Ocean acidification
As acidification of the oceans, the decrease in the pH of sea water is called. It is caused by the absorption of carbon dioxide ( CO2) from the atmosphere. The process factors in addition to global warming, one of the main consequences of human emissions of the greenhouse gas carbon dioxide. While carbon dioxide in the atmosphere physically leads to rising temperatures on Earth, it acts in the sea water chemically. Acidification caused by gases can be exclusively attributed to CO2 emissions of other greenhouse gases such as methane and nitrous oxide do not help. In addition, acid deposition as dilute acid and other environmental contaminants play a certain role.
The consequences of this acidification initially concern kalkskelettbildende beings, their ability to form protective covers or internal skeletons, decreases with decreasing pH. Because these species often form the basis of food chain in the oceans, to more serious consequences for many of them dependent marine species and as a consequence can it also provides for the need of these people.
The acidification of the oceans is also the subject of the Ocean project the future.
The pH is defined for ideal dilute solutions and is therefore not directly applicable to the salty sea water. To specify average values for sea water, models must also be applied to simulate a chemical equilibrium of the ocean. For this purpose, three different models are currently being used, with consequent economies of scale, which are separated by up to 0.12 units. Average values can therefore be compared only in the context of the underlying model.
The sea water is slightly alkaline with a pH of around 8. After a summary of the British Royal Society, the surface water of the oceans today to a depth of 50 m typically pH values from 7.9 to 8.25 on, with an average value of 8.08. The main causes of this difference by 0.25 units, the temperature of the water, the local buoyancy of kohlenstoffdioxidreichem deep water, and biological productivity, where it is high, in the form of marine life binds much carbon dioxide and transported into deeper water layers.
One way to reconstruct previous pH values , providing the analysis of sediment. From the isotopic composition of Borhydroxiden can be determined that the pH at the ocean surface before about 21 million years ago, about 7.4 ± 0.2 was until he before about 7.5 million years ago to the value of 8.2 ± 0.2 rose. Since the pH of the oceans is coupled through the Henry coefficients directly with the carbon dioxide concentration of the atmosphere, even paleo- CO2 concentrations can thus be determined. Until the beginning of the oceanic acidification due to the onset of industrialization in the 18th century and the rising carbon dioxide emissions, this value remained constant.
A study from Stanford University, according to that takes a pre-industrial pH of the near-surface seawater of average 8.25, the pH should have reduced to the present value of an average of 8.14 by the uptake of carbon dioxide. A common overview from the United States 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 of 8 16 was, as he is now 8.05. In both cases, the acidification is due to the human emissions of carbon dioxide and estimated at 0.11 pH units.
Oceans as a carbon sink
The oceans play in the carbon cycle of the earth as a carbon sink an important role, as 70 percent of the earth is covered by water. Throughout the hydrosphere 38,000 gigatonnes (Gt ) of carbon is stored. The carbon dioxide enters the ocean due to the difference in partial pressure of CO2. A gas always flows from the region of the higher partial pressure (atmosphere) into the area of lower pressure (ocean). Carbon dioxide is dissolved in the sea so long, 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 less than at sea. The temperature of the sea also affects the absorption of carbon dioxide, since water can absorb less carbon dioxide with increasing temperature.
The absorbed from the atmosphere carbon is distributed in the ocean within a few years of screened from the sun layer of the sea. To go into even greater depths, there are two mechanisms. Most important is the so-called physical carbon pump, wherein the carbon-rich surface water cools in the Arctic and heavier, falls and is distributed over the cold deep current of the global conveyor belt widely in the depths of the oceans. Less important, but still not insignificant that the biological carbon pump in the carbon as marine snow ( biogenic shower of particles ) is falls into deeper regions. It takes hundreds to thousands of years until the captured anthropogenic CO2 from the atmosphere is penetrated and distributed from the oceans into the deepest layers of the water, now it's down to a water depth of 1,000 m on average detectable. In Seamounts at the continental slopes and in shallow seas (for example, in parts of the Weddell Sea ), the anthropogenic CO2 can already reach to the sea floor.
