Sea-level rise

The volume of water in the world's oceans has varied considerably over geological time, influenced by the quantity of ice bound up as ice on land; and the temperature of the oceanic waters. During the last ice age sea levels were as much as 150m lower than today due to the very large volumes of water held on land in ice-sheets, combined with the lower temperatures of oceanic water (making it denser). The actual observation of sea level rise in any one place is somewhat confused by movements in the level of the earth's surface (isostatic movements) which mean that, in different areas, land may be sinking or rising relative to sea level.

By the end of the 20th century it was estimated that global average sea level had risen by 10-20cm, and a further rise of 9-88cm has been predicted by 2100. Such changes in sea level have little or no impact on oceanic, pelagic or deep benthic ecoystems. They are, however likely to be of increasing significance in the shallowest of benthic communities and even more so on intertidal communities. Coral reefs and seagrass beds may be particularly threatened as these are dependant on high levels of solar radiation, whilst corals in particular may already have reduced growth rates resulting from chemical changes .

Intertidally, flooding and erosion are both likely to result from rising sea levels. On average it has been estimated that a 1 cm rise in sea level erodes beaches about 1 m horizontally. In reality this figure varies based on the profile both of the beach and of the offshore waters over a considerable distance. Rising sea levels will further reduce friction from the underlying sediments, increasing the impacts of storm surges. This influence will clearly affect coastal development, coastal agriculture, and, importantly, coastal ecosystems including beaches, saltmarshes and tidal flats, and mangrove forests. Natural ecosystems typically back directly onto human landscapes and natural retreat of coastal ecosystems is typically limited.