Climate change and the coasts
Unbridled carbon dioxide emissionsCoasts are adversely affected by many stressors. These include not only local construction or pollution. In addition, coasts are increasingly facing global threats from climate change – sea-level rise, ocean acidification and ocean warming. These trends are primarily due to the still unrestrained burning of the fossil fuels natural gas, petroleum and coal, which adds large quantities of the greenhouse gas carbon dioxide (CO2) to the atmosphere. Since the beginning of the Industrial Revolution, the atmospheric CO2 concentration has risen from 280 parts per million (ppm) in 1800 to a level of 400 ppm today. This increase has resulted in gradual climate change, with the attendant consequences.
Climate inertiaDue to the inertia that is inherent in our climate system, many impacts of human-induced global warming are slow to become apparent. Even if we managed to completely stop all carbon dioxide emissions today, near-surface air temperatures would continue to increase for at least another hundred years. The sea level would even continue to rise for several centuries. What is the cause of this? One factor is that due to slow deep sea warming the ocean waters are only gradually expanding. At the same time, the continental ice sheets in Greenland and the Antarctic probably react very slowly to atmospheric warming. As a result, the melting of the glaciers is a long drawn-out process that will continue for thousands of years.
Increasing ocean warming will substantially alter the conditions faced by many marine organisms. These processes are likely to result in sustained changes to the composition of the oceans’ biotic communities (biocoenoses) and food webs. Such changes will be further amplified by ocean acidification, which effectively alters the chemistry of marine waters. This acidification is a result of the increasing uptake by seawater of carbon dioxide from the atmosphere. Simply put, when carbon dioxide dissolves in water it forms acid.
In recent years, the number of research projects investigating the impacts of climate change on the oceans has increased rapidly. Many of these studies primarily deal with the impacts on coasts and coastal waters. They also address the question of how far the impacts of ocean warming and ocean acidification are similar in coastal waters and the open sea, or whether they differ significantly between those two marine realms.
- 3.1 > In the ocean, a thermocline often forms between the warm surface water and cool water at greater depth. This distinct layer can be seen with the naked eye as the water’s density changes with the temperature and certain particles concentrate at the thermocline as can be seen here off the Thai island of Ko Phangan.
- OCEAN WARMING
Warmer water, increased stratificationWhile it is already possible to fairly accurately predict which coastal areas will be affected by a specific amount of sea-level rise, it is much harder to appraise the impacts of ocean warming. Enhanced stratification of ocean waters in future is however deemed a certainty. It will be more difficult for oxygen-rich layers at the water surface to mix with colder, deeper layers. This may result in a lack of oxygen at greater depths, as has already been observed in various marine regions of the world.
The stratification of waters is a natural process: during the summer months sea surface water warms up and forms a layer of water close to the surface that covers the heavier, colder deep water like a lid. The transition from the warm surface layer to the colder water below is quite abrupt, which is why the line separating warm and cold water is called the thermocline. These thermoclines range in thickness from only a few decimetres to many metres in different marine regions, with thermoclines in the open ocean with deep waters being considerably thicker than those in coastal areas.
At the thermocline, a warm and less dense water layer rests over a colder water body of higher density. The thermocline thus functions like a barrier. The greater the temperature differential, the greater the difference in density and the more stable the thermocline. Ultimately hardly any oxygen-rich surface water can be mixed into deeper layers by means of wave motion, with the lack of light in the deep ocean also precluding oxygen production through photosynthesis. As the decomposition of organic material by microorganisms in the deeper water layers continuously consumes oxygen this is a serious issue which results in oxygen deficiency in the deeper layers of many coastal seas.
Today, ocean warming further exacerbates oxygen deficiency in deep waters. This is due to the fact that biochemical processes generally run faster at higher temperatures as the biochemical substances involved are more reactive. This is also true for the metabolism of bacteria. Bacteria decompose the remains of dead plankton that has sunken into greater ocean depths and use oxygen in the process. The higher the temperatures, the faster the bacterial metabolism and the more oxygen will be used up.
Unique measurements spanning six decadesFor the German Baltic Sea, scientists have detected the current specific impacts of ocean warming by analysing a unique time series, the data points of which go all the way back to 1957. The scientists regularly measure the water’s temperature, nutrient and oxygen contents as well as other parameters at the same location in the Eckernförde Bay. The data show that the water’s nutrient content has decreased in recent years, very probably due to lower nutrient loads from terrestrial sources. Surprisingly, however, the deeper water layers are nonetheless affected by oxygen deficiency during the spring and summer months. At a depth of 25 metres, oxygen concentrations in the Eckernförde Bay have decreased significantly, with the lowest values found between May and September. At times oxygen is completely absent from the deep-water areas.
- 3.6 > Scientists have regularly studied the water at a certain location in the Eckernförde Bay on the Baltic Sea coast since 1957. Nowadays they use modern water samplers that take water samples at different depths.
