Anja Krieger, Maike Nicolai

Planetary Boundaries: Oceans are becoming more acidic

Oceans are becoming more and more acidic; their pH value is decreasing. This is because of more and more carbon dioxide entering the atmosphere, which dissolves in oceanic water, turning into carbonic acid. This phenomenon is impacting sea creatures and marine ecosystems, which in turn will have consequences for human beings. However, the planetary boundary for acidification has not yet been exceeded.

The nine planetary boundaries according to Rockström, Stockholm Resilience Center
The nine planetary boundaries according to Rockström, Stockholm Resilience Center
Julia Blenn / Helmholtz-Klima-Initiative

The oceans absorb more than 25 percent of human-emitted CO2 annually, turning it into carbonic acid. The pH value of the water falls as a result, which is more acidic than it was over the past two million years (though it is not actually ‘acidic' in the sense of a pH under 7, see further down). Oceanic surface water today is nearly 30% more acidic than when industrialization began. The Intergovernmental Panel on Climate Change has reported that such a rapid rise has not been seen for the last 26,000 years or more of Earth's history.

This development could pose a major problem for plankton, corals, clams, snails and other marine animals with calcareous skeletons and shells, for more acidic water means there are fewer carbonate ions available, which are necessary for forming their bodies and shells. The internal acid-base balance of organisms can furthermore become disturbed, requiring additional energy to regulate that can no longer be devoted to growth or other processes. If water acidification should proceed to a certain level, their very shells and skeletons could disintegrate as a result.

Among the calcifying organisms affected are certain types of phytoplankton that use sunlight to convert CO2 in the water into oxygen. Phytoplankton are also an important food source for zooplankton, which are larger plankton of an animal variety. If these organisms die, their bodies sink down in the water column together with the carbon they contain. This contributes to the oceanic surface water further absorbing CO2.

pH value as specifier of acidification

The pH value of an aqueous solution indicates whether it is basic or acidic. A pH value of 7 is neutral, i.e., the solution is neither acidic nor basic. Below a pH value of 7 is acidic; above is basic, or “alkaline”.

Prior to industrialization, seawater was slightly alkaline, with an average pH of 8.2. While still alkaline, the pH value has now declined to 8.1. That may not sound like a lot, but it is. The pH value scale is logarithmic, meaning it is mathematically compressed. Thus, the seemingly slight difference between a pH value of 8.1 and 8.2 means that oceanic surface water has become 26% more acidic due to human-induced emissions.

If carbon dioxide emissions are not reduced, the pH value could decline by another 0.3–0.4 through the year 2100, at which time the water would be 100–150% more acidic. The acidification effect is particularly strong at the poles, as cold water absorbs more CO2.

How to stay in the safe zone

The steps we can take to fight climate change will also help halt oceanic acidification.

Reducing our net emissions to zero will keep ocean pH from falling further so as to threaten marine life. A turnaround will be necessary in many areas of modern life, including electrical power generation, road and air traffic and home heating. One common thread is that fossil fuels must be abandoned in favor of renewable energy sources.

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