Interview about the 5th IPCC report

"We are talking about a deadly trio"

For the first time the IPCC has given the ocean the importance it deserves. Two entire chapters were devoted to the largest ecosystem on our planet in the second part of the Fifth Assessment Report of the IPCC. This part summarises the observed and expected impacts of climate change and was adopted in Yokohama, Japan, in March 2014. Prof. Hans-Otto Pörtner from the Alfred Wegener Institute was responsible for one of these chapters. In this interview the biologist explains how important the oceans are for our climate system, why the climate debate is focusing on them only now, how they change in the course of warming and what adaptation options are still open to their dwellers and humankind.

Professor Pörtner, how important are the oceans for the climate of the Earth?

The oceans are indispensable for the heat balance of the globe and regulation of the climate system. They absorb over 90 percent of the heat that currently builds up on Earth. This way they balance the temperatures in the atmosphere and stabilise the climate. They transport the heat via the oceanic currents and thus influence climate processes on the continents.

Why does the IPCC only now devote its attention to the oceans?

In the past the Assessment Report placed its focus more on changes in terrestrial areas and their impacts on people. The oceans were not ignored, but were not taken into account to an extent appropriate to their significance. This has now changed in the current report on climate change. Two new articles were included that take a special look at the oceans and oceanic regions. Carbon dioxide is one of the main driving forces of climate change, also in the oceans.

The ocean - A changing ecosystem

Warming, acidification, lack of oxygen - a deadly trio

What impact does the greenhouse gas have on life in the ocean?

Carbon dioxide impacts on life in the ocean through three processes. First of all, there is a trend towards warming in most marine ecosystems, particularly in the upper water layer. Because the uppermost layer heats up, it expands and attains a different density. This difference in density leads to increasing stratification of the oceans. That causes a lower exchange of gases and nutrients between the water layers.

How does that impact ocean dwellers?

Organic material accumulates at the boundary between these two water layers. Predominantly microbes decompose this material. They consume oxygen in the process and produce carbon dioxide. This results in an oxygen deficiency in this boundary layer. At medium sea depths these oxygen deficiency zones have always been a natural part of the oceans at low latitudes, but they expand as warming increases. In addition, there are a growing number of oxygen deficiency zones close to the coasts. Human-induced warming and nutrient deposition are the triggering factors here.

And what is the third process that carbon dioxide initiates?

A third and important aspect is the direct input of carbon dioxide into surface waters. There the gas reacts with seawater and forms carbonic acid. Consequently the seawater acidifies, while its pH value declines.

Which of these three factors is most noticeable at the moment?

The strongest of the three factors at the moment is certainly warming. It leads to significant changes in ecosystems. According to forecasts, acidification of the oceans will increase more and more rapidly in the coming decades and finally have considerable impacts at the global level and in specific ecosystems. Growing oxygen deficiency will initially impact coastal areas. Expansion of oxygen deficiency zones on the open sea remains minimal for the time being, thus we will not feel the effects of that as much as of the other processes.

Can ocean warming, oxygen deficiency and acidification also mutually reinforce one another?

When these three factors interact, ocean dwellers – especially animals – react extremely sensitively to climate change. With regard to the combination of these three factors, we can also talk about them as a deadly trio that may have drastic impacts on organisms, depending on the extent of the changes.

IPCC videos

Climate Change 2014: Impacts, Adaptation, and Vulnerability

IPCC AR5 Synthesis Report

Fact Sheets about the changes in the ocean

Adapt or become extinct


Climate changes in the past led to mass extinctions in the ocean. Can we compare current climate change with these events in Earth’s history?

The current climate change may not appear to be especially rapid to us on a human time scale. In comparison to periods in the Earth’s history in which the climate also underwent change, however, it is very rapid. This applies to the last 65 million years and probably even to the last 300 million years. To this extent current climate change is at least as fast as the climate events that, in the end, led to the occurrence of mass extinction in the Earth’s history. The deadly trio mentioned above was very probably involved in these events.

That sounds like we face another occurrence of mass extinction.

Together with some of my colleagues, I believe we can definitely compare the current events to the beginning of a mass extinction. Climate change is certainly not the only cause. Other human influences also play a role here: diminution of habitats and displacement of species from their habitat. For ecosystems in the ocean, however, there are still no clear findings indicating that a species has become extinct due to climate change.

Creatures that do not want to die out have to adapt. How do ocean dwellers adapt to rising water temperatures?

