2B.7A: Long-Term Sea Level Change

• Eustatic (global) changes

  Global-scale sea level change caused by a change in the volume of water in the ocean store.
• Isostatic (local) changes

  Local-scale sea level change caused by a change in the level of the land relative to the level of the sea
• How does isostatic change occur?

  • During glacial period, the weight of ice in ice sheets and glaciers adds weight to the world‘s crust - isostatic subsidence
  • Causes the crust to sink lower into the rock (mantle) below
  • Apparent rise in the level of the sea
  • Ice melts at the end of the glacial period and weight is lost from the crust, causing it to slowly rise - post-glacial isostatic adjustment
  • Sea level will then appear to fall
  • However, in the south and east (which was not as covered in ice) the weight of the sediment is causing sinking
• How does eustatic change occur?

  • Decrease in global temperatures leads to more precipitation occurring as snow
  • Eventually this turns to land ice and so water is stored on land rather than being returned to ocean store
  • Global fall in sea level
  • If global temperatures rise, glaciers retreat and ice melts, causing a rise in global sea level
  • Enhanced by thermal expansion
• Thermal expansion

  • The warming of the earth is primarily due to accumulation of heat-trapping greenhouse gases
  • More than 90% of this trapped heat is absorbed by the oceans
  • As this heat is absorbed, ocean temperatures rise and water expands
  • This thermal expansion contributes to an increase in global sea level
• Synoptic link: impact of tectonics

  • Principle tectonic processes responsible for changing the ocean basins are measured in millions of years and are so slow long-term processes that short-term global satellite records do not consider them
  • Tectonic plate movement raises and lowers the lithosphere crust and can reduce/increase the height of the sea levels relative to the land