it gets warmer.

This has significant implications for the ability of the oceans to mop up the carbon dioxide emitted by human activities. As discussed in the report of The Royal Society, it is estimated that the oceans have absorbed almost half of the total carbon dioxide emitted in the last two hundred years.  However, as the seas become warmer, their ability to absorb carbon dioxide will decline.

The other factor affecting how much carbon dioxide can dissolve in the oceans is the volume of seawater available. Now the oceans are vast and deep, so they would seem to have almost infinite capacity, but this is not the case. The problem is that water in the oceans is stratified. That is to say, it forms layers that mix very little. Obviously, only the surface is in contact with the air above, so that is where carbon dioxide dissolves, up to the limit defined by Henry’s Law.

This upper layer – 50 to 100 meters in depth – gets churned up by waves and currents, so the carbon dioxide dissolves throughout this volume, but there is very little mixing with the much greater depths. This means carbon dioxide concentrations are highest nearer to the surface. Again referring to the Royal Society report, evidence suggests that as the oceans get warmer, stratification will increase, so in the future less mixing will take place. This means a smaller volume of water will be available as a solvent for carbon dioxide, further reducing the quantity that can be taken up.

The oceans have been sucking up the carbon dioxide produced by man for the last two hundred years, and so helping to keep the planet habitable. However, as the waters get warmer less carbon dioxide will dissolve, with the result that atmospheric concentrations will rise even faster than at present.