- This is most likely caused by ocean warming which on the Baltic coast gives rise to two interconnected phenomena. Firstly the warming of the upper water layers results in a more pronounced thermocline which hampers oxygen transportation to greater depths during the summer months. Secondly this is accompanied by a biological phenomenon. Small filamentous algae that settle on macroalgae such as bladder wrack thrive particularly well in warmer waters. Normally such filamentous algae are grazed by small crustaceans. But when water temperatures increase, the crustaceans become more sluggish and hardly feed at all. This allows the filamentous algae to proliferate and ultimately overgrow the bladder wrack and other macroalgae. Bladder wrack, which is dependent on sunlight for photosynthesis, dies off, thus generating unnaturally large quantities of dead biomass which then sinks to deeper water layers where it is decomposed by bacteria. This increases the oxygen demand, with oxygen already being in short supply due to the more pronounced thermocline. These processes can give rise to oxygen-depleted zones, especially during July and August.
For several years now the scientists have observed a collapse of the biocoenoses in the water layers near the bottom of the Eckernförde Bay at the height of summer.
Cyanobacteria are a group of bacteria that are able to photosynthesise. For this reason they were originally considered to be plants and were called blue-green algae. The term “blue” refers to the fact that some types of cyanobacteria contain the bluish plant pigment phycocyanin instead of the green plant pigment chlorophyll.
- These observations in the Eckernförde Bay are congruent with measurements that have been analysed for the entire Baltic Sea. US American weather satellites have been measuring Baltic Sea surface temperatures several times per day since 1990, thus building up a very good set of temperature data. These data show that the Baltic Sea surface temperature has increased by 0.6 degrees Celsius per decade since 1990. This figure is based on annual averages, as the Baltic Sea is subject to strong seasonal fluctuations and also displays clear regional differences. Over the study period of 27 years the surface temperature has therefore increased by 1.62 degrees Celsius. The increasing temperatures particularly favour the growth of cyanobacteria. In calm summer weather periods during which the water heats up particularly swiftly, these algae rise to the sea surface where they form mats, primarily in the central Baltic Sea. Winds can wash such algal mats onto the beaches. From the human point of view this is a problem because many species of cyanobacteria produce toxic substances. Overly rapid growth of cyanobacteria can result in toxic carpets of Harmful Algal Blooms (HABs). Swimming is prohibited in affected coastal areas. Moreover, HABs can poison marine animals such as fish, thus resulting in potentially significant losses for coastal fisheries.
- 3.7 > In the Eckernförde Bay, the number of months per year in which the water at 25 metres of depth is oxygen-deficient has increased since the late 1950s. This is thought to be due to the warming of the Baltic Sea waters.
Corals under heat stressTropical coral reefs are one of the coastal ecosystems particularly at risk from ocean warming. Not only are they sensitive to a rise in water temperatures, but in many areas they suffer additional pressures, particularly as a result of the pollution of coastal waters with toxic substances, nutrients and suspended solids. While globally only approximately 1.2 per cent of the continental shelves are covered by coral reefs, these reefs are enormously species-rich. It has been estimated that they host more than 1 million species of fish, bivalves, corals and bacteria.
Coral bleaching – a symbiosis is failingCorals are marine animals in the Cnidaria phylum living in symbiosis with unicellular plants. These single-celled organisms, the zooxanthellae, reside in the tissue of corals. They are green-brown in colour and are able to photosynthesise. It is these organisms that provide corals with much of their colour. They also provide their hosts with sugars and in return they receive various nutrients. Coral bleaching occurs when this symbiosis fails and the zooxanthellae leave the corals, which as a result lose much of their colour. Recent research has been able to identify the various factors contributing to the failing of this symbiotic relationship. Ocean warming evidently plays a key role.
The optimum water temperature range for many tropical coral species is between 25 and 29 degrees Celsius. For many species, an increase of as little as 1 to 3 degrees Celsius can trigger bleaching. This appears to be caused by changes in the zooxanthellae’s metabolism. At higher temperatures, many metabolic processes, such as photosynthesis, run faster and result in the production of increased amounts of cell-damaging radicals, i.e. aggressive molecules, a proportion of which enters the corals from the zooxanthellae. As soon as the corals register an increase in the production of radicals they trigger a protective reaction, expelling the zooxanthellae into the water column. Bleaching is therefore a mechanism protecting corals from cell damage.
- 3.9 > Corals bleach when they come under stress – such as this stony coral in the Indonesian Raja Ampat Archipelago. The corals then expel the zooxanthellae, pigmented single-celled organisms with which they live in symbiosis.
- Coral bleaching is a natural and reversible phenomenon. Once the stressor abates, for example if water temperatures drop, the corals once again take up the zooxanthellae from the surrounding water into their tissues and recover. However, in many coral reefs bleaching now occurs much more frequently than in the past due to ocean warming in combination with other stressors. While in the past a reef may have experienced a bleaching event roughly once in twenty years, in many areas the phenomenon now tends to occur at intervals of only a few years, leaving the corals hardly any time to recover. Once the zooxanthellae have been expelled they can no longer provide the corals with sugars. The corals then begin to starve and weaken and become more susceptible to being attacked by pathogens such as bacteria.
Approximately 20 per cent of corals have been killed and a further 30 per cent are severely damaged as a result of ocean warming and other stressors. Moreover, a total of 60 per cent of all tropical coral reefs are locally at risk due to at least one of the following local aspects:
- destructive fisheries practices that destroy the reef, such as anchored boats or nets;
- coastal development (construction);
- pollution of marine waters due to riverine inputs of pollutants or suspended solids;
- local pollution of marine waters due to direct inputs of wastewater along the coast or from merchant vessels and cruise ships as well as destruction resulting from bottom-contact by ferries or tourist vessels.
