A term used rather sloppily to describe an important process of Geomorphology. In its purest meaning, "Sea level change" refers to the variation of the equipotential surface representing the level of the Earth's oceans through time. Two factors affect this surface:
- The amount of liquid water on the Earth in its ground state (i.e. in the Earth's oceans). The principal process affecting this amount is global climate change, most importantly Earth's cycle of glaciation. During an ice age, vast amounts of water are locked into ice sheets, making the level of the oceans drop, exposing continental shelves; as the ice sheets melt, sea level rises to cover the continental shelves. Recent findings of global warming raise the likelihood of sea level change from the melting of the Greenland and Antarctic ice sheets.
- The size of the world ocean basin, dependent upon how much continental crust there is. Of course, this factor is far less imporant in the short term (over a few million years or so), as it takes a very long time for the amount of continental crust to change enough to squeeze the oceans into a smaller basin.
- Thermal expansion/contraction of sea water is thought by some to contribute up to half of all sea level rise/fall in an episode of global warming/cooling. This is an extremely complicated effect, dependent upon the way heat diffuses through the world ocean, and the way that different layers of the ocean interact.
However, "sea level change" is often used to mean variations in the surface of the ocean relative to land in some localized area. Other processes affect these levels:
- Tectonic uplift or subsidence from the buckling of tectonic plates;
- Isostasy, reacting to large lava flows, denudation, or the advance and retreat of continental ice sheets;
- localized fault activity, which can produce dramatic changes in a short period of time (witness the demise of the people who drew the Nazca Lines).
It is often difficult to distinguish between global causes of sea level change and local ones. The ice age cycle provides us with a perfect illustration: Global changes from the advance and retreat of ice sheets are complicated with regional changes from isostasy. Northern North America and Scandinavia are still rebounding from the last Ice Age, while the Mid-Atlantic is falling. All the while, glaciers everywhere are getting smaller and smaller.
It is, after all, usually pointless to make a distinction between these global and local causes, as they have the same geographic effects. As the shoreline moves around:
- More and more of the land is exposed to the forces of coastal erosion. Wave-cut terraces and fossil beaches are the result.
- The transition zone between the transportation of sediment and its deposition changes. New river deltas are formed. The various zones of particle sorting governing the formation of different types of sedimentary rock migrate. Barrier islands move inland as sea level rises, or are left high and dry when it falls, and quickly erode away.
- Islands, as well as vast expanses of continent, can be drowned or exposed. Numerous drowned Coral reefs mark the former locations of atolls and islands. 50,000 years ago, Great Britain was connected to the European continent; and Indonesia was a continent, rather than thousands of islands. 12 million years ago, the opening to the Mediterranean Sea was closed off and it dried up. 200 million years ago, a vast shallow ocean covered central North America; in its place today, we have the Great Plains and the Rocky Mountains.
- Coastal ecosystems are forced to migrate, die off, or evolve into something else. 50,000 years ago, the floodplain of the lower Susquehanna River was a dry savanna, far different from the marshy, temperate Chesapaeake Bay that is there now. Since the last Ice Age, the northern end of the Persian Gulf has retreated southwards. The marshy areas of Iraq that were once a cradle of civilization are now barren desert, and the Shatt al-Arab now flows across a marshy plain that was once open sea.
|
