WOR 1 Living with the oceans. A report on the state of the world’s oceans | 2010

Altering the coasts


Climate change alters the coasts

In order to accurately predict the future fate of coastal regions, researchers must first determine whether the present measurable changes are actually a consequence of climate change or an expression of natural climate variability. We can only speak of climate change if climate-related changes are statistically discernable from natural fluctuations. Climate change is thus not equivalent to climate variability. Scientists hence need to have measurements and observations covering representative time intervals. We already know that global warming will not lead to uniform increases of air and water temperatures everywhere, and the change is not restricted to temperature changes. The consequences of climate change can be highly variable. This is clearly illustrated in the following examples.

Melting of sea ice and
thawing of permafrost grounds

Sea ice in the sub-polar and polar coastal waters acts as a buffer between the atmosphere and seawater. It prevents storms from creating waves that would roll into the coasts and remove sediments. If the ice masses shrink by thawing, this buffering effect is lost. Sediments that were previously protected by the ice cover are also more strongly eroded. Permanently hard-frozen soils, called permafrost grounds, thaw out. These are also more easily eroded by wind and waves on the coasts than the frozen land masses. On the other hand, erosion typically caused by icebergs and glaciers no longer occurs.

Changes in freshwater balance, precipitation and sediment input

Climate change will presumably lead to the melting of continental glaciers, while at the same time the amount of new snow required for maintaining the glaciers will probably decrease. Over time this will also lead to a decline in the amount of freshwater flowing from the mountains. Water shortages could result. People could respond by increasing the amount of water held in reservoirs. This, however, would result in less fresh water and sediments being transported to the ocean. At the same time, in other areas, increased precipitation is expected as a consequence of global warming, for example, in the monsoon regions of the world. The strong monsoon rains and water discharge will lead to increased flooding and the transport of large amounts of sediments and nutrients by rivers into the coastal seas.

High tide and spring tides Mean high tide refers to the average high-water level at a particular location on the coast. Flood tides that reach especially high above the mean high tide are referred to as spring tides. These occur regularly, corresponding to certain alignments of the sun and moon. It is particularly dangerous for the German North Sea coast when heavy storms from the west coincide with the spring tides.

Island and coastal flooding

The rise in sea level caused by climate change will lead to flooding in many coastal areas and island groups. It is expected that these regions will not be just temporarily, but permanently submerged. These floods thus cannot be equated with the short-term, episodic flooding of land areas that will occur more frequently in the near future. Sea-level rise could reach the two-metre mark as early as the next century or soon thereafter. This scenario, however, is based on topographic data alone. It does not take account of dikes and other protective structures. Simulations simply allow the water to flow over the coastal contours. This model also does not consider the increased removal of land by coastal ero- sion, which will probably accompany rising sea level. Complete coastlines, along with their surf zones, will likely shift landward due to the erosion. The destructive power of the water will then be unleashed on areas that were previously protected. Today, storm floods are al- ready tearing out protective vegetation. These negative effects will only intensify in the future. The very gently rising coastal slopes, where the surf can gradually roll up to the land, are becoming steeper. The steeper coastal foreland presents a greater surface area of vulnerability for future storms to assault. Erosion gains dynamic momentum. The buffering capacity of the coastal fore­land decreases. Threatened regions also include many areas that are presently protected by dikes. For dike structures on the North and Baltic Seas the crown height is designed to be 30 to 90 cm above the maximum storm event height, as a safety factor in consideration of future sea-level rise. This will not be sufficient, however, for a sea-level rise of 2 metres. Many densely settled areas in the North Sea area today already lie below the mean high-tide level, and in the Baltic Sea area at around the present-day sea level.
Other coastal areas are characterized by complex and important ecosystems. These produce biomass that sometimes has a direct impact on the shape of the coast. For example, the growth of corals can create new islands. Coral banks are also important bulwarks that break the surf. In some cases the growth of corals can even compensate for the rise of sea level. Whether the formation of new corals will be able to keep up with the rise of sea level in the future depends on the rate of the rise as well as on water temperature. Researchers are concerned that living conditions for the adaptable but sensitive corals will become worse; firstly, because the water temperature in some areas is already too high today and, secondly, because the corals will not be able to keep up with the projected sea-level rise or possible subsidence of the coasts. Other shallow coastal segments, such as estuarine areas, mangroves and marshes, which themselves provide natural protection against storm floods, are also threatened by flooding. In case the mangroves and marshes sink, waves can encroach onto the land and cause considerable damage. Only in very rare and uncommon cases could such changes be compensated by sediment input from inland.
3.11 > The storm surge of 1976 is notable as the most severe storm flood ever recorded on the German North Sea coast, and failure of the dike, as seen here on the Haseldorfer Marsh on the Elbe River, caused extremely severe damage. The water level in Cuxhaven reached a record high of 5.1 metres above normal. Nonetheless, the consequences were less severe than those of the flood of 1962 because many segments of the dike had been reinforced in the meantime.
3.11 > The storm surge of 1976 is notable as the most severe storm flood ever recorded on the German North Sea coast, and failure of the dike, as seen here on the Haseldorfer Marsh on the Elbe River, caused extremely severe damage. The water level in Cuxhaven reached a record high of 5.1 metres above normal. Nonetheless, the consequences were less severe than those of the flood of 1962 because many segments of the dike had been reinforced in the meantime. © Werner Baum/dpa Picture-Alliance

Extreme water levels

It is now believed that extreme weather events such as tropical storms and storm surges will occur more frequently due to global warming. These will likely intensify the impacts of sea-level rise because when sea level is higher the destructive capacity of a storm on the coast is much greater. Scientists expect to see increased storm activity particularly in the temperate and tropical regions. There is still no consensus as to whether the frequency of storm activity will increase worldwide, because dif- ferent scientific computer models and measurement data have yielded different results.
Storm floods originate through the interplay of storm systems and tides. When storm winds push the water toward the coasts during flood tide, especially during spring flood, the risk of flooding for large land areas is greatly increased. Such storms can last for several days and cause the water to rise so high that it does not abate even during the ebb tide. Storms can also have severe effects in marginal seas such as the Baltic Sea, where the tidal ranges are minimal. Just like in a bathtub, the wind piles water masses up in one part of the basin, and when the wind abates or changes direction they slosh back. If the wind then blows in the opposite direction, the two factors can amplify the effect. As a consequence the water level on the German Baltic Sea coast can rise by more than 3 metres. Heavy precipitation can even intensify this situation because the rainwater or high water from the rivers cannot flow off due to the already high water level on the coast.

Increasing incidence of high water

There are other factors related to sea-level rise besides just increased water levels. It is of critical importance that unusually high storm-surge levels are occurring more often, as the example of the threat for Germany illustrates. With a sea-level rise of 1 metre, dangerous storm surges will occur more often because the base level is then a metre higher. In this case, a 100-year highwater level, like the storm flood of 1976 on the German North Sea coast, could take place every ten years. The probability of recurring severe storm surges would increase significantly. On the German Baltic Sea coast with its lower storm-flood levels, this effect would be even more pronounced. A 100-year high-water level with an elevation of 2.5 metres above mean sea level (German: Normalnull, NN) would occur every 2 to 5 years there. Textende