Adaptation to warmingFortunately corals can adapt to rising ocean temperatures to a certain extent. Recent studies have shown that some coral species selectively incorporate other species of zooxanthellae following a bleaching event. This form of adaptation is called adaptive bleaching. The corals appear to prefer species of zooxanthellae that only moderately increase their metabolism under conditions of rising water temperatures and thus produce fewer radicals. However, these zooxanthellae tend to have a lower metabolic rate which means they also produce less sugar. If in the course of the year temperatures drop again, this may put the corals at a disadvantage, as the zooxanthellae will then be less productive and provide lower quantities of sugar due to their lower metabolic rate. Research is currently underway into the consequences this may have. The insufficient supply of sugars might slow down the corals’ growth. Moreover, adaptive bleaching has limits. If the water temperatures are constantly too high, the symbiosis may fail nevertheless, resulting in renewed bleaching. This may be due to the production of radicals in the zooxanthellae or to other metabolic processes that are as yet not fully understood.
In addition, eutrophication of coastal waters with nutrients from agricultural or aquacultural sources may contribute to the failure of the symbiosis. Nitrogen plays an important role in this context as it is a vital nutrient for zooxanthellae. If a lot of nitrogen is available, the zooxanthellae increase their metabolism and display strong growth. However, if in the course of this growth phosphorus, another important plant nutrient, is missing, problems may arise.
Phosphorus is an essential component of cell membranes. If it is in short supply during cell growth, insufficient amounts of phosphorus are integrated into the membranes, making them more permeable. As a result, increased amounts of free radicals can transfer from the zooxanthellae into the coral tissue, which in turn leads to the zooxanthellae being expelled, and to coral bleaching.
- 3.10 > Corals are basically colourless. Single-celled organisms (zooxanthellae) residing in the coral tissue are responsible for making them appear colourful. Zooxanthellae engage in photosynthesis and are of a greenish or reddish colour. If the coral comes under stress, for example due to elevated water temperatures or water pollution, it expels the zooxanthellae and bleaches as a result. Moreover, it now lacks the essential sugar compounds normally provided by the zooxanthellae. This weakens the coral.
- Efforts are now underway to restore dead coral reefs. To this end, fragments of living corals are attached to the dead ones in the hope that they will grow and reproduce. Experts have also been searching for particularly stress-resistant coral species suited to these efforts. The Red Sea appears to host particularly robust species. Due to the seasonal variations in water temperatures – just over 20 degrees Celsius in winter and often more than 30 degrees Celsius in summer – many corals in this region are adapted to fluctuations in water temperature and would therefore be suitable for the restoration of damaged reefs. However, it is important to consider that there are several hundred species of corals worldwide. Experts consider that probably only very few species will be suitable for reef restoration efforts in that they are sufficiently robust to exist in other oceanic regions with other environmental conditions. Even if reef restoration for purposes of coastal protection was to be successful, the reef’s species diversity will have been irretrievably lost upon its destruction.
Bleaching is not the only impact of ocean warming. There are numerous diseases that can cause corals to die. Bacterial infections in particular are on the increase, Acropora White Syndrome (AWS), for example, or the Black Band Disease (BBD), both of which quickly kill the Cnidaria upon infection. These diseases are therefore clearly more dangerous than bleaching, as the latter is reversible while the infections are generally lethal. These infectious diseases primarily affect reefs in the Caribbean where they can spread several metres in just a few days. It is thought that in such cases the corals are weakened by ocean warming and are not able to produce sufficient quantities of antibodies which would normally help them to keep the pathogens at bay.
Too warm for fish offspringMarine organisms are adapted to limited temperature ranges. Changes in temperature can cause massive species shifts within the marine food web. The encroachment of marine organisms from warmer southern regions to cooler regions in the north has already been observed over a number of years. For many species, sensitivity to warming is somewhat variable through the individual life-cycle stages. The tolerance range of young developmental stages, particularly the egg and larva, is often very narrow and thus critical with regard to the impact of climate change on a particular species. This is the case for the codfish, native to the Northeast Atlantic and one of the most important food fishes worldwide. Cod spawn in the spring, with each fish releasing up to 5 million eggs in water temperatures between 3 and 7 degrees Celsius because embryo development in the eggs is most successful within this range. The most important spawning areas in the Northeast Atlantic are located near the coast of Norway around the Lofoten Islands, and in the Skagerrak and the Kattegat between Denmark, Norway and Sweden.
- 3.11 > Economically, cod is one of the most important fish in the Northeast Atlantic. Ocean warming could create unfavourable growth conditions for the cod eggs and larvae. This could cause a significant decline in the large cod stocks north of Norway.
- Past experiments have shown that the embryos of cod are highly sensitive to water acidification. Now, for the first time, studies are being carried out to determine how the added factor of ocean warming affects their development. Fertilized cod eggs are held at different water temperatures and acidities in aquariums until the fish larvae hatch. These simulate ocean conditions that could develop during this century. The results show that a temperature increase of around 3 degrees leads to the death of the eggs or to larval deformity. Embryos in the fish eggs appear to react sensitively to warmer water, particularly during the early stages of their development. The experiments also indicate that this situation is exacerbated when the acidity of the water is increased. The number of damaged or dead embryos then escalates by 20 to 30 per cent.
