Glacial isostatic adjustment (GIA) describes the adjustment process of the earth to an equilibrium state when loaded by ice sheets. The ongoing adjustment of the earth's body to the redistribution of ice and water masses is evident in various phenomena, which have been studied to infer the extent and amount of the former ice masses, to reconstruct the sea level during a glacial cycle and to constrain rheological properties of the earth's interior.
... described as deformation of the surface of equal gravitational potential, the geoid. The state of isostatic disequilibrium in regions with past ice-mass loss results in a geoid low; the adjustment process, meaning an influx of mantle material, reduces the geoid low. A well-known example is found in north-eastern Canada, where a geoid low of about 40 m indicates that the earth has not fully regained isostatic equilibrium after the retreat of the Laurentide ice sheet. However, in this region, at least half of the geoid depression is attributed to a density anomaly driving mantle convection (e.g. Mitrovica & Vermeersen, 2002) . The geoid-height changes associated with present-day glacial changes are dominated by the direct attraction of the ice and water masses involved. Changes due to the earth's elastic response are comparatively small.
A well-studied example is the land uplift taking place in the Gulf of Bothnia, Fennoscandia, an area which was depressed by an ice sheet of 2 to 3 km thickness at the LGM. The ongoing adjustment following the ice sheet's retreat has been monitored by GPS studies, such as the Baseline Inferences for Fennoscandian Rebound Observations, Sea Level and Tectonics (BIFROST) project (Davis & members of BIFROST, 1996; Scherneck et al., 2003). The results determine the vertical and horizontal surface displacements, indicating land-uplift rates of up to 8 mm/a close to the former load centre. In contrast, the deformations in Antarctica has to be separated into recent ice-thickness changes and GIA induced vertical motion.
... where, in the formerly glaciated regions, the sea-level falls at rates up to 1 cm/yr. But the relative sea level is rising in the surrounding areas like the Netherlands by a few mm/yr. The present melting of Greenland results in a global sea-level rise. But in the vicinity of Greenland, the sea level drops.
... which is the motion of the earth's centre of mass relative to the earth's surface. For this quantity, GIA contributes the dominant secular motion which is directed towards NE America with less than 1 mm/yr velocity, but which is masked by seasonal mass redistributions in ocean, hydrology and atmosphere.
... axis with respect to an earth-fixed system: the true polar wander (TPW) and the duration of a single rotation: the length of day (LOD). Mass redistributions within and on the earth change the inertia tensor of the earth, which is visible as TPW. The LOD is proportional to the moment of inertia about the earth's spin axis, which, in turn, is proportional to the zonal harmonic of degree two of the geopotential. At present, this results in an acceleration of the earth's rotation, i.e. a shortening of the LOD (e.g. Nakada & Okuno, 2003). However, GIA-induced variations of the earth's rotation parameters are masked by other processes, e.g. the convective flow in the mantle, the direct effect of the present redistribution of ice and water masses and changes in the pressure fields at the core-mantle boundary (z.B. Mitrovica & Vermeersen, 2002).
Climate dynamic on millenial time scale is affected by changes of ice sheets and of sea level. Both act as loads which deform the solid earth. Vice versa, the deformation affects climate relevant processes in the atmosphere (change in topography), in the ocean (change of bathymetry and coast line) and in the ice sheets (change of sea level at ice margin and bedrock topography).
In response to climate variations, we focus on the numerical modelling of ice-sheet dynamics, assess how the solid earth responses to the respective surface loading and consider, gravitationally consistent, the sea-level change due to mass conservation.