To understand isostatic changes, you need to consider the fact that huge amounts of water can be stored as ice during colder periods in Earth’s history (many times more than today). When the planet warms and ice melts, this water is returned to the ocean basins (causing a rise in sea level). When ice sheets and glaciers covered the land during the ice ages of the Pleistocene, the weight of the ice depressed the elevation of the land. Over the 20,000 years since the last glacial maximum, the land masses, relieved of their burden of ice, have gradually rebounded. This rebound is called Glacial Isostatic Adjustment or GIA. The level of the land relative to the sea level increases. This can cause a regional sea level change effect and is still impacting parts of Alaska and other northern coasts. These are the emergent coasts we met in Module 2.
This short, but silent (1:40) video animation illustrates how changes in sea and land level take place in response to the onset and departure of glacial conditions, and the melting of polar ice as the planet warms. It also documents the erosion of sediment from the land and deposition in the ocean basin at each sea level stand. This erosion leaves a signature of each sea level (the erosional notch shown), which is evidence of these changes.
Sea level changes over time. Video shows how water level changes compare to land.
Before the last ice age (more than 30,000 years ago): erosion notch one forms.
During the last ice age: sea level drops, ice forms on land. The Ice is 1 mile thick and erosion notch 2 forms below notch 1 and a new sediment layer forms.
The weight of the ice pushes down on the land: forming erosion notch 3 between notch 1 and 2 and a new sediment layer forms.
As the ice melts: the sea level rises forming erosion notch 4 above notch 1 and a new sediment layer forms.
With no ice to hold it down the land begins to rise again (rebound)…and it’s still rising very slowly: Erosion notch 5 forms between notch 1 and 4 and a new sediment layer forms.
Polar ice caps melt: sea level rises as a result of melting ice. Erosion notch six forms above all other notches. New sediment layer forms.
Albedo Feedback Mechanism
Albedo is a measure of the reflectivity of the Earth's surface. Ice-albedo feedback is a strong positive feedback in the climate system. Warmer temperatures melt persistent ice masses in high elevations and upper latitudes. Ice reflects some of the solar energy back to space because it is highly reflective. If an equivalent area of ice is replaced by water or land, the lower albedo value reflects less and absorbs more energy, resulting in a warmer Earth. This effect is currently taking place - for example, as the Greenland ice sheet melts there is less bright white, reflective ice and more, darker less reflective water and land surfaces. This decreases the albedo effect and increases warming. Conversely, cooling tends to increase ice cover and hence the albedo, reducing the amount of solar energy absorbed and leading to more cooling.