Acidification of the Seas

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Estimated decrease in the pH value at the sea surface due to anthropogenic carbon dioxide in the atmosphere between approx. 1700 and the 1990s
Estimated decrease in the concentration of carbonate ions (CO 3 2− ) in surface water between the 1700s and the 1990s

The decrease in the pH value of the sea ​​water is called acidification of the oceans . It is caused by the absorption of carbon dioxide (CO 2 ) from the earth's atmosphere . In addition to global warming , the process is one of the main consequences of human emissions of carbon dioxide. While carbon dioxide in the earth's atmosphere physically leads to rising temperatures on earth, it has a chemical effect in seawater by forming carbonic acid from CO 2 and water . The sea water is slightly basic . The "acidification" does not make it acidic, but less basic.

The consequences of this so-called. "Acidification" relate first kalkskelettbildende organisms whose ability exo or endoskeletons to form, with decreasing pH decreases. Because these species often form the basis of the food chains in the oceans, this can have further serious consequences for the numerous sea creatures that depend on them and, as a result, for the people who depend on these animals.

pH of the ocean

The pH value is defined for ideally diluted solutions and therefore not directly applicable to salty sea water. In order to be able to provide average values ​​for sea water, models must also be used to simulate a chemical equilibrium of the ocean. For this purpose, three different models are currently used with the resulting scales that are up to 0.12 units apart. Average values ​​can therefore only be compared within the framework of the underlying model.

The sea water is slightly alkaline with a pH value around 8. According to a summary of the British Royal Society , the surface water of the seas today typically has pH values ​​between 7.9 and 8.25 down to a depth of 50 m, with an average value of 8.08. The most important causes for the fluctuations within this range are the temperature of the water, the local upwelling of carbon dioxide-rich deep water, as well as the biological productivity, which, where it is high, binds a lot of carbon dioxide in the form of marine life and transports it into deeper water layers.

The analysis of sediments offers one possibility to reconstruct earlier pH values. From the isotopic composition of boron hydroxides it can be determined that the pH value at the sea surface was approximately 7.4 ± 0.2 approximately 21 million years ago, until it reached a value of 8.2 ± approximately 7.5 million years ago 0.2 rose. Since the pH value of the oceans is directly linked to the carbon dioxide concentration in the atmosphere via the Henry coefficient , paleo-CO 2 concentrations can also be determined in this way. Until the beginning of oceanic acidification as a result of the onset of industrialization in the 18th century and increasing carbon dioxide emissions, this value remained roughly constant.

As a result of human carbon dioxide emissions, around a quarter of which is absorbed by the world's oceans, the acidity of the oceans has increased by almost 30% since the beginning of industrialization (as of 2016). Without a reduction in current CO 2 emissions, the acidity of the world's oceans would more than double by 2100. According to the Fifth Assessment Report of the IPCC, acidification is proceeding faster than any similar acidification over the past 65 million years, possibly over the past 300 million years. According to a 2005 study by Stanford University , which assumes a pre-industrial pH value of shallow seawater averaging 8.25, the pH value decreased to the then value of 8.14 on average due to the uptake of carbon dioxide. A joint survey from the USA by the National Science Foundation (NSF), the National Oceanic and Atmospheric Administration (NOAA) and the United States Geological Survey (USGS) concludes that before industrialization, the average pH was 8, 16, down from 8.05 today. In both cases, acidification is attributed to human emissions of carbon dioxide and is quantified at 0.11 pH units.

Acidification also occurs in the vicinity of the coast or ships through acid input caused by sulfur oxides and nitrogen oxides (see acid rain ). These come mainly from the use of fossil fuels and from agriculture. Globally, these inputs hardly contribute to the acidification of the oceans.

Oceans as a carbon sink

The oceans play in the carbon cycle of the earth as a carbon sink an important role, since 70 percent of the earth's surface is covered by water. An estimated 38,000  gigatons (Gt) of carbon are stored in the entire hydrosphere . The carbon dioxide passes due to the difference in CO 2 - partial pressure in the ocean. A gas always flows from the area of ​​higher partial pressure (atmosphere) to the area of ​​lower pressure (ocean). Carbon dioxide is dissolved in the sea until the partial pressure in the atmosphere and in the sea are the same. Conversely, it also escapes again when the pressure in the atmosphere is lower than in the sea. The temperature of a sea also influences the uptake of carbon dioxide, since water can absorb less carbon dioxide as the temperature rises.

