Why are oceanic dead zones expanding

Imagine walking by the sea and finding a dead fish, a dented plastic bottle, some seaweed and seaweed on a small dune that has formed in the wind. With which of these nowadays not exactly unusual finds on a walk on the beach can you definitely rule out that it has anything to do with the current climate changes?

Human and animal unrest and disruptive factors in the marine ecosystem.
Photos: by NOAA, Michelle Johnston (left) and Louise Giuseffi (right)

Right, all of these things might, at least for the foreseeable future, be related to climate change in some way ... or they might not. The very fact that flotsam would not only be found in the immediate vicinity of the water, but in a higher area, could perhaps be due to the rising sea level due to climate change in 10 or 20 years. As the oceans warm, they expand and absorb meltwater that comes from thawing ice sheets. According to Hans-Otto Pörtner, who on June 7, 2019 at the Bremen State Representation, gave the prelude to this year's conference on World Oceans Day 2019, which also this time saw Bread for the World, Fair Oceans and the Environment Forum Development in cooperation with the German Society for the United Nations had invited a considerable capacity of potential melt water, which is enough to raise the sea water level across the board by 66 meters. A dead fish could be the first vanguard of a multitude of other dead marine comrades, which the sea washes ashore due to the increasing lack of oxygen in the coastal region. As the oceans, which absorb more than 90 percent of the heat the globe absorbs, warm up, their dissolving capacity for oxygen decreases. They outgass the life-giving substance and develop a stronger stratification in the process. This development is increasingly restricting the habitat for marine animals, with extremely drastic consequences for those who are dependent on "living resources".

Seaweed and seaweed could already come from helpless mitigation attempts or controversial geo or climate engineering measures in which macroalgae plantings or sea grass meadows were planted on the sea floor in order to bind "blue carbon" or CO₂ on the sea floor and thereby permanently affect the climate revoke. Of course, the dying marine plants that were artificially imposed on this region have themselves caused further eutrophication and an unacceptable excess of nutrients for the system, which temporarily led to increased productivity, i.e. a greater abundance of living beings, and ultimately not due to the demand more covering oxygen concentration to massive deaths.

Perhaps the plastic bottle still has the least share in the warming of the sea and its inhabitants, which are already plagued by overfishing and littering. It warns of the currently much discussed problems of marine animals that are strangled, poisoned or internally injured and perish miserably from these human legacies. However, one does not see from the rudiment of civilization that it is one made from the "good" biodegradable plastic material that, when broken down into its basic components by sunlight, releases numerous volatile climate-relevant substances, such as the greenhouse gas that is 28 times more potent than CO₂ methane. But also classic, more durable plastic debris could disintegrate more quickly in a chemically acidic sea and also release climate-relevant substances.

This small example is only given in advance to show how diverse, unpredictable and dramatic even inconspicuous changes, which in any case lead to ever more intensive use and pollution of the world's oceans by humans, ultimately also to the effects of climate change on the marine ecosystem can be involved. From the global movement of goods, 90 percent of which is handled by shipping, to tourism, to fishing and the use of other marine resources, all human activities contribute to climate change because they consume a large amount of energy and are associated with CO₂ emissions . Acidification as a result of the direct dissolution of CO₂ in surface water is just as threatening for the oceans as an increase in temperature.

The rise in temperature in particular has already led to shifts in the occurrence of species and thus to a change in marine ecosystems. Plankton, jellyfish, turtles and sea birds have already migrated ten degrees of latitude towards the cooler poles, says marine biology. A decline in fish stocks is to be expected in tropical regions. If greenhouse gas emissions remain unchanged, catch yields in Southeast Asia by 2050 could be 10 to 30 percent below the average for 1970 to 2000.

Both the stocks themselves and their distribution - both commercially important and non-commercially used fish species - will change in an unpredictable manner. Stocks that are already overfished could react more sensitively and present future fisheries management with even greater difficulties than before. Temporary cessation of fisheries for certain target species may be necessary.