The increased amount of carbon dioxide in the atmosphere has led in the past 200 years, to 118 ± 19 Gt of carbon, or 27% were added to 34 % of anthropogenic CO2 by the oceans. In 2006 came the world 36.3 Gt of CO2 produced additional people or about 9.9 Gt of carbon into the atmosphere. Including the natural sources takes the hydrosphere present to about 92 pbw of atmospheric carbon per year. About 90 Gt of which are submitted by the seas again, and 2 ± 1 Gt save it. A published 2003 study estimates the uptake of carbon in more detail in the period 1980-1989 to 1.6 ± 0.4 Gt between 1990 and 1999 to 2.0 ± 0.4 Gt per year.
Chemical process of acidification
Carbon dioxide from the air to dissolve in sea water and then lies mostly in the form of various inorganic compounds whose relative ratio reflects the pH of the oceans. Inorganic carbon is found in the ocean and about 1% into carbon dioxide and carbon dioxide to about 91 % in bicarbonate ions (HCO3-) and about 8 % in carbonate ion ( CO32 - ). In water, dissolved carbon dioxide is through the following reaction equations with bicarbonate, carbonate and hydronium ions ( hydronium ions ) in equilibrium:
The incurred in this process oxonium ions ( H3O ) result in a declining pH, as the negative logarithm of the molar concentration (more precisely: the activity ) is defined by oxonium ions.
The acidification from dissolved CO2 is counteracted by the presence of calcium carbonate ( CaCO3), which interacts with bicarbonate and carbonate ions as a chemical buffer system (→ buffer solution) and binds protons:
Like all carbonates of alkaline earth metals calcium carbonate is only slightly soluble in water. The calcium carbonate in sea water comes primarily from two sources, sediments on the sea floor and the entry by inflow of fresh water. In the latter carbonate passes through weathering of calcareous rocks. Thus, the sediment can contribute to neutralize the acidification, the calcium carbonate contained therein must be dissolved and carried into higher layers of the water by circulation from the ocean floor. If in model calculations of weathering -related entry assumed to be constant ( 0.145 Gt carbon per year in the form of carbonate ), the acidification of the oceans would lead within a few hundred years into a reversal of the sedimentation rate. Only in a period of about 8,000 years of weathering -induced entry of calcium carbonate could offset this effect again.
Significant amounts of calcium carbonate in the sediment caused by calcite -forming plankton, especially of Globigerina (a group of foraminifera ), coccolithophores (a group of calcareous algae ) and pteropods. Smaller quantities are formed, for example, in coral reefs. Plankton may be at the bottom of the sea in the form of a carbonate-rich, biogenic sediment ( lime mud ) deposited when the water depth is not too large. If, however, the calcite and aragonite compensation depth exceeded for calcium carbonates calcite and aragonite, then they dissolve completely. This compensation depths wander in the course of acidification upwards, and so, a large quantity of limestone on the seabed in solution. For aragonite could already be found in the Atlantic, an increase since preindustrial times to 400 m to 2,500 m today. By 2050, a further increase to 700 then m is expected. 300 to 800 m above the calcite compensation depth is the Lysokline, the area from which the resolution process begins in the water. Consequently, even in the less deep areas can be solved as in calcium carbonates additionally fixed, 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 sea water with elevated CO2 levels, a process plays, which is very similar to the dissolution of CO2 in the ocean. CO2 as a gas can freely wander through cell membranes, thereby changing the pH of the body's cells and the blood or the Hämolymphes. The change in the natural acid-base budget must be compensated by the organism, which for some species succeed better and some worse. A permanent shift in the acid-base parameters within an organism can affect the growth or reproductive capacity, thus endangering the survival of a species in the worst case.
Damage to coral
Although the solution of carbon dioxide slows down the global warming, the subsequent slow acidification of the oceans but can have serious consequences, among others, for animals with a protective shell of calcium carbonate ( lime or plain ) by yourself. As described above, the chemical balance of the oceans shifts at the expense of carbonate ions. Their conjunction with calcium to form calcium carbonate in seawater however, is of vital importance for forming carbonate shells of marine life. An acidic expectant ocean hinders the biomineralization of coral as well as micro-organisms such as tiny sea snails and zooplankton, although some of these creatures increase the pH value of the water specifically, by reducing the dissolved amount of carbon dioxide in the production of lime crystals in their own cells.
