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Postglacial land uplift

The postglacial land uplift in Sweden affects our geodetic reference frames and is an important component when reducing coordinates to a certain epoch.

Here we refer to postglacial land uplift as the return of the earth crust to its state of equilibrium after having been heavily loaded by the kilometre-thick ice during the last Ice Age. When the ice started to melt approximate 20 000 years ago, the pressure on the earth crust was relieved and the the land started to rise. The last ice disappeared from Scandinavia about 10 000 years ago. Because of the viscous interior of the Earth it will take very long before it has returned to its state of equilibrium. Until now, the land has rised several hundred metres and it is estimated that there are several tens of metres to go. In Sweden, land uplift varies; it is largest in the north along the coast of the Baltic Bay (around 10 mm/year) and smallest in the south (around 1 mm/year).

As early as the 18th century it was observed that the Earth's surface was rising in the Nordic area. At that time it was believed that this was due to a lowering of the sea level and the phenomenon was called 'water diminution'. Following a suggestion by the scientist Anders Celsius, water level marks were cut into cliffs and rocks along the Swedish coast. Today the sea level is automatically recorded by permanent mareograph stations.Land uplift can also be determined by other types of observations, such as repeated levelling, GNSS and observations of ancient shore lines (which now lie on dry land).

Depending on what the land uplift is related to, we distinguish between:

  • Absolute land uplift: land uplift relative to the centre of the Earth
  • Levelled land uplift: land uplift relative to the of climate effects undisturbed sea level (the geoid)
  • Apparent land uplift: land uplift relative to the actual mean sea level.

The postglacial land uplift, which is also accompanied by mass changes inside the earth, also affects the gravity field of the Earth, even though this change is rather slow. This causes both the geoid and gravity to change. This means that gravity values must to be referred to a certain epoch.

The land uplift model NKG2016LU

In 2016 the Nordic Geodetic Commission (NKG) released the land uplift model NKG2016LU. It is used as an official land uplift model in Sweden, as well as the other Nordic and Baltic countries. The model is based on repeated precise levelling and data from permanent GNSS reference stations. For the area outside the Nordic countries the land uplift has been computed using a geophysical model, which is based on theories concerning the thickness of the ice and the properties of the mantle and the crust.

Map showing levelled land uplift according to the NKG2016LU land uplift model.
Levelled land uplift [mm/yr] according to the NKG2016LU land uplift model.

The figure shows levelled land uplift [mm/yr] according to the NKG2016LU land uplift model. By levelled land uplift we refer to the land uplift relative to the of climate effects undisturbed sea level (the geoid).

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