The carbon absorbed from the atmosphere is distributed in the ocean within a few years in the layer of the sea illuminated by the sun . Two mechanisms ensure that it reaches even greater depths. The most important thing is the so-called physical carbon pump : carbon- rich surface water cools down in the Arctic , becomes heavier and sinks, then the carbon-rich water is distributed over large areas in the depths of the oceans via the cold deep currents of the global conveyor belt . Less important, but not insignificant, is the so-called biological carbon pump , in which carbon sinks into deeper regions as sea ​​snow (biogenic particle rain). It takes hundreds to thousands of years for the anthropogenic CO 2 absorbed from the atmosphere to penetrate the oceans into the deepest water layers and distribute them. Today it is detectable down to an average water depth of 1000 m. At seamounts , on continental slopes and in shallow seas (for example in parts of the Weddell Sea ), the anthropogenic CO 2 can already reach the sea floor.

The increased amount of carbon dioxide in the earth's atmosphere has resulted in 118 ± 19 Gt carbon or 27% to 34% of anthropogenic CO 2 emissions being absorbed by the oceans over the past 200 years . In 2006, 36.3 Gt of additional CO 2 produced by humans or approx. 9.9 Gt of carbon were released into the atmosphere worldwide . Including natural sources, the hydrosphere currently absorbs about 92 Gt of atmospheric carbon per year. Around 90 Gt of this is released from the oceans and 2 ± 1 Gt is stored. A study published in 2003 estimated the uptake of carbon somewhat more precisely in the period 1980–1989 at 1.6 ± 0.4 Gt and between 1990 and 1999 at 2.0 ± 0.4 Gt per year.

Chemical process of acidification

Distribution diagram for the dissociated forms of carbonic acid as a function of the pH value in seawater

Carbon dioxide from the air can dissolve in seawater and is then largely in the form of various inorganic compounds, the relative proportions of which reflect the pH of the oceans. Inorganic carbon is found in the ocean to approx. 1% in carbonic acid and carbon dioxide, approx. 91% in hydrogen carbonate ions (HCO 3 - ) and approx. 8% in carbonate ions (CO 3 2− ). Carbon dioxide dissolved in water is in equilibrium with hydrogen carbonate, carbonate and oxonium ions (hydronium ions) via the following reaction equations :

The oxonium ions (H 3 O + ) produced in this process cause the falling pH value, which is defined as the negative decadic logarithm of the molar concentration (more precisely: the activity ) of oxonium ions.

The acidification caused by dissolved CO 2 counteracts the presence of calcium carbonate (CaCO 3 ), which works with hydrogen carbonate and carbonate ions as a chemical buffer system (→ buffer solution ) and thus binds protons:

Like all carbonates of the alkaline earth metals , calcium carbonate is only sparingly soluble in water. The calcium carbonate in sea water essentially comes from two sources, namely sediments on the sea floor and the input from the influx of fresh water . Carbonate gets into the latter through the weathering of calcareous rocks. In order for the sediment to help neutralize acidification, the calcium carbonate it contains must be dissolved and carried by circulation from the sea floor to higher water layers . If the weathering-related input is assumed to be constant in model calculations (with 0.145  Gt per year of carbon in the form of carbonate), the acidification of the oceans would lead to a reversal of the sediment formation rate within a few hundred years. The weather-related input of calcium carbonate could only compensate for this effect again after a period of approx. 8000 years.

