Winners and losers in ocean acidification
While 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 residents
Residents 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 location
For 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 rise
Local 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 wetlands
Many 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 losses
In 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.