Ocean acidification was frequently called the evil twin of climate warming. Have the current findings made it less frightening?
Ocean acidification is more the evil little brother of climate warming. This is based on our investigations, which indicate that the rising temperature has severe consequences. Ocean acidification is thus an additional effect. It aggravates the impacts of warming in most cases, but not always.
However, acidic water may impact animals in a completely different way from warmer water. It may, for instance, alter the behaviour of an organism.
An important question at the end of the previous CBD report on ocean acidification was the determination of so-called tipping points – has research been able to define these up to now?
Tipping points, or sensitivity levels, as we also call them, are incredibly difficult to formulate because they are extremely species-dependent and diverge between the individual life stages. There is a certain limit, for example around 1,500 ppm, where a reversal of certain effects takes place. There is a phenomenon in this connection known from fish farming: when the pH value drops a little, fish grow significantly better.
We have also ascertained this in laboratory experiments with the Atlantic cod. In fish that we have kept at a carbon dioxide concentration of around 1,000 ppm, i.e. at double to triple the figure compared to today, the declining pH value has a stimulating effect on their growth. This effect reverses if we work with concentrations around 2,000 ppm. The animals then suddenly stop growing. That means the threshold is somewhere between these concentrations. A limit is reached here, as of which the organism can no longer adapt, and it is presumably somewhere between a pH value of 7.8 and 7.6. For egg and larval stages of the same species, however, these limits may be reached much earlier. And there are plenty of other exceptions. Some sea urchins, for example, are unbelievably sensitive and their tipping point is much lower.
And finally there is the question of where the tipping point of an ecosystem is? Up to what point can an ecosystem adapt to a situation where some species disappear and others spread? And as of what point is there a long-term detrimental effect on an ecosystem?
Without being able to indicate a certain figure – do you think the ocean will get to a point where its ecosystem is fundamentally altered?
According to what we are observing at the moment, it looks like we are heading for far-reaching changes. The question is actually, therefore, no longer whether, but when we will reach this threshold. The time factor is incredibly important here since the more time organisms have to adapt, the less will presumably happen in the end. Gradual shifts in the ecosystem are easier to cope with than abrupt changes.
However, we are also aware that some marine regions will change to a greater extent than others. In ecosystems in which key species react sensitively, the consequences will naturally be dramatic. The polar cod, for instance, is a key species for the Arctic ecosystem and our results indicate that it reacts relatively sensitively to the new living conditions. Thus, the structure of the ecosystem in the Arctic may shift substantially.
Kristina Bär conducted the interview.