Eustatic refers to worldwide variations of sea level resulting from climate (and so hydrological cycle) change. For example, during an Ice Age more precipitation falls as snow. Therefore, water cycled from sea to land (through evaporation, condensation and then precipitation) is now stored as snow and ice and does not return to the oceans. Consequently, sea levels fall. When the glaciers and ice sheets melt, sea levels rise again.
Isostatic adjustments may result from tectonic uplift or changes caused by the enormous weight of ice depressing the crust during an Ice Age, only for the land subsequently to uplift (rebound) on melting.
Eustatic and Isostatic Changes
Both eustatic and isostatic changes can occur at the same time, resulting in the flooding of the existing coastline or exposure of new land previously covered by the sea. For example, if isostatic recovery is faster than eustatic sea level rise, then emergent features are produced. If, however, eustatic rises dominate, then submergent features result.
While current temperatures and sea levels can be measured across the globe, the prediction of future changes using computer global climate models (GCMs) is less certain. GCMs are highly sophisticated but not infallible. By simulating atmospheric conditions using past climatic records, ‘proxy’ evidence from ice cores and a wealth of corroborative information, they allow researchers to predict future climatic scenarios within broad ranges. These projections depend upon data input. Very often ‘best’ and ‘worse case’ scenarios are published, which frequently allow those using the data to select the scenario best suited to their point of view, and so bias is introduced.