Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research: Everything you need to know about acidifying oceans at a glance.
This text was originally published in the collection “Switzerland and the Oceans” by the Swiss think tank Denknetz.
More acidic is not the same as acidic
With an average pH value of 8.2, seawater is typically slightly alkaline. Over the past 200 years, this value has fallen to 8.1. Since pH values are logarithmically compressed, this corresponds to a decrease of almost 30 percent. By 2100, the pH value of the oceans is expected to fall by a further 0.3 to 0.4 units, making seawater 100 to 150 percent more acidic. This does not mean that the oceans are actually acidic, because even at values around 7.7 they remain alkaline, but they are – relatively speaking – more acidic than before.
Naturally acidic
The pH value of seawater is subject to natural fluctuations. The pH value can change depending on the season and region. At so-called champagne sites, for example, large amounts of carbon dioxide escape from natural volcanic sources. These marine regions therefore serve as a window into the future. This is because they show which marine organisms can adapt to a low pH value – and which cannot.
The colder, the more acidic
Carbon dioxide dissolves particularly well in cold water. This is why ocean acidification is progressing particularly rapidly in the polar regions. By the middle of this century, the acidification of the Arctic Ocean could lead to a shortage of aragonite, an important building material for calcareous shells.
In bad company
One evil rarely comes alone. In addition to ocean acidification, rising water temperatures and falling oxygen levels are also forcing marine life to adapt to new living conditions. A deadly trio. When these three factors combine, ocean creatures are particularly vulnerable. In addition, the marine habitat is often polluted and overfished.
Everyone reacts in their own way
Not all marine life is equally sensitive to the falling pH value of seawater. For example, while calcareous organisms reach their limits even at low carbon dioxide concentrations, the more acidic water has little effect on others. In some cases, animals and plants also differ within a single species, which is why scientists suspect that some parent generations have already succeeded in equipping their offspring for the challenges of ocean acidification – a so-called epigenetic effect.
Danger in early life stages
Ocean acidification poses a particular threat to marine animals in their early life stages, such as eggs or larvae. Some larvae, for example, do not grow and develop as well in more acidic water. Unlike their adult counterparts, they have not yet developed all the internal mechanisms needed to successfully protect themselves against external influences.
Sensitive calcareous shells
If the water becomes more acidic, this is particularly bad news for all marine organisms that build calcareous shells, such as mussels and winged snails. From then on, they will have to expend more energy to build and maintain their calcareous shells. One possible consequence is that their shells will become thinner or possibly dissolve, offering less protection against predators.
Too light for deep transport
If the shell walls of calcareous phytoplankton species become thinner and smaller in more acidic water, this can affect the entire marine carbon reservoir. This is because thinner shells are also lighter and therefore lose weight. However, this additional ballast has so far caused even the shells of the smallest organisms to sink to the depths – and with them the carbon in their shells. This has allowed carbon to be stored at the bottom of the sea for thousands of years. Ocean acidification could therefore lead to significantly less carbon being transported to the depths.
Corals at risk
Even today, the most species-rich ecosystems in the oceans, the coral reefs, are suffering from excessively warm and acidic living conditions in some regions. By the end of this century, only 30 percent of all corals may have sufficient building materials for their skeletons.
This also has consequences for us humans: 400 million people owe their food and protection from storm waves to coral reefs that are still intact.
Energy deficit
Marine life is in very close contact with the water in which it lives. If the pH value of seawater drops, the pH value in the body fluids of most living organisms also drops, which can lead to an acid imbalance. More highly developed organisms such as fish can regulate their acid balance within hours or days. However, this costs energy – energy that may be lacking elsewhere, for example in growth and reproduction.
When acidification gets on your nerves
Fish are relatively insensitive to ocean acidification. Nevertheless, it can affect their senses and thus influence their behavior. In laboratory experiments, clownfish swam toward their predators instead of fleeing. Scientists also suspect that acidification impairs the vision of fish. Their ear bones, on the other hand, grow well in more acidic water – this could strengthen their hearing and orientation, or confuse them, as they may overestimate the distance of signals.
Boosted photosynthesis
Not all marine life is sensitive to falling pH levels. Seagrasses, macroalgae, and phytoplankton, which do not form calcareous shells, actually benefit. On the one hand, they live mainly in coastal regions that are naturally subject to pH fluctuations. On the other hand, the additional carbon dioxide aids their photosynthesis. Seagrasses can even positively influence the chemistry of surrounding waters through their primary production.
Learning from the past
The ocean has repeatedly become more acidic in the past – often with serious consequences, especially for calcareous organisms. During the last ocean acidification event 56 million years ago, many of the coral species that existed at the time disappeared from the oceans forever. Scientists can learn a lot from these past geological eras about how marine life has reacted to more acidic water. However, the pH value is falling ten times faster today than in the past.
Expensive consequences
The consequences of ocean acidification for corals and mussels alone will cost 1,000 billion US dollars. Scientists have calculated this using forecasts.
Only one way out
There is only one effective way to combat ocean acidification: we humans must reduce our carbon dioxide emissions. But even if we could stop all emissions overnight, it would take thousands of years for the ocean to fully recover.