Ocean Acidification – Emergence from Pre-Industrial Levels

Australia: The Land Where Time Began

Ocean Acidification – Emergence from Pre-Industrial Levels

The ocean acts as a giant buffering system in which exchange of heat and gas takes place at the interface between the ocean and the atmosphere, and this helps to regulate the climate of the Earth. The oceans have absorbed ⅓ of all the anthropogenic CO₂ that has been released to the atmosphere, which has significantly mitigated the impacts on climate of the increasing anthropogenic emissions. There is a cost to this service, however: Shifts in the chemistry of seawater have resulted from the absorption by the ocean of increased levels of CO₂. Acidity of the ocean has increased and the saturation state of carbonate minerals, from which the shells of marine organisms are built, has decreased. This effect, ocean acidification, has impacted the ocean in such ways as the dissolution of shells of marine organisms and the alteration of food webs, as well as other changes that have yet to be studied fully.

It has been shown by observations over the long term that there has been an ocean-wide increase in acidity and decreases in the saturation state of aragonite, a form of carbonate mineral. Regional patterns of variability may, nevertheless, differ and it is ultimately these changes on a local level that have the most effect on marine communities. Large seasonal and sub-seasonal fluctuations in the chemistry of carbonate could lead to the adaptation of ecosystems to a greater range of conditions. Therefore, the relating of variability at the present to the range of conditions that existed in pre-industrial times provides and important context for understanding the response of marine organisms to acidification of the ocean.

Adrienne Sutton et al. compiled a set of high-frequency observations from 12 different moorings across 3 different regions of the ocean: open ocean (subtropical and subarctic), coastal ocean and coral reefs, in order to constrain the sub-seasonal to interannual carbonate chemistry variability. They determined the monthly pH of surface seawater and the saturation state of aragonite of these sites since 2010, and compared them with estimates of pre-industrial conditions. They identified regions where current variability exceeds pre-industrial bounds (Biogeosciences 13, 5065-5083; 2016).

Sites in the open ocean and coral reefs experience seawater levels of pH and aragonite saturation states that are completely outside the pre-industrial envelope for most of the year. Contrasting with this, as a result of large natural variability observations at the present at the coastal sites still overlap with pre-industrial conditions, falling outside these only during the winter months. Though overlap at these sites can offer coastal ecosystems some respite thresholds that are biologically important for marine life, such as the growth and survival of shellfish, may still be exceeded. In coastal sites conditions at the present fall below dangerous aragonite saturation states more frequently than in pre-industrial times.

The gradual acidity increase, the overlap with pre-industrial range and the timing and duration of exposure to conditions that are biologically unfavourable, in combination, need to be considered to determine the overall impact on marine life. Biological impact studies on a local scale and the improvement of regional projections of ocean acidification will be informed by pinning down the sub-seasonal, annual and interannual variability of seawater carbonate chemistry. But ocean acidification is not the only threat to the oceans: some of the changes that are also putting pressure on marine ecosystems are increasing temperatures, decreasing oxygen and rising sea levels. In order to carry out a full assessment of vulnerability of marine ecosystems the challenge remains to understand how the complex interaction between multiple anthropogenic stressors will pan out.

Sources & Further reading