Corals produce calcite with aragonite the next most abundant lime- in the sea. Aragonite is a particularly easily detachable by carbonic acid form of lime, which increases the risk to corals by become increasingly acid oceans. In an experiment at the Israeli Bar - Ilan University corals were exposed to artificially acidified water with a pH value of 7.3 to 7.6. These are values that are viewed by some scientists as possible in a few centuries, provided that the atmospheric content of CO2 quintupled. After a month in the water more acidic, the carbonate shells began to peel away from the coral, and as a result they disappeared completely. Surprising to the researchers was that the polyps of the corals survived. When, after 12 months, the pH was raised to 8.0-8.3, the polyps began again with the calcification. This result may explain why the corals could survive in spite of earlier eras with a less favorable for them pH of seawater. Despite this finding, the researchers speak only of a possible " retreat " of the coral and emphasize the serious consequences of decalcification on the affected ecosystems. A negative effect of acidification on the growth was also detected for the genus Lophelia pertusa coral stone. In one experiment, the calcification occurring in the wild in depths of 60 m to 2100 m cold water corals decreased at a reduced pH by 0.15 and 0.3 units by 30 % and 56%.
Even more significant for the Riffbildung creatures likely to suffer in the acidification. In a seven-week experiment were red algae of the family of Corallinaceae, which play an important role in building coral reefs exposed to artificially acidified seawater. Compared to the same group, the rate of reproduction and growth fell sharply in the algae in the water more acidic. Under the terms of a further decrease in pH in the oceans, this means there may be significant consequences for affected coral reefs.
Impairment of other marine life
The Intergovernmental Panel on Climate Change (Intergovernmental Panel on Climate Change, IPCC) are 2007 Fourth Assessment Report of a scientific " medium security " for negative consequences from the acid becoming the world's oceans for calcareous shells forming organisms and their dependent species at. In a survey conducted at Kyoto University study sea urchins grew in artificially acidified water significantly slower, compared to a control group kept under normal conditions, or lost weight. They were less fertile and their embryos increased significantly slower increase in size and weight. In sea urchins the type Heliocidaris erythrogramma that are native to the waters of South Australia, introduced experimentally by 0.4 units to a presumably reduced reproductive capacity, determined to 7.7 lowered pH at significantly reduced speed and mobility of sperm. This could reduce the number of offspring by a quarter. Problems are expected even with clownfish whose larvae their sense of smell only limited in artificially acidified water at a pH value of 7.8 and could no longer use at a value of 7.6. This can result in severe impairment of postlavalen juveniles in finding suitable habitats and thus draw a decreasing population by itself.
The calcification rate of mussels could decrease by 25% and the Pacific cupped oyster by 10 % by the end of the 21st century. These values were scientists, by following a specific scenario of the IPCC, which provides an atmospheric CO2 concentration of about 740 ppm by 2100. Above a threshold value of 1.800 ppm, the conch shell even begins to dissolve, thus biodiversity is generally vulnerable to coastal and significant economic loss or damage.
The oceanic food chain is based on plankton. Especially calcareous algae ( so-called Haptophyta ) are dependent on the formation of a calcareous shell, to survive. If this is not possible due to the acidification, far-reaching consequences for the food chain of the oceans would thus possibly connected. A 2004 study published in the former Leibniz Institute of Marine Sciences points out the many complex effects that can have on plankton, a lower pH, including compared to the poorer starting position for calcifying organisms with animal phytoplankton ( floating algae ). At the same time the uncertain state of research is emphasized, which allows currently no far -reaching predictions about the development of entire ecosystems. A decreasing Kalzifierungsrate could be detected in the order Globigerinida foraminifera in the Southern Ocean. The unicellular foraminifera are responsible for a quarter to half of the total oceanic carbon flux. In studies, a decreased by 30 to 35% by weight of the calcareous shell compared to dead, recovered from sediments specimens was found for the foraminifer Globigerina bulloides. The consequences of further decreases in pH are considered to be uncertain.