It is already foreseeable today that the growth of corals and other calcifying organisms will be impaired because of the acidification and because of the rise in temperature. A complex interplay of human influences - including the rise in carbon dioxide in the atmosphere - threatens coral reefs. In addition, there are indications that more plants and animals become sick in the warmer water. Pathogens that are dangerous to humans could also spread faster as a result.

Marine biologist Prof. Hans-Otto Pörtner from the Alfred-Wegener Institute, Helmholtz Center for Polar and Marine Research in less than 20 minutes. The scientist, who is also co-coordinator of BIOACID and co-chair of Working Group II of the International Panel on Climate Change (IPCC), has been known for many years for his advocacy of immediate action, which is the goals set for reducing emissions concerns.

On the occasion of the publication of the IPCC's special report on global warming of 1.5 degrees, Pörtner and the other authors called for a radical rethink in climate policy, especially in the energy sector, transport and agriculture, in order to overcome the climate crisis.

In the course of the event he was ready to shed light on the complex climate problem, especially for the oceans, with the shadow view.

"For some ecosystems, even the ambitious emission reduction measures will come too late to achieve the 1.5 degree target" (Hans-Otto Pörtner)
Photo: © 2019 by Schattenblick

Schattenblick (SB): You have impressively summarized the various effects of climate problems and their interactions with ocean acidification, lack of oxygen and the living beings in the sea today. The increasing stratification of the water masses seems to have a key function, which will increase further in the future as a result of the warming. If I understand correctly, the oxygen-rich layers on the water surface will mix less and less with the cold, deep water layers. Could you explain to us again how this stratification affects other ecosystems or other physical conditions in the sea?

Hans-Otto Pörtner (HOP): This stratification means first of all that there is a greater expansion of the thermocline, i.e. the barrier between the warm, oxygen-rich surface water and cold deep water in the oceans, whereby the oxygen deficiency phenomena increase in the depths. [1] More precisely, the layer expands longitudinally, i.e. in the direction of the higher latitudes, and hinders the mixing of the cold water body with oxygen-rich surface water in a much larger area. This thermocline stabilizes with the increasing warming and the greater temperature difference, so that finally hardly any more oxygen-rich water gets down from the surface, for example due to the wave movements, where no oxygen can be produced by photosynthesis due to the lack of light. When organic material, which is consumed by microorganisms while consuming oxygen, sinks and these biochemical processes generally run faster at higher temperatures, the oxygen deficiency in the deeper layers becomes more pronounced than before.

These two things come together. Then there is also the phenomenon that the nutrients that are bound in the sinking organic material are not recycled as quickly and are no longer available as such. Ocean productivity therefore decreases noticeably in these areas. We reckon, and the projections also show it, that this will shift towards higher latitudes. This can have different regional effects. But the net effect will be a decrease in ocean productivity as it extends south.

The areas in the ocean where there is no oxygen at all are increasing.
Graphics: 2012 by Fährtenleser CC BY-SA 4.0 via Wikimedia Commons

SB: That means, if areas in which there is less productivity expand, is this phenomenon also accompanied by growing oxygen deficiency zones?

HOP: That is partly true. The North Pacific already has severe oxygen deficiency phenomena, some of which are quite natural because all large-scale ocean currents end in the Pacific. But the combination of warming and lack of oxygen also affects the fish fauna. So recently it was shown in a modeling that the habitats of the fish are getting smaller and smaller. In the geographical distribution of the species in the oxygen-poor areas, we expect a further, quite noticeable reduction in species diversity. All in all, the mentioned phenomena of warming, stratification, acidification, lack of oxygen and their consequences limit the productivity of the oceans. You can say that quite clearly.

SB: Does this stratification, which initially only seems to be a physical density phenomenon, also have an effect on the ocean currents, which in my opinion are also subject to different temperatures, density gradients and similar physical variables?

HOP: To what extent the Gulf Stream is affected by global warming is actually discussed again and again among scientists. For example, it is believed that this will weaken the drive in the North Atlantic and that the Gulf Stream has already decreased in the course of climate change. The initial predictions even said that this ocean current could collapse and that Europe would become cooler as a result. But so far there is no evidence of these extreme changes. To expect a weakening of the flow of the Gulf Stream would, in my opinion, be quite realistic with the ongoing changes in climate change.

