Ocean acidification

The evil twin of climate warming

Ten years before the professional world had even found a name for the phenomenon, scientists at the Alfred Wegener Institute were already exploring the causes for ocean acidification. They recognised early on that the growing content of carbon dioxide in the air would also change the chemistry of the oceans. They now wish to find out how the acidic seawater affects its inhabitants and ultimately also humans.

The oceans are the blue lungs of our planet.  Every year they absorb more than 25 per cent of the carbon dioxide we release into the air. Since the start of the industrial revolution, the oceans have absorbed around one half of this greenhouse gas which we humans have blown into the atmosphere through exhaust pipes and chimney stacks. Without this natural store, the carbon dioxide concentration in the air would today be far higher and it would be a great deal warmer on the Earth.

But even the expansive oceans cannot absorb unlimited quantities of carbon without this having consequences. In the same way as all gases, carbon dioxide dissolves in water and unlike most gases it also reacts with it to produce carbonic acid. The more carbon dioxide penetrates into the oceans, the more carbonic acid is created. This process impacts the chemistry of the seawater and reduces its pH value.

Declining pH value

The pH value specifies whether a liquid is acidic, basic or alkaline. With an average pH value of 8.2, seawater is typically slightly basic. However, this value has declined to 8.1 over the past two hundred years. “This may not sound like a lot, but the pH values are logarithmic, i.e. mathematically compressed. This means that if the pH value drops by 0.1 units, the seawater becomes 30 per cent more acidic," explains Professor Jelle Bijma, biogeochemist at the Alfred Wegener Institute. By 2100, the pH value of the oceans is expected to drop by a further 0.3 to 0.4 units so that the seawater would become 100 to 150 per cent more acidic. This does not mean that the oceans would actually turn into acid. With values around 7.7, they would still remain basic but from a relative point of view they are more acidic than they were before. This is why reference is made to ocean acidification.

The extent to which the constantly increasing carbon dioxide output chemically influences the oceans can be predicted with great certainty by scientists such as Jelle Bijma. “This is a simple chemical balance which can be calculated with the assistance of global ocean models”. Researchers know far less about how the ocean acidification will impact the sea organisms and the “ocean ecosystem”.

Effects on calcifying sea inhabitants

The researchers assume that above all organisms with lime shells and skeletons such as corals and mussels will suffer from increasing ocean acidification. “Carbonic acid reduces the content of carbonate ions amongst other things, which is one of the components of lime. But the lower pH value is also suspected of impeding the calcification of mussels and other organisms. They then need to expend more energy on building their housings," explains the biologist Dr Björn Rost from the Alfred Wegener Institute. However, as from a certain pH value not only the building material is absent, the calcification process itself becomes increasingly more difficult and the lime shells even start to dissolve. In the same way as a house where the load-bearing pillars have been taken away, the shell housing then collapses.

How ocean acidification impacts individual organisms has far reaching consequences for the entire ecosystem: starting from the food chain through to the carbon content in the atmosphere. “Calcifying algae, for example, build biomass by photosynthesis. But they also store carbon in the lime scales of their housing. When they die, they drop to the bottom of the ocean. The heavy lime material acts as ballast and permits the transportation of more carbon to great depths," says Dr Sebastian Rokitta, biologist in Björn Rost’s working group. A part of the carbon reaches the ocean bed and forms lime deposits over geological periods such as the chalk cliffs of Rügen. However, if the ocean acidification attacks the lime housing of the algae, the ballast effect is reduced and the carbon storage function of the oceans weakened.

More highly developed sea inhabitants such as fish also react to lower pH values. The acidic water influences their development primarily in the first stages of life, i.e. when the fish in the egg and as larva have not yet developed any mechanism to protect them from ocean acidification. “Australian researchers have discovered that acidic water impairs the sense of smell of young clown fish. Instead of fleeing from predators, they feel attracted to them all of a sudden," says biophysicist Dr Christian Bock from the Alfred Wegener Institute.

The carbon cycle becomes imbalanced

Ocean acidification is therefore the evil twin of climate warming in view of its far reaching impacts. In actual fact, both processes are attributable to the same problem: the destruction of the natural carbon cycle. This describes the constant exchange of carbon between the atmosphere, plants on land and the oceans. 

It is with this cycle that nature creates a balance between air, land and water. This carbon balance has now been disrupted by man burning fossil raw materials such as oil and coal and clearing forests and continuing to do so on a grand scale to this very day.

At the beginning of the nineties scientists of different disciplines at the Alfred Wegener Institute were investigating how the carbon cycle has altered by anthropogenic influence and how this affects the oceans. But it took ten years after the working group started its research for the term ocean acidification to be coined, thereby also attracting more attention to the phenomenon.


Ocean acidification in the polar regions

Since then scientific and public interest has grown - just in time, it could be said, to explore the first deep changes in the polar seas. “How ocean acidification is altering the ecosystems will first be demonstrated in the Arctic and Antarctic because carbon dioxide dissolves best in cold water," explains physiologist Dr Felix Mark from the Alfred Wegener Institute. Scientists presume that the sea in parts of the Arctic will have absorbed so much carbon in this century that the housing of calcareous organisms will slowly dissolve.

However, ocean acidification does not affect only the polar regions. Very soon tourism and fishing will be noticing the effects when coral reefs disappear and fish stock alters. This is why ocean acidification is placed “in the focus” of many scientists at the Alfred Wegener Institute. Some of their projects will be presented in this column.

(Kristina Bär)