Studies on the effect of lower pH on larger marine animals revealed that, for example, the spawning and the larvae may be damaged. The experiments were made at much lower pH values , as is to be expected in the near future, so that they have only a limited value.
Not all marine life acidification is a limitation of their habitat. First, the increased amount of carbon dioxide in the ocean leads among other things to a better carbon dioxide fertilization of marine plants. Since the effect of different impact in different plants and is connected to the rising water temperature and the decreasing pH, may in turn change the species composition. In some species surprising response to the decreasing alkalinity of the sea have been identified. For the Emiliania huxleyi Kalkalgenart a study showed as expected at an atmospheric CO2 concentration of 750 ppm in the oceans, paradoxically, a possible doubling their calcification and photosynthetic rate measured at pH values. At the same time a significantly decreasing growth rate is expected. E. huxleyi has a share of almost 50 percent in the biological carbon pump in the ocean and provides a third of the sea- bound production of calcium carbonate, is therefore a key species in the ecosystem. As a result of the fallen already by 0.1 pH units at the sea surface, the average weight of these calcareous algae have increased by 40 % over the past 220 years. A further investigation revealed an increased calcification rate under acidic water conditions, compensate for the brittle stars for the more adverse conditions by means of which for brittle stars the type Amphiura filiformis. This adaptation is, however, associated with decreasing muscle mass, in the words of the authors in the long run probably not a sustainable strategy.
Current and Future Development
Situated near a detailed, ongoing for over eight years study before the U.S. Tatoosh Iceland, near the Olympic Peninsula in Washington State, the local pH fluctuated day as well as the year progressed significantly stronger than previously thought, namely by up to one pH unit within a year and by 1.5 units over the period 2000-2007. Parallel took the pH significantly decreased total concentration, with an average -0.045 units per year more quickly calculated as models. On the biology spot these reductions had a discernible effect. The California mussel, mussels and barnacles decreased in the sequence, while various barnacles and some types of algae increased.
Without the sink effect of the oceans, the atmospheric concentration of carbon dioxide would be higher today to 55 ppm, ie at least 435 ppm instead of 380 ppm at currently. Calculated over the period of centuries, the oceans should be able to receive between 65 % and 92% of anthropogenic CO2 emissions. However, phenomena such as an increasing Revelle factor ensure that with rising temperatures and increasing atmospheric CO2 concentration decreases the absorption capacity of the oceans for carbon. Until 2100 should accordingly be reduced by about 7-10% of the capacity of water for CO2. The warming of the sea water also leads to reduced carbon dioxide uptake, by the end of the 21st century, probably around 9-14 %.
Overall, the sink capacity is expected to decline by the end of the 21st century by about 5-16 %, according to the seas model calculations. There is evidence that this process may have already begun. Relative to the theoretically expected recording of the Southern Ocean has apparently 0.08 Gt of carbon per year recorded 1981-2004 too little. This is particularly significant because the seas south of 30 ° S ( the Southern Ocean, south of 60 ° S ) take between one third and one half of the world's oceans bound of carbon dioxide. In the North Atlantic, the capacity weakened not only theoretically, but they fell in effect between 1994-1995 and 2002-2005 to about 50 %, or by about 0.24 Gt of carbon. This indicates a significantly detached buffer capacity of the ocean for atmospheric carbon dioxide. In both cases probably are changing winds or decreasing mixing of surface and deep water with the cause of the decline.
A doubling of atmospheric CO2 concentration compared to pre-industrial level of 280 ppm (parts per million, parts per million) is expected to further decrease the pH to 7.91, with a tripling to 7.76 or by about 0.5 points. By the end of the 21st century so that such a low pH in the oceans is expected, as it is not happened since at least 650,000 years. If the period of the estimate by a few centuries expanded in the future, lowering the pH by up to 0.7 points seems possible. This worst-case scenario assumes that most of the remaining fossil fuel is consumed, including non- arable scattered occurrences. This would probably be a greater acidification than ever before. During the past 300 million years, with the possible exception of rare and extreme disaster events Such a hypothetical state would hardly reversible within human time scales; it would take at least several thousand years, until the natural way of the pre-industrial pH was reached again, if ever.