In addition, climate models are being applied to determine possible changes in the geographical distribution of the cod that could occur due to future warming and acidification of the ocean. Investigations are focussing on whether critical temperatures will be reached in the known spawning areas. The results of the studies are alarming. They suggest that up to 40 per cent fewer cod larvae will hatch along the Norwegian coast. This would very probably have severe consequences for the entire ecosystem and for the cod fishery in the Barents Sea to the north of Norway. For centuries this stock has ensured the livelihood of Norwegian and Russian fishermen who take in around 2 billion euros from the fishery each year. A collapse of the cod population is a potential catastrophe that would threaten the livelihoods of a majority of the human population in this region.
The total magnitude of the consequences of climate change on cod offspring, however, is difficult to assess. Cod release their eggs in open water. The eggs and subsequently hatched larvae are then transported by ocean currents into areas that provide optimal conditions for development of the young fish. If ocean warming causes a northward shift of the cod populations, they may end up spawning in marine areas with different current patterns. It is not known whether these will provide optimal conditions for development of the offspring.
- 3.12 >On the left is a healthy cod larva, on the right a deformed one. This clearly illustrates the destructive impact of increased temperature and acidification on young life stages.
- According to current studies, not only the cod but also other marine organisms will migrate northward or become scarcer in the south. The coastal waters of Great Britain are thus expected to suffer huge losses in their fisheries. Investigations here are assessing how the fisheries for cod and sea bass, as well as cockles, scallops and mussels will develop. Together these five species presently account for around half of the total fish catch in Great Britain. The analyses, again, are based on the four RCP scenarios of the Intergovernmental Panel on Climate Change. According to the RCP2.6 scenario, by the end of this century a decline of around 30 per cent in the catch volumes of mussel species is expected, while under RCP8.5 the expected loss would be about 60 per cent. The regional situations, however, would differ somewhat for England, Northern Ireland, Scotland and Wales. For cod and sea bass, the expected changes would range from negligible to slight losses under scenario RCP2.6. If RCP8.5 proves to be the future reality, however, the volumes of cod and sea bass caught are likely to decrease by as much as 20 per cent by the year 2100. England, lying farthest to the south, would be especially hard-hit under this scenario. England, therefore, would have to look to other species to compensate for the losses, possibly to Mediterranean species that could spread northward with ocean warming.
Hypersaline river deltasWarming of the Earth due to human-induced amplification of the greenhouse effect can also have an indirect impact on the fish communities in coastal waters. This is illustrated by the situation in the Sine-Saloum Delta on the coast of the West African country of Senegal. Senegal is located in the transitional area between the dry Sahel zone to the north and the more humid tropical forest belt further to the south. Because precipitation in the Sahel has decreased considerably since the 1960s, only very limited amounts of fresh water now flow into the delta from the landward side. Consequently, salt water from the Atlantic has penetrated deeply into the delta. In the upper reaches of the tributaries, as a result of evaporation, salinity can be as much as three times the normal concentration of seawater. Fish species that can only survive in relatively low salinities have thus disappeared from the delta. These include, among others, the very popular food fish tilapia. Today, in its place, large areas of the delta are dominated by smaller herring-like fish such as bonga shad, which have a considerably lower market value than the tilapia. The total fishery yield is thus decreasing. In general, fewer fish species are found in the Sine-Saloum Delta than in comparable Western African deltas located in the humid tropical belt to the south that still have a strong input of river water.
Sudden mass proliferation after a half centuryNot only can the composition of biotic communities in coastal seas change during the course of ocean warming through migratory shifts of species, but also through direct introduction – when organisms or larvae are transported unintentionally in the ballast water of ships or as incrustations on their hulls from one marine area to another. Non-native species can also be introduced into new areas when organisms are released or escape from an aquarium. This introduction of new animal species (neozoons) and new plant species (neophytes) is also known as bioinvasion.
Some introduced organisms are able to establish themselves and proliferate in their new environment. If conditions are favourable they can even supplant native species and thus significantly alter the habitat. There is now evidence that ocean warming can also contribute to such a change, as illustrated by the example of Austrominius modestus. This Australasian barnacle species was probably introduced into British waters in the 1940s by warships or sea planes from Australia and spread from there across the entire North Sea. It was first observed on the German island of Sylt in 1955. It was also able to propagate there, but for several decades it occurred only in very low numbers. The native Sylt barnacle species Semibalanus balanoides and Balanus crenatus predominated. This relationship was reversed in 2007 with the first massive proliferation of the Australasian barnacle.
- 3.13 > The Australasian barnacle Austrominius modestus enjoys ideal living conditions in the North Sea thanks to an increasingly mild climate. On the island of Sylt it has almost completely supplanted the native species Semibalanus balanoides.
- Barnacles in the area around Sylt preferentially colonize on mussel beds. In 2007, the mussels were predominantly covered for the first time by young Australasian barnacles. The barnacle population density was 70,000 individuals per square metre. For comparison, in 1997 there were just 70 individuals of this species per square metre. The reason for the sudden enormous increase is presumably related to the changing climate through the preceding years. For some time there had been a general trend toward warmer summers and milder winters. For example, the average air temperature on Sylt between April and August today is 2 degrees higher than it was in 1950. Now, decades after its initial introduction, the Australasian barnacle is evidently living under ideal conditions for mass proliferation.
