Ocean Acidification

Since the beginning of industrialization, the Earth's oceans have absorbed just under half of the carbon dioxide (CO2) stemming from the burning of fossil fuels. While CO2 generally behaves in a chemically neutral fashion in the atmosphere, i.e., does not react with other gases, it is chemically active in the oceans. Dissolved CO2 contributes towards a reduction in the pH. The oceans become more acidic! This effect can already be measured: The pH of the oceanic surface waters has been reduced from 8.2 by an average of 0.1 units.

If the global carbon dioxide emissions (CO2) continue to rise at the current rate, Kiel scientists estimate that the pH of the oceans will be reduced by a further 0.4 units by the year 2100. The oceanic pH would then probably be lower than at any point over the past 20 million years and exhibit a rate of change of approx. 100 times greater than ever before during this period..

Ocean water chemistry also changes as a result of ocean acidification. In addition to the continuous reduction in pH and increasing CO2 concentrations, these changes are, above all, characterized by a reduction in carbonate ion concentrations and therefore in the carbonate saturation point. Carbonate is the building block used by all marine organisms that produce calcium carbonate. Corals, shell fish/bivalves and snails/gastropods, but also planktonic producers of calcium carbonate, such as calcareous algae, rotalids and pelican's foot require this material in order to build their calcareous skeletons . All calcium carbonate producers tested to date reacted to reductions in carbonate saturation with reduced calcium carbonate production, even resulting in malformations to their calcium carbonate skeletons.

Calcium carbonate is present in marine organisms mainly in the form of aragonite and calcite. Organisms like corals, for example, will suffer particularly from acidification as they produce aragonite and this form of calcium carbonate is highly soluble and therefore susceptible to changes in pH. For example, by 2070, the increasing flattening off of the depth profile for carbonate saturation will result in half of the existing cold-water coral reefs recorded to date being exposed to conditions under which aragonite, the main building block for their calcium carbonate skeletons, dissolves. However, corals only account for a minor portion of global marine calcium carbonate production. Around three quarters is produced by planktonic organisms. Unicellular algae, the coccolithophorids, export enormous quantities of calcium carbonate into the deep sea, resulting in massive calcium carbonate accumulations on the sea floor. The White Cliffs of Dover or the chalk cliffs on Rügen are evidence of such depositions in past times. Experiments have demonstrated that the frail calcium carbonate plates in coccolithophorids become progressively thinner with increasing CO2 concentrations, finally resulting in drastic deformations in their calcium carbonate structure.

Ocean acidification will also change the geographical distribution of important groups of marine organisms. An example: Unabated CO2 emissions will result in decreased carbonate saturation in the northern and southern polar regions by the end of the century. This will render these areas uninhabitable for calcareous organisms such as, e.g., the pelican's foot that produces aragonite. This means that the base of the marine food chain is being damaged.

In parallel with ocean acidification, temperature increases due to climatic change are also to be expected. These two effects are not independent: increases in CO2, for example, can reduce animal tolerance towards temperature. Coral ecosystems, in particular, are an example for this coupled effect of simultaneous decreased carbonate saturation and increasing water temperatures.


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