Significant amounts of calcium carbonate in the sediment arise from calcite- forming plankton , especially from globigerins (a group of foraminifera ), coccolithophores (a group of calcareous algae) and pteropods . Smaller amounts are formed in coral reefs , for example . Plankton can be deposited at the bottom of the sea in the form of carbonate-rich, biogenic sediment (lime sludge) if the water depth is not too great. If, however, the calcite and aragonite compensation depths for the calcium carbonates calcite and aragonite are exceeded, then they dissolve completely. These compensation depths move upwards in the course of acidification, and so large amounts of limestone dissolve on the sea floor . For aragonite, an increase of 400 m to 2500 m today has been determined since industrialization. A further increase of 700 m is expected by 2050. 300 to 800 m above the calcite compensation depth is the lysocline , the area in which the dissolution process begins. As a result, solid carbonates, such as calcium carbonate, can also be dissolved in shallower areas until the solution is again saturated with carbonate ions. The reaction equation for the lime solution is:

Consequences for marine life and the ocean ecosystem

In marine organisms that are exposed to seawater with increased CO 2 content, a process takes place that is very similar to the dissolution of CO 2 in the ocean. CO 2 can migrate unhindered through cell membranes as a gas and thus changes the pH value of the body cells and the blood or the hemolymph . The change in the natural acid-base balance must be compensated for by the organism, which some animal species do better and others worse. A permanent shift in the acid-base parameters within an organism can impair growth or fertility and in the worst case endanger the survival of a species. In the geological past, acidification events, which were less pronounced than today's man-made acidification, repeatedly led to severe declines in biological diversity or mass extinctions .

Damage to corals

A coral island in the Pacific. The oceans, which are becoming more acidic, pose a risk for corals because they depend on the formation of calcareous shells

The solution of carbon dioxide slows down global warming, but the resulting slow acidification of the oceans can have serious consequences for animals with a protective layer of calcium carbonate (lime), among other things. As described above, the chemical equilibrium of the oceans is shifting at the expense of the carbonate ions. Their connection with calcium in seawater to form calcium carbonate is of vital importance for marine life that forms lime shells. An ocean becoming more acidic hinders the biomineralization of corals as well as microorganisms such as tiny sea snails and zooplankton , although some of these organisms specifically increase the pH of the water by reducing the dissolved amount of carbon dioxide when the lime crystals are generated in their own cells.

Along with calcite, corals produce aragonite, the most common form of lime in the sea. Aragonite is a form of lime that is particularly easy to dissolve through carbonic acid, which increases the risk to corals from the increasingly acidic oceans. In an experiment at Israel's Bar Ilan University , corals were exposed to artificially acidified water with a pH of 7.3 to 7.6. These are values ​​that some scientists consider possible in a few centuries, provided that the atmospheric CO 2 content increases about fivefold. After a month in the more acidic water, the calcareous shells began to peel off the corals and as a result disappeared completely. It was surprising to the researchers that the polyps of the corals survived. When the pH was raised to 8.0–8.3 again after 12 months, the polyps began to build up again. This result could explain why the corals were able to survive despite earlier epochs with a pH value of the sea water that was less favorable for them. Despite this finding, the researchers only speak of a possible “refuge” for the corals and emphasize the serious consequences of decalcification on the ecosystems concerned. A negative effect of acidification on growth was also demonstrated for hard corals of the genus Lophelia pertusa , which occur in the wild at depths of 60 m to 2100 m. In one experiment, the calcification rate of these cold-water corals decreased by 30% and 56%, respectively, when the pH was reduced by 0.15 and 0.3 units.

Other organisms that are important for reef formation are also likely to suffer from acidification. In a seven-week experiment, red algae from the Corallinaceae family , which play an important role in the development of coral reefs, were exposed to artificially acidified seawater. Compared to the comparison group, the algae in the more acidic water showed a sharp decrease in the rate of reproduction and growth. If the pH value in the oceans continues to fall, this will likely have significant consequences for the coral reefs affected.

Impairment of other marine life

The larva of the orange clownfish ( Amphiprion percula ) reacts to oceanic acidification with an impaired or completely interrupted sense of smell, which could make it difficult or even impossible for them to find suitable habitats.