Heavy encrustation of their shells by the Australasian barnacle is not a problem for the mussels. This example illustrates, however, how rapidly a massive proliferation of invasive species can occur. When the invasive species supplant or even prey on the native species, an ecosystem can be degraded quickly and severely.
- OCEAN ACIDIFICATION
Carbon dioxide alters the pH value of waterWhile global warming as a result of human activity has been a topic of intensive discussion within scientific and public circles for several decades, acidification of the oceans has been largely ignored. It was only a decade ago that researchers first began to point out that increasing CO2 in the atmosphere is accompanied by significant changes in the chemistry of ocean water.
Chemists determine the acidity of a liquid based on its pH value, whereby a more acidic liquid has a lower pH value. The pH scale ranges from 0 (very acidic) to 14 (very alkaline). A value of 7 is considered to be neutral and marks the transition from acidic to alkaline. Since the beginning of the Industrial Revolution near the middle of the eighteenth century, the average pH value of the oceans has dropped from 8.2 to 8.1. Strictly speaking, with a value of around 8 ocean water is a weak base and not an acid. But because the pH value of seawater is shifting toward the acidic side of the scale with continuing CO2 absorption, this development is nevertheless considered to represent an acidification of the water. By the year 2100 the present pH value of 8.1 could decrease by an additional 0.3 to 0.4 units. This may sound like an insignificantly small change, but not when one considers that pH is measured on a logarithmic scale. This means that it is mathematically compressed. So, in fact, even with this small numeric change, the ocean would then be 2 to 2.5 times as acidic as it was in the year 1860. The cold Arctic and Antarctic waters are especially impacted by acidification. Because CO2 dissolves more easily in cold water, these marine regions acidify more readily than warmer regions.
For the high seas and non-coastal regions, the trend of continued acidification, which is already well documented, can be reliably predicted for the future because relatively constant conditions prevail here in terms of water chemistry. On the other hand, it is more difficult to determine how CO2 will affect coastal waters. Water chemistry near the coasts is strongly influenced by substances brought in from landward areas, particularly carbonate anions and bicarbonate (hydrogencarbonate) ions, which are the fundamental components of numerous minerals. As the rocks are weathered by rain, these materials are washed through rivers into the coastal waters. They are also the major component of lime, which is applied, for example, to neutralize acidic soils. Large amounts of carbonate anions and bicarbonate ions entering the coastal waters can have a buffering effect on the acidification. The term alkalinity is used as a measure for this buffering property.
The acidity (pH value) of a liquid such as seawater can be changed by adding alkalinity – by introducing a high-alkaline liquid, for example. This buffers the acidity and is referred to as the acid-binding capacity. The degree of alkalinity, and thus the acid-binding capacity, is determined by the content of carbonate anions and bicarbonate ions, which have an alkaline effect and thus counteract the acidity. Carbonate anions and bicarbonate ions have a high affinity for hydrogen ions, which generally make liquids acidic. They buffer the acidity by capturing a certain portion of the hydrogen ions.
Complex interactions between the land and coastal seasInteractions between the land and coastal seas have been intensively studied in the Baltic Sea. It is considered to be an inland sea because it is surrounded by land and has only a single narrow outlet to the North Sea, and thus to the Northeast Atlantic. An analysis carried out over the past 20 years indicates that, depending on the season and area of the Baltic Sea, the input of carbonates from the land either partially or totally compensated for the acidification – as a function of alkalinity in the water.
he alkalinity, in turn, is dependent upon many different factors, including the amount of precipitation on land. When rainfall is stronger, weathering of the rocks is more intensive, so that more carbonate and bicarbonate ions are carried to the rivers. Alkalinity is also increased in the rivers and the sea as a result of the liming of farmland in agricultural areas around the Baltic Sea.
Most climate studies for northwest Europe assume that climate change will be accompanied by increased precipitation because warming of the atmosphere will enhance evaporation at the sea surface. The prevailing winds in northwest Europe will then bring in more moisture from the North Atlantic. If precipitation increases, more water will flow from the land into the sea, thus bringing more alkalinity into the sea. Acidification in the area of the Baltic Sea could therefore be partially or totally buffered in the future because of its geographical position and the strong influx of water from the land. With an increase in precipitation, of course, more alkalinity would also be introduced into the Northeast Atlantic. But in the small, inland Baltic Sea, the impact would be recognizable much more quickly than in the open ocean with its significantly greater volumes of water.
Withstanding acidification?In recent years many studies have been undertaken to investigate how marine organisms react to acidification. Pictures of calcareous algae, called coccolithophorids, showing the calcareous plates slowly dissolving with decreasing pH values have become familiar. The studies, based on laboratory experiments, consistently supported the conclusion that large numbers of organisms could perish under conditions of increasing acidification, and some species could become extinct. Now, however, some contrasting results have been obtained which show that this may not necessarily be the case. It has been shown, for example, that certain groups of organisms apparently have the ability to adapt to the acidification. Experiments on the coccolithophorid species Emiliania huxleyi have shown that after about 500 generations a certain degree of resistance is developed and calcite formation improves again in more acidic seawater. Because Emiliania reproduces rapidly, the 500th generation is achieved after about six months. Ongoing investigations are attempting to discover what kinds of metabolic changes are at the root of this adaptation to acidification.
