- Eustatic sea level change – caused by a change in the volume of water in the sea, or by a change in the shape of the ocean basins.
- Causes:
- Climate change.
- Tectonic movements.
- Isostatic sea level change – caused by vertical movements of the land relative to the sea.
- Causes:
- Uplift or depression of the Earth’s crust due to accumulation or meling of ice sheets.
- Subsidence of land due to shrinkage after abstraction of groundwater.
- Tectonic processes.
- Sea level has risen in the last 10,000 years:
- Sea level varies daily with the tidal cycle. Onshore winds and low atmospheric pressure systems cause the sea surface to rise temporarily.
- During the last glacial period, water was stored in ice sheets, so sea level was lower than present. At the last glacial maximum, sea level was about 130m lower than present.
- As temperatures started to rise, ice sheets melted, and sea level rose rapidly.
- Over the last 4,000 years, sea level has fluctuated around its present value.
- Since about 1930, sea level has been rising.
Climate Change Causes Changes in Sea Level
- Last century: global temperature has increased rapidly – global warming.
- Temperature increase has been very fast; changes in climate are a result of human activities.
- Activities increase the concentration of greenhouse gases in the atmosphere. Concentration increases, more energy is trapped with global warming.
- Rising temperatures cause increased sea level, through melting of ice sheets and thermal expansion of water.
- Global sea level is currently rising at almost 2mm a year; predicted to increase to 8-16mm a year by 2100.
Climate Change Has Impacts on Coastal Areas
- Storms are likely to be more frequent and more intense.
- More frequent and more severe coastal flooding of low-lying areas.
- Submergence of low-lying islands.
- Changes in the coastline, islands are created.
- Contamination of water sources and farmland – salt water may enter bodies of fresh water.
- Sea level fall results in coastlines of emergence:
- Landforms can be created:
- Raised beaches – when the fall in the sea leaves beaches above the high tide mark.
- Wave-cut platforms are exposed.
- Cliffs above raised beaches are no longer eroded and slowly get covered by vegetation – relict cliffs.
Sea Level Rise Result in Coastlines of Submergence
- Rias:
- Formed where river valleys are partially submerged.
- Gentle, long- and cross-profiles.
- Wide and deep at their mouth.
- Fjords:
- Drowned glacial valleys rather than drowned river valleys.
- Straight and narrow with steep sides.
- Shallow mouth formed by deposition of material.
- Dalmatian coasts:
- In areas where valleys lie parallel to the coast, an increase in sea level forms a dalmatian coastline.
- Valleys are flooded, leaving islands parallel to the coastline.
- Processes create and alter landforms and landscapes over time:
- Individual landforms combine to form landscapes.
- Processes operating in coastal systems can create new landforms or change existing landforms:
- A change in one factor can lead to changes in others.
- Relict landforms can still experience coastal processes.
- Coastal landscapes are therefore often made up of a mixture of active and relict landforms that reflect different periods of change.
- Changes occur over a range of spatial scales and temporal scales.