The Intergovernmental Panel on Climate Change ( Intergovernmental Panel on Climate Change, IPCC ) are 2007 Fourth Assessment Report a scientific "medium security" for negative consequences from the acid becoming the world's oceans for calcite shells producing organisms and dependent of them species at. In a study carried out at Kyoto University , sea ​​urchins grew or lost weight significantly more slowly in artificially acidified water compared to a control group kept under normal conditions. They were less fertile and their embryos gained size and weight much more slowly. In sea urchins of the species Heliocidaris erythrogramma , which are native to the waters of South Australia, an experimental pH value reduced by 0.4 units to 7.7 led to a presumably reduced reproductive capacity, which was determined by the significantly reduced speed and mobility of the sperm. This could reduce the number of offspring by a quarter. Problems are also expected with clownfish , whose larvae could only use their olfactory sense to a limited extent in artificially acidified water at a pH value of 7.8 and no longer use them at all at a value of 7.6. This can seriously impair the post-larval juvenile fish in their search for suitable habitats and thus lead to a decreasing population.

The rate of calcification of mussels could decrease by 25% and that of the Pacific oyster by 10% by the end of the 21st century . Scientists arrived at these values ​​by following a specific scenario by the IPCC, which envisages an atmospheric CO 2 concentration of around 740  ppm by 2100 . Above a limit value of 1,800 ppm, the mussel shell even begins to dissolve, which generally endangers the biodiversity of the coast and also threatens considerable economic damage.

Turquoise color of the water off the coast of Cornwall , caused by a bloom of the calcareous alga Emiliania huxleyi . While E. huxleyi could benefit from the acidification of the oceans, the calcareous alga Gephyrocapsa oceanica , among others, is of great importance for the ocean ecosystem and is threatened by acidification.

The oceanic food chain is based on plankton . Particularly coralline algae (so-called Haptophyta ) are dependent on the formation of a calcareous shell, to survive. If this is no longer possible due to acidification, it could have far-reaching consequences for the oceans food chain. A study published in 2004 by the former Leibniz Institute for Marine Sciences points to the numerous complex effects that a lower pH value can have on plankton, including the poorer starting position for calcifying animal organisms compared to phytoplankton (floating algae). At the same time, the uncertain state of research is emphasized, which currently does not allow any far-reaching predictions about the development of entire ecosystems. A decreasing rate of calcification was found in foraminifera of the order Globigerinida in the southern ocean . The unicellular foraminifera are responsible for a quarter to half of all oceanic carbon flux. In the investigations, the weight of the calcareous shell of the foraminifera Globigerina bulloides was reduced by 30 to 35% compared to dead specimens recovered from sediments. The consequences of a further drop in pH are considered uncertain.

Studies on the influence of a lower pH value on larger marine animals have shown that, for example, the spawn and larvae can be damaged. The tests were carried out at much lower pH values ​​than can be expected in the near future, so that they are of limited informative value. In a study on puff adder cat sharks it was shown that the increasing acidification of the oceans could have a negative effect on the scale structures of sharks .

Acidification does not mean a restriction of their habitat for all marine life. First of all, the increased amount of carbon dioxide in the sea leads, among other things, to better carbon dioxide fertilization of marine plants. Since the effect has different effects on different plants and is associated with increasing water temperature and decreasing pH value, the species composition can in turn change. Surprising reactions to the decreasing alkalinity of the seas have been observed in some species. For the calcareous algae Emiliania huxleyi , a study paradoxically showed a possible doubling of its rate of calcification and photosynthesis, measured by pH values ​​as expected at an atmospheric CO 2 content of 750 ppm in the oceans. At the same time, a significantly lower growth rate is expected. E. huxleyi has a share of almost 50 percent in the ocean's biological carbon pump and makes a third of the ocean-bound calcium carbonate production, making it a key species in the ecosystem. As a result of the pH value at the sea surface, which has already fallen by 0.1 units, the average weight of these calcareous algae has increased by 40% over the past 220 years. A further investigation revealed for brittle stars of the type Amphiura filiformis increased calcification under acidic water conditions by which the brittle stars to compensate for the more adverse conditions. However, this adjustment goes hand in hand with decreasing muscle mass, a strategy that is unlikely to be sustainable in the long term.