Interesting field studies in this context were carried out off the Swedish Baltic Sea coast, investigating how phytoplankton, the base of the marine food web, reacts to acidification. Here, over a six-month period, CO2 gas was introduced into Baltic Sea water so that it corresponded approximately to a level that would be produced if the present CO2 content of the atmosphere were doubled. Amazingly, only minor changes in the plankton associations could be recognized at specific times in their development when compared to seawater without CO2 introduced. The increased CO2 had a slightly negative effect on some groups of organisms in the plankton community and a slightly beneficial effect on others. The researchers propose that many of the organisms are able to tolerate lower pH values because of the natural fluctuations of pH in the Baltic Sea due to alkalinity.
Meta-analyses, however, in which the results of several hundred publications were analysed and integrated, indicate that there are still organisms in other coastal regions that definitely react to acidification, especially in marine regions where the chemical conditions of the water are fairly constant. Besides many areas in the open ocean, these are primarily coastal waters in hot and dry regions where no rivers flow into the sea. The marine organisms most strongly impacted are those that form calcareous shells or skeletons. It is evident that carbonate formation by corals, clams and snails, depending on the group studied, is reduced by 22 to 39 per cent in acidified water. Changes are also seen in the growth of organisms. Taking all carbonate-forming marine organisms together, it can be shown that on average they are up to 17 per cent smaller than those living in water with normal pH values.
Lower species diversity in coral reefsStudies by Australian researchers illustrate how increased acidification affects coral reefs in Papua New Guinea. In these areas, CO2 escapes from the sea floor through volcanic vents, producing a natural acidification of the seawater. Coral colonies have developed here that are able to cope with the increased CO2 content of the water and relatively low pH values. The area can be seen as a kind of field laboratory for anticipating ocean acidification. The closer the corals are to the CO2 sources, the more acidic the water. Thus, depending on the distance to the source, conditions are found that could be prevalent globally in the ocean 20, 50 or 100 years from now. Here, instead of the delicate, branching stony corals, which react especially sensitively to acidification, the robust and plump Porites corals with an outward appearance similar to cauliflower are more common. Overall species diversity in these reef areas is significantly lower than in areas with normal pH values. In water with a pH value of 7.7, which could actually be reached by the year 2100, the living conditions are so unfavourable that even the Porites corals can no longer grow.
Winners and losers in ocean acidificationWhile calcareous organisms are at a disadvantage, the cyanobacteria, previously called blue-green algae, may possibly be among the winners. Very much like plants, cyanobacteria require CO2 to produce sugar with the help of photosynthesis. They can thus carry out metabolic processes that concentrate CO2 in their body and make it available for photosynthesis. But these Carbon Concentrating Mechanisms (CCMs) consume energy. If there is abundant ambient CO2 available, the strain on the CCMs is lightened and cyanobacteria and plants can save energy. This energy can then be used to enhance growth. The ancestors of today’s cyanobacteria existed as early as 2 billion years ago, at a time when the Earth’s atmosphere contained abundant carbon dioxide and sparse oxygen. Cyanobacteria living today are therefore still well adapted to high CO2 concentrations and low pH values in the water.
- 3.15 > Scientists expect that, with global warming, thawing of the Greenland ice sheet will intensify in the future. Particularly acute melting was observed in the year 2012. Due to exceptionally mild air temperatures in this year, thawing on the surface of the glaciers persisted for many more days over large parts of the island than the annual average of the years from 1979 to 2007.
- 3.16 > Melting of the Greenland glaciers during the summer months, as seen here near the town of Qaanaaq, is a natural process. For the past several years, however, the thawing of the ice masses appears to be intensifying.
- SEA-LEVEL RISE
Imminent danger for coastal residentsResidents of many coastal regions will probably notice the impacts of climate change primarily in the form of sea-level rise, because it will cause a great loss of land in the form of residential areas, industrial and economic centres, and farmland. Furthermore, due to the rising sea level, storm floods today surge to higher levels. It should be noted that not only human-induced global warming influences the level of the water, but that natural processes also play a part. Generally a distinction is made here between:
- eustatic, climatically induced, globally acting causes that lead to an increase in the water volume in the oceans, such as rising sea level when ice melts after a glacial period;
- isostatic, primarily tectonically controlled causes that have an essentially regional impact. These include the subsidence or uplift of land masses that occurs with the alternation of cold and warm periods. The immense weight of ice sheets formed during the ice ages causes the Earth’s crust to sink in certain regions, resulting in a sea-level rise relative to the land. When the ice thaws, the land mass begin to rebound, or uplift, again, a phenomenon that can still be observed in the Scandinavian region today.
- Over millions of years the height of sea level can fluctuate by more than 200 metres. But it can also change significantly within relatively short time periods. Changes of around 10 metres can occur within a few centuries. After the last glacial period, around 15,000 years ago, temperatures on the Earth began to rise strongly again, and since that time sea level has risen by around 125 metres. At first the rise was relatively rapid. This phase lasted until around 6000 years ago. For a long period of time after that, sea level varied only slightly with fluctuations of a few centimetres per century. Compared to the relatively minor changes during the past 6000 years, however, the rise has now started to accelerate again. Between 1880 and 2009 sea level rose by 21 centimetres, fairly weakly through the first half of the twentieth century but with increasing speed during more recent decades. Since 1990 sea level has risen annually by about 3 millimetres. The following factors are presently contributing to sea-level rise:
- 15 to 50 per cent is due to the expansion of seawater as a result of temperature increase;
- 25 to 45 per cent to the melting of mountain glaciers outside the polar regions;
- 15 to 40 per cent to the melting of the ice sheets on Greenland and in the Antarctic.