SB: And in what period of time do you now expect this warming-related stratification? Assuming the scenario of current greenhouse gas emissions and "business as usual", then developments so far have led to an increase in the global average atmospheric temperature of one degree and could rise to 2.5 to 5 degrees in a given time. At what point in development does this affect the sea so much that these stratification effects become evident?

HOP: These oxygen minimum layers are a natural phenomenon. It has always existed since time immemorial and the reinforcement effects have already been demonstrated more frequently in the last few decades. More precisely, it has been known for a long time that this hypoxia increased in the center of the layers, that the oxygen deficiency zones expanded towards the surface of the water or to greater depths, that the layers spread to higher or lower latitudes and that the oxygen deficiency zones also increased in number.

In some cases - due to shifting ocean currents - for example on the coast of North America, there was an oxygen deficiency phenomenon in the immediate vicinity of the coast, which was accompanied by massive mortalities of the species resident there. It is well known that these upwelling areas, which are found on the western edges of the continents, bring oxygen-poor deep water to the surface. But at the same time these upwelling areas also ensure that the surface water is mixed with the nutrients from the depths, which is essential for productivity. It's a delicate balance. As soon as a trend prevails, that is, if, for example, the lack of oxygen becomes too great, the increased nutrient intake does not help the animals. They die because they don't have enough oxygen to breathe. And when there are El Niño years - and this is particularly prominent on the coast of South America - when the upwelling breaks off briefly, warm surface water sloshes onto the coasts with massively reduced productivity. Even then, there is a collapse in biodiversity, which is sustained by buoyancy. Ultimately, we should always keep in mind that we are dealing with an extremely delicate balancing act in the sea, which from a global perspective with climate change and many other disruptive influences can be completely unbalanced or even crash.

SB: You mentioned that climate change goes hand in hand with an increase in species loss, not only on land, but also in the sea. Could you be more specific? I mean, of the estimated 10 million species that are supposed to populate the oceans, not all of them are really known. Do you also count species among the losses if you have not yet been able to discover and assess them individually?

HOP: I have to correct you at this point, because I only spoke of the possibility of triggering or at least intensifying the sixth mass extinction in the history of the earth with climate change. This statement applies equally on land and in the oceans. This not only causes the influence of climate change, but also other human influences such as overfishing, the interruption of food chains, littering and environmental pollution and others.

If one only thinks of the prognoses with the shifting biogeography, then this is accompanied by a stronger mix of species in the ecosystems, which brings great unrest in these communities. Since each species reacts a little differently to changes and the ecosystems are not completely relocated, the relocations suddenly bring together species that have never had anything to do with one another before. New food relationships or competitive relationships can emerge and this unrest is generally understood as a possible cause of decreasing biodiversity, which explains the prognosis. This can be clearly seen when there is a lack of oxygen. This immediately leads to a decline in biodiversity, because when there is little oxygen, only specialists who are adapted to it can survive until the conditions become so extreme that they too can no longer exist. So we are dealing with a complex structure.

In fact, neither on land nor in the oceans can the extinction of a species be linked to climate change alone. There have been population shifts, relocations and population decreases, but an extinction of a species due to climate change has not yet been proven. With one exception, where the aim is to attribute the extinction of a small marsupial on the coast of Australia to climate change.

Otherwise one cannot argue with a single cause. You have to take the entire complexity of all negative human influences together and this results in a very great threat to biodiversity. If species have become extinct in human history, then these events have always been a direct result of human influences.

In addition to pollution and overfishing, climate change is another problem that can kill a coral population.
Graphic: 2018 by NOAA, How does climate change affect coral reefs?

SB: Among other things, you mentioned in the lecture that 50 percent of the coral population in the Great Barrier Reef has already died due to the current changes. Isn't that enough evidence that this is clearly a consequence of climate change?

