GRACE and GRACE-FO constraints on 3D GIA models

Glacial isostatic adjustment is the response of the solid Earth to past ice sheets. It is the largest secular gravity increase in GRACE and GRACE-FO data. Models of GIA are used to correct GRACE observations for ice mass and sea level change, but in areas where GIA is the main signal GRACE data can be used to constrain GIA models. Global GIA models increasingly use 3D variations in viscosity which can have large uncertainties resulting from the conversion of seismic velocity anomalies to viscosity. By ‘anchoring’ the GIA model to GRACE observed values in some regions confidence in its performance in other regions can be increased.

Here, we compare predicted gravity rate from a global GIA model with 3D viscosity to gravity rate in North America and Scandinavia derived from gravity rate trends estimates from GRACE and GRACE-FO data from 2003-2023. 3D viscosity maps are derived from upper mantle model WINTERC-G with a flow law for mantle material olivine, in which material parameters such as grain size and water content are varied. We use a GIA model based on the commercial FEM package ABAQUS and use the ICE-7G ice loading history as forcing.

The GIA simulations mostly result in gravity rates larger than observed for North America, while in Scandinavia they are mostly too small. Both regions prefer a dry mantle rheology, which is expected for the relatively colder mantle in the regions. The best fitting GIA models are used to correct GRACE and GRACE-FO data and estimate global sea level change due to land ice melt of about 1.6 mm/year, which is at the lower end of the predictions for different GIA models including traditional models with only radially varying viscosity.