A question of locationFor the coasts, sea-level rise is surely the gravest consequence of climate change, but in this century it will not lead to permanent flooding of coastal areas like an overflowing basin. Furthermore, sea-level rise does not affect all coasts to the same degree. The climate scenarios of the Intergovernmental Panel on Climate Change (IPCC) usually refer to a global average sea-level rise. But regionally, in fact, there will be large differences in sea-level rise relative to the land surface. So today a differentiation is recognized between the global sea level, regional sea level and local sea level.
- 3.17 > Sea level is presently rising by an average of around 3 millimetres each year. Whether the rate escalates or becomes weaker depends on how much the greenhouse effect increases in the future.
Different regions, different sea levelRegional sea level is mainly determined by regional conditions, such as the uplift or subsidence of land masses or changes in regional wind and ocean-current patterns. For example, on the Pacific coast of South America the El Niño climate phenomenon causes a deviation in sea level of up to 40 centimetres from the normal average level. El Niño occurs at irregular intervals every 3 to 10 years in the Pacific between Indonesia and Peru, when the surface ocean currents reverse due to a weakening of the prevailing trade winds. Normally the strong trade winds drive the surface water from the Pacific coast of South America out into the open sea. During El Niño events, however, the trade wind is weaker and water piled up in the West Pacific swashes back toward America. The effect of this current reversal can then be observed in the water level at the coast.
The thick continental glaciers in Greenland and the Antarctic also have a large regional influence. The masses of these glaciers are so great that the gravitational force is stronger there than in other marine regions. The physical principal that bodies with greater mass have a stronger gravitational attraction applies here. Seawater is thus more strongly attracted in the vicinity of the glaciers, so that sea levels around Greenland and the Antarctic are a few decimetres higher than the global average. With the melting of the glaciers as a response to climate change, however, the glacial mass will decrease, and in the coming centuries Greenland and the Antarctic will likely experience a regionally falling sea level while the average global level rises each year.
Regional sea levels are also influenced by other phenomena. These include, for example, the present-day uplift of Scandinavia or other areas that were covered by ice in the past. During the last glacial period several thousand years ago the large ice load depressed the Earth’s crust down into the mantle. As the ice thawed the land mass began to rebound and is still now rising relative to the sea, which is observed on the coasts as a fall in sea level. The uplift today amounts to several millimetres each year.
Homemade sea-level riseLocal changes in sea level often result from the construction of high-rise buildings or the extraction of groundwater for drinking water (see Chapter 2). River deltas, on the other hand, subside under their own weight. In many places today the construction of dams prevents adequate compensation for this subsidence due to the reduced amount of new sediment being transported in by the rivers. With rising sea level many delta regions will likely be more frequently flooded in the future.
For the 33 large delta regions of the world, it is presently assumed that the surface area threatened by flooding due to sea-level rise will increase by around 50 per cent by the year 2100.
More than 6 metres in 500 years?Regardless of the present state of local and regional sea-level rise, failure to significantly curb the emission of greenhouse gases will result in a substantial rise in the average global sea level during this century and beyond. If the Earth’s population and its energy consumption increases greatly, as illustrated by scenario RCP8.5, average sea level could rise more than 6 metres by the year 2500. This would be exacerbated by additional threats to the coasts as were summarized in the latest report of the Intergovernmental Panel on Climate Change. The report finds that the following consequences can very probably be expected during this century:
- an increase in wind speeds and precipitation during tropical cyclones, which will likely lead to more flooding and damage, whereby the heavy flow of rainwater from the land and high ocean water levels caused by strong winds occur concurrently;
- higher storm-flood surges. The average surge of storm floods today is already higher than it was 100 years ago;
- higher extreme water levels due to higher wind speeds. Subsiding coastal regions are especially hard-hit;
- stronger erosion of the coasts as a result of more frequent flooding and surging waves breaking higher than normal on the beach.
- 3.18 > Since the melting of ice-age glaciers the Scandinavian land mass has been rising. This motion continues even today. Northern Germany, on the other hand, is sinking. The boundary runs approximately along a line from southern Denmark to the island of Rügen.
Sinking beaches and wetlandsMany natural coastal habitats will be destroyed irretrievably through permanent flooding and erosion, or will shift inland. This loss of land is already happening today. On the coasts of northern Alaska and Siberia, for example, the permafrost soil is breaking off in many places at a rate of several metres a year. The reason for this is milder and longer summers. In addition, large expanses of sea ice are melting, allowing the wind to create stronger waves, which can then, in turn, easily erode the thawing soil on the shore. Beaches and dunes have also been more strongly eroded on many coasts in recent years, such as those along the east coast of the USA. Scientists attribute this to stronger winds and higher-surging storm floods.
