Why does sea level change?
Long-term geological effects
One of the main long-term geological contributions to sea level is Glacial Isostatic
Adjustment (GIA) - which is also known as Post-Glacial rebound (PGR). The ice sheets of recent ice ages compressed the earth's mantle as they advanced causing the mantle to subside and also to squeeze material out in front of the ice sheet, producing what is known as the "forebulge". Since the ice sheets retreated the mantle has been slowly returning towards its
original configuration. This has produced the following effects:
- regions which were under the ice sheets (e.g. much of northern Eurasia and North America)
are rising - in some cases by up to 7mm/year.
- regions which were on the forebulge (e.g. the east coast of the U.S.) are sinking, typically
at rates of 1mm/year or slightly more.
- regions further away are moving vertically at smaller rates as part of the overall adjustment
that this causes. For example, Australia is rising at ~0.3-0.4 mm/year.
These effects contribute to changes in measured sea level through the vertical movements at
tide gauge sites and also through the change in volume of the ocean basins as this long-term
geological adjustment goes on.
Plate tectonics also contributes, but this is, generally, a much smaller effect
Short-term geological effects
A number of geological processes contribute to short-term changes in measured sea level. A few examples are:
- earthquakes and other small-scale geological events
- sinking of land through compaction of sediments and/or withdrawal of ground water
- sinking of land through withdrawal of oil