HOP: For this I had shown a diagram with various influences that all together cause the coral to die. On the one hand there is the parasitic starfish, the Crown of Thorn-Seastar or in German: Crown of Thorns, the intensity of the cyclones and finally the coral bleaching. We can now clearly say that coral bleaching is climate-induced or warming-induced. The intensity of the cyclones could also be attributable to it. And the intensity of this influence will increase due to climate change, as will the frequency of these extreme events. She has already gained weight.

So in these two cases you can relate to the climate with a slightly lower level of confidence. And we don't yet know about the parasitic starfish. Of course, other human activities, such as the increasing cloudiness of the water, could promote the spread of this parasitic or predatory starfish.

Could human activities favor the proliferation of the predatory starfish?
Corals have no defenses against these invaders.
Photo: 2018 by Chloé Girard as CC BY-SA 4.0 via Wikimedia Commons

The remains of a colony after being infected with "The Crown of Thorns".
Photo: 2012 by JSLUCAS75 as CC BY-SA 3.0 via Wikimedia Commons

SB: Could the increasing temperature also weaken the corals in such a way that a parasite like the crown-of-thorn starfish has an easy time of it, or maybe just microbes give the corals the fatal blow, just as the weakened immune system of humans collapses through infestation with a simple E-Coli pathogen if it is already heavily used by other infections? [2]

HOP: I don't know of any relevant work there. But that would be conceivable.

SB: With increasing climatic stress, could the multifactorial influences gradually come to a head towards a single cause for the extinction of the species?

HOP: We are of course always very careful in such cases and proceed according to the principle of "detection and attribution". This means that a phenomenon is first identified and verified with a high level of confidence. Then we ask whether we can actually attribute this process to climate change. On the one hand, this happens on the physical side and affects weather phenomena and individual extreme events. Progress has now been made here. Nevertheless, only probabilities are used as an argument.

For example, they say that an extreme summer like the one we had last year is 30 times more likely with climate change than without it. And with regard to the Japanese extreme summer of last year, it can be clearly stated that this would not have existed without climate change. In this respect, the consequences of climate change are slowly emerging from the natural variability of the phenomena.

In the oceans there are already observations that we can clearly assign to ocean acidification and we are finding further indications on the coral reefs that increasingly point towards the climate.On the one hand, there is the rather sharp temperature limit, the exceeding of which is accompanied by the bleaching; on the other hand, it is the massive spread of the bleaching phenomenon and the increasing frequency with which it occurs because the global average temperature is approaching this threshold value more and more. In short, one must be careful with the assignment of individual events and consciously check in the prognosis whether one has really understood the causes and effects in order to be able to say in which direction something is developing and to what extent a development is certain risk thresholds and similar approaches.

SB: Given these specifications, what led to your prognosis that even reaching the 1.5 degree target may - I am careful - not be enough to prevent 70 to 90 percent of the corals from dying off?

HOP: That is a prognosis that can be derived from the phenomena with a relatively high degree of confidence.

SB: In return, were calculations made as to what braking force would be needed for climate change in order to save the corals or to protect them better?

HOP: You could only influence that locally. You would first have to abolish global warming and, in principle, combat ocean acidification locally with more alkalinity. I'm afraid the coral loss is already built in, so there is nothing we can do about it. In the medium term, there are considerations to apply genetically modified varieties, i.e. to apply somewhat less temperature-sensitive algae and symbioses in the areas in question. But whether this could really work has not even been investigated so far.

SB: A study recently proposed a new GWP * (Global Warming Potential *) [3], ie a better equivalence factor than the earlier GWP without a star, which should also include short-lived greenhouse gases in climate forecasts. Could there also be unconsidered factors that influence climate change and its consequences in the sea?

HOP: A not unrelated factor could be methane hydrates, which are found in the deeper ocean. They dissolve and release methane. There is still great uncertainty about how much of it could actually get into the atmosphere or whether the methane will still be converted into CO₂ in the water column. In addition, the distribution areas of algae and seagrass meadows will shift, which could play a role. And there are certainly numerous individual phenomena that are not yet fully understood.

SB: Prof. Pörtner, thank you very much for the informative interview.