Many of the world’s coasts are characterized by wetlands, salt marshes, or seagrass growth in shallow waters. These are vital habitats for many organisms, including specialized plants and insects, birds that stop to rest and breed, or for fish. Many of these areas have already been destroyed by construction or pollution of the coastal waters. Due to rising sea level, combined with higher-surging floods and strong winds, these areas are severely threatened by erosion. Salt marshes, for example, are more strongly eroded on the water side. With higher water surges in the future, new salt-marsh areas could possibly form further inland. This will only be possible, however, in locations where the hinterland is not protected by dikes and cut off from the salt marshes on the sea side. Where the salt marshes have no room to retreat they will be lost as a valuable habitat as erosion increases. The same is true in many regions for wetlands or shallow-water seagrass. Because seagrass can only take root in relatively sheltered, shallow-water areas with low wave activity, many populations will be battered and destroyed by stronger currents or waves.
Can corals keep pace?With regard to the consequences of sea-level rise for coastal habitats, the fate of coral reefs appears to be not yet sealed. Current studies on Indonesian coral reefs, for example, indicate that they can react quite flexibly to rising or falling sea level. Tropical stony corals live in shallow coastal waters suffused with light because their symbionts, the zooxanthellae, need sufficient light for photosynthesis, which is not available below certain depths. If sea level rises the deeper water layers become darker. As the studies show, however, the corals are able to keep pace with the water by growing the reef vertically at the same rate that the water rises. New corals colonize at the top while the corals at greater depths die.
Studies on ancient coral reefs show that corals were also apparently able to cope with the intermittent, very rapid sea-level rises after the last glacial period. There were phases during which sea level rose at rates up to 40 millimetres per year – 13 times more rapidly than today. If even more CO2 is emitted in the future, with the growing world population and increasing energy consumption, the rate of sea-level rise could increase to as much as 15 millimetres per year by the end of this century. It is conceivable that the corals would be able to keep up with that rate. This observation, however, requires qualification. Due to acidification and the warming of coastal waters, corals are already highly stressed in many regions to the extent that carbonate formation and growth are seriously hampered. It is not yet known whether the corals can keep up with rising sea level under these conditions. Current studies in the USA indicate that coral reefs that are under pressure from stressors such as destructive fishery, disease or water pollution cannot always grow fast enough, and in fact, on the contrary, are even being eroded by breaking waves. In field studies, the present-day state of reefs in Hawaii, off Florida, and in the American Virgin Islands in the Caribbean were compared with their condition in the 1930s, 1960s and 1980s. The comparisons revealed that the reefs have been eroded by 9 to 80 centimetres since the 1930s. The researchers were only able to find actively growing reefs in protected areas or on especially secluded segments of the coasts.
Densely populated coasts, heavy lossesIn its most recently published report the Intergovernmental Panel on Climate Change compiled the results of many scientific publications on the consequences of climate change for populated coastal areas. The results indicate the extent to which livelihoods will be lost. Furthermore, they present an estimate of the financial burden that can be expected in terms of how much coastal protection will cost in the future. It is evident that with the continuing population influx to coastal regions, increasing numbers of people are threatened by extremely intense high-water events. The economic damages will be enormous. Many could lose their homes and property, or even their lives by drowning, drinking polluted water or by epidemics.
Estimates are now available for the numbers of people who will be affected by a 100-year flood, i.e. a flood which is statistically likely to occur on the average every 100 years. In the year 2010 around 270 million coastal residents were at risk globally. In 2050 it will be up to 350 million and in 2100 between 500 and 550 million, based on world population estimates of 9.7 and 11 to 12 billion, respectively. The flooding in 2100, according to the estimates, would likely result in losses of up to 9.3 per cent of the global gross domestic product. Up to 71 billion US dollars would have to be allocated in order to prevent this. Such coastal protection measures are critically needed because even isolated events can cause immense damage.
The extent of damage that can result is illustrated by the destruction caused in 2005 by Hurricane Katrina in the Gulf of Mexico and in 2012 by Hurricane Sandy on the east coast of the USA. US researchers estimate that Katrina caused damage totalling around 150 billion US dollars in the most severely affected states of Louisiana and Mississippi. Hurricane Sandy also caused huge damage in 2012 on the highly developed east coast. Sandy made landfall near New York City, causing damage of up to 50 billion US dollars within a few hours.
With the strength of hurricanes and higher-surging waters in the future, the damage could be even significantly greater if appropriately designed coastal protection systems are not erected. It has been estimated for the US coast of the Gulf of Mexico that, with an average rise in global sea level of 1 metre, along the 750 kilometre stretch between the coastal cities of Mobile and Houston about one-third of all streets would be permanently flooded and 70 per cent of all harbours would be practically useless.
Without massive investments in coastal protection many other coastal regions and cities worldwide will be similarly threatened by flooding. The Intergovernmental Panel on Climate Change notes that the greatest population influx to coastal regions today is occurring in developing countries and newly industrialized countries where coastal protection measures are less well developed. These primarily include India and China, but also Vietnam, Bangladesh and Indonesia, where especially severe losses from high-water levels can be expected. Because protection measures in the form of dikes or dams are rare, it is anticipated that more people with drown in storm floods in coastal regions in the future. Furthermore, the lack of coastal protection will lead to great economic losses, which the weak national economies will scarcely be able to compensate for.