Animals and plants in the ocean have various ways of reacting to rising water temperatures. The easiest way is to follow the temperature to which the organisms have adapted. We currently observe in large sections of the ocean that species migrate to other areas and a shift occurs in a range of organisms. There is a trend that habitats are shifting predominantly poleward. The best-known example is perhaps the Atlantic cod, which is vanishing from the North Sea and spreading in the north. However, there is also a trend that organisms submerge to greater water depths where they find their preferred water temperature.

On sufficiently long time scales species can also adapt in an evolutionary manner, for example through change in their genetic make-up. In this way they can change major processes in their cells and are able to survive in spite of increased body temperatures. The ability to adjust to changes in temperature is limited, however, and the adaptability of species varies. Moreover, the pace of climate change is so high that the time for such adaptation processes may not suffice for many species.

Traces of the past - Climate change in Earth's history

The danger of acidifying waters


The second great challenge faced by marine dwellers is ocean acidification. What effect does this have on organisms?

In the process of ocean acidification carbon dioxide accumulates in seawater and enters the bodies of ocean dwellers. There it lowers the pH value of the bodily fluids and causes disorders in metabolism as well as, in the case of mussels, snails, sea urchins and corals, for example, disorders in the formation of calcareous shells. Among shell-forming organisms the so-called calcification decreases and thus also their ability to create powerful shells as supporting elements or as protection against predators. Corals in particular are important in this context as they build large-scale ecosystems. They are the engineers of the seas that create habitats for other species. These habitats are currently threatened by warming, acidification as well as by local pollution and invasive species.

Do some species also profit from ocean acidification?

There are species that benefit from ocean acidification. They primarily include species that take up carbon dioxide, carry out photosynthesis and, like macroalgae and seagrasses, do not build calcareous shells. If an algae species forms calcareous structures at the same time, its capabilities are impaired.

In the climate debate the Arctic is often compared to the “canary in the coal mine”. Does this metaphor also apply to ocean acidification?

We can assume that the concentration of atmospheric carbon dioxide is roughly balanced out worldwide. However, we have to take into account that the solubility of carbon dioxide varies according to water temperature. The gas not only dissolves extremely well at low water temperatures, but also in places where the seawater is diluted, or freshened so to speak, due to melting ice or increased precipitation. These two findings indicate that the Arctic is particularly hard hit because the two processes come together there. The Antarctic seas, too, can be described as significantly affected due to low temperatures.

You already said that different species react differently to the effects of climate change. Can you say anything about how the changes will impact ecosystems overall?

The varying geographic shift of species results in increasing mixing of ecosystems. At higher latitudes, for instance, species diversity increases. At the same time there is a displacement of the previously native species because they react sensitively to climate change and because many new species migrate into their habitat and compete with them. If, in addition to that, the oxygen-deficient zones spread, the areas in which bigger animals, including fish, can live, become smaller. This leads to a greatly reduced community of species and a dominance of unicellular organisms and bacteria.

Atlantic and polar cod under stress – Ocean acidification in the Arctic

Consequences for us humans

What do these changes mean for us humans?

The impacts of the three factors – ocean warming, oxygen deficiency and acidification – on the ecosystems of the seas are absolutely crucial for human economic activities, from fishing all the way to tourism. For instance, we expect that fishery focal points will shift from the moderate and low latitudes to high latitudes. At the same time there is a risk that the productivity of fishing will decline, particularly at lower latitudes. Generally, forecasts indicate that warming will result in a reduction in the size of fish and thus in the available biomass of fish.

Are these consequences inevitable?

We have to keep in mind that we live in a world that we have already changed and will continue to change. We will not be able to restore the climate to its preindustrial level during our lifetime. To achieve that, one would have to reduce the CO2 concentration in the atmosphere in a targeted fashion. The natural processes we have set into motion will have an impact on time scales of up to tens of thousands of years. That means we humans finally have to adapt. However, we can attempt, in particular, to curb further changes so as to minimise future consequences.

By means of what measures could this be achieved?

If you look at the protection of ecosystems, we could place extensive marine areas under protection. It is important to keep in mind, however, that we are only buying time with these measures. This means we will not be able to prevent the changes in the end. By means of far-reaching protection, we would make ecosystems and perhaps individual species as well more resistant to climate change. Then the protected marine areas would ideally have to be set up in such a way that they could be shifted with the protected species that migrate elsewhere. Stationary protected areas, as we know them today, are rather limited in their protective effects. However, it is of great importance and top priority to reduce emissions as efficiently and rapidly as possible in order to minimise the scope of climate change and its impacts.

The interview was conducted by Kristina Bär.