It is clear from the projections depicted in Video 1.4.5 that there will be dramatic changes in the chemistry and biology of the oceans in coming decades, even if conditions do not change to the extent that coral reefs and the shells of other organisms in the surface oceans actually dissolve. It is for this reason that the planetary boundary for cap omega sub arag is set at ≥80% of the preindustrial average of 3.44. At the time of writing (2025), the best estimate of this measure is around 2.8, approximately 81% of the preindustrial value and just a fraction above the boundary of 2.75. This is one planetary boundary that is on the verge of being breached, and it is only a matter of time before that happens.

Values of cap omega sub arag greater than 1 favour precipitation of aragonite, while values less than 1 favour dissolution. The following video shows historic and projected global trends in surface water aragonite saturation state. Some parts of the oceans (primarily the poles) have historically low cap omega sub arag , but most of the temperate and tropical oceans have values greater than 3 (colour blue in the video). This changes over time, with much of the oceans forecast to be below 3 by the end of the century. Although cap omega sub arag <1 favours inorganic dissolution of aragonite, values <3 make the production of aragonite by marine organisms energetically much more expensive.

So, the ocean acidification planetary boundary relates to the saturation state of aragonite in the surface waters. The aragonite saturation state refers to the concentration of dissolved carbonate ions in relation to the solubility of aragonite. It is referred to by the symbol normal cap omega sub a times r times a times g (where normal cap omega is the Greek letter capital omega). It is calculated using the formula: cap omega sub arag equals left square bracket cap c a super two postfix plus right square bracket times left square bracket cap c cap o sub three super two postfix minus right square bracket divided by cap k sub sp super prime

where left square bracket cap c a super two postfix plus right square bracket and left square bracket cap c cap o sub three super two postfix minus right square bracket are the concentration of their respective ions and cap k sub sp super prime is the ‘apparent solubility product’ – the equilibrium constant for the dissolution of the compound, in this case aragonite. The important thing to take away from this is that, other things being equal, the saturation state is dependent on the concentration of calcium and carbonate ions, which, as you learned in Study session 1.3.1, vary with changing CO2 concentration and pH. Furthermore, cap k sub sp super prime increases with temperature, so in warmer seas (as expected with climate change), if calcium and carbonate ion concentrations stay the same, cap omega sub arag would decrease. Overall, however, changes in ion concentrations are expected to be the main influence on cap omega sub arag as our climate changes.

The crystal structure of the two minerals differs. Calcite forms blocky crystals while aragonite forms needle-like crystals. Calcite is the more stable form of CaCO3 in most conditions and is by far the most abundant form in rocks. It is the major component of most limestone. However, the presence of magnesium ions in solution in seawater alongside calcium ions favours the formation of aragonite. Although many organisms can form both calcite and aragonite in their shells and exoskeletons, going against the energetically favoured form in any environment requires greater energy expenditure by the organism. Marine conditions in Earth’s oceans have favoured organisms that use aragonite predominantly over calcite in their hard structures. This is important in understanding the effects of ocean acidification because aragonite is less stable and more prone to dissolution than calcite. Over geological time and under certain conditions, aragonite can convert to (or dissolve and re-precipitate as) calcite, which is one reason why limestone rocks, made from the bodies of marine organisms, predominantly contain calcite.

The global picture of ocean hypoxia matches the patterns evident from the two examples above. Coastal zones which drain large areas of croplands and those in shallow seas are where most hypoxia are found (Figure 1.4.7).

The second part of the interview (from 7 minutes 5 seconds) describes the processes involved in causing hypoxia at this location. The questions that follow focus on this part of the interview. You may wish to make notes on this part to help you answer them.

The first part of this interview reviews what ocean hypoxia is and describes some of the effects on marine organisms. Listen to the first part (up to 7 minutes 5 seconds) to set the scene for the activity.

series of thought experiments and mathematics. It is an example of deductive reasoning

many observations. It sought to explain the variety observed in nature at small and large scales. It is an example of inductive reasoning