Current and future development

Because of the different solubility depending on the temperature, the acidification of the oceans is highest in the polar regions, since cold water can dissolve more carbon dioxide than warm water (see: Temperature dependence of Henry's constant ). The pH value can also be subject to regional and seasonal fluctuations, for example due to changes in ocean currents or biogeochemical processes. These influences must be separated from the trend of individual measurement series caused by greenhouse gas emissions. In a detailed eight-year study off the US Tatoosh Island , near the Olympic Peninsula in Washington State , the local pH value fluctuated significantly more during the day and during the year than previously assumed, namely by up to one pH unit within one year and by 1.5 units in the study period 2000–2007. At the same time, the overall pH decreased significantly, with an average of −0.045 units per year, significantly faster than calculated by models. These reductions had a noticeable effect on the local biology. The California mussel , mussels and barnacles decreased in the sequence, while various barnacles and some species of algae increased.

Without the sinking effect of the oceans, the atmospheric concentration of carbon dioxide would be 55 ppm higher today, i.e. at least 466 ppm instead of the current 411 ppm. Over a period of centuries, the oceans should be able to absorb between 65 and 92% of anthropogenic CO 2 emissions. Phenomena such as an increasing Revelle factor mean, however, that with rising temperatures and a growing proportion of atmospheric CO 2, the ocean's ability to absorb carbon decreases. By 2100, the capacity of water to absorb CO 2 is likely to decrease by around 7–10%. The warming of seawater also leads to reduced carbon dioxide uptake, likely by 9–14% by the end of the 21st century.

Overall, according to model calculations, the ability of the oceans to sink is likely to decrease by around 5–16% by the end of the 21st century. There is evidence that this process may have already started. Relative to the theoretically expected uptake, the Southern Ocean apparently took up 0.08 Gt of carbon per year between 1981 and 2004 too little. This is particularly important because the seas south of 30 ° S (the Southern Ocean is south of 60 ° S) absorb between a third and half of the carbon dioxide bound by oceans worldwide. In the North Atlantic , the absorption capacity not only weakened theoretically, but it actually decreased between 1994–1995 and 2002–2005 by more than 50% or by around 0.24 Gt carbon. This indicates a significantly reduced buffer capacity of the sea for atmospheric carbon dioxide. In both cases, changes in winds or a decreasing mixing of surface and deep water are likely to be responsible for the decline.

If the atmospheric CO 2 concentration doubles compared to the pre-industrial level of 280 ppm (parts per million), a further decrease in the pH value to 7.91 is expected, with a tripling to 7.76 or um about 0.5 points. By the end of the 21st century, the pH value in the oceans is expected to be lower than it has been for at least 650,000 years. If the period of the estimate is extended by a few centuries into the future, a lowering of the pH value by up to 0.7 points seems possible. This worst-case scenario assumes that most of the remaining fossil fuels is consumed including non-arable scattered occurrences . This would likely be more acidification than ever before in the past 300 million years, with the possible exception of rare and extreme catastrophic events. Such a hypothetical state would hardly be reversible within the framework of human time scales; it would take at least several tens of thousands of years before the pre-industrial pH value was naturally reached again, if at all.

Ocean acidification and mass extinction events in geological history

Three of the five major mass extinctions in the Phanerozoic were associated with rapid increases in atmospheric carbon dioxide concentrations, which were likely due to the intense volcanism of magmatic great provinces combined with the thermal dissociation of methane hydrate . Geoscientific research initially focused on the consequences of possible climatic effects on biodiversity, until a study in 2004 pointed to the connection between mass extinction at the end of the Triassic and reduced calcium saturation in the oceans as a result of greatly increased volcanic CO 2 concentrations. The mass extinction at the Triassic-Jurassic border is a well-documented example of a marine extinction event due to ocean acidification, as volcanic activities, changes in the carbon isotope ratio, decrease in carbonate sedimentation and marine species extinction coincide precisely in the stratigraphic sequence and also the expected selectivity occurred in the extinction pattern, which mainly affected species with thick aragonitic skeletons. In addition to the end-Triassic mass extinction, ocean acidification is also discussed as a cause of marine extinction at the end of the Permian and on the Cretaceous-Paleogene border .

Further articles

Publications

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Individual evidence

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This article was added to the list of excellent articles on March 24, 2008 in this version .