GRACE detects the total signal of the changes in the gravitational field. These changes are caused due to mass redistributions within the entire Earth system. The geosciences want to understand the various causes of the redistributions and to identify the individual processes responsible for them. One of these processes is glacial isostatic adjustment (GIA), i.e. the redistribution of mass in the Earth's interior due to changing loads on the Earth's surface.
Dr. Meike Bagge, GFZ
Signal separation and uncertainties
It is important to identify, quantify and separate these mass changes caused by GIA in the GRACE data. The less accurate the correction of the GIA signal is, the more difficult it is to interpret the remaining measured GRACE signal. For example, if GRACE measures a decrease in mass in the Antarctic, this can be related both to mass transport in the Earth's interior and to the loss of ice mass from the ice sheets. In most cases, the measured signal is a combination of both (and other) processes. This is because the melting of the ice sheets and the associated reduced load leads to time-delayed mass redistributions in the Earth's interior and a subsequent uplift of the Earth's surface.
Computer models for estimating the GIA signal
With geodetic observations we can measure the movements of the Earth's surface, but the mass redistributions in the Earth's interior are not directly accessible to us. In order to estimate the GIA-induced mass redistributions, we need computer models. The standard model currently used for the GIA correction of GRACE data is that of Peltier et al. (ICE-6G(vm5)), which calculates the GIA signal using observational data and a simple radially symmetric (i.e. one-dimensional) Earth structure. However, this and other GIA models have large uncertainties.
GIA model ensemble for estimating uncertainties
Using the numerical model VILMA developed at the GFZ, we can simulate the global GIA mass redistributions of the solid Earth and also include more complex 3D Earth structures (Figure 1).
The GIA deformation of the solid Earth depends in particular on two parameters: The temporal and spatial distribution of the ice sheets, which describes the load history, and the rheology of the solid Earth, which controls the time-delayed response of the solid Earth to the load history. Both parameters are not precisely known, but there are indications from geology, for example, about the temporal and spatial expansion of the ice sheets, or from seismology, about the structure of the Earth's interior.
Our GIA model ensemble considers various load histories from the literature as well as different 3D Earth structures that we have developed at the GFZ. We used a seismic tomography model and considered uncertainties for the conversion from seismic velocities via temperatures to viscosities, resulting in an ensemble of 3D Earth structures. Figure 3 shows the geoid change due to GIA, calculated using the GIA model ensemble.
The GIA model results of the ensemble can help to better estimate the uncertainties in the interpretation of the residual GRACE signal.
Further literature
- Bagge, M., Klemann, V., Steinberger, B., Latinović, M., & Thomas, M. (2021): Glacial-Isostatic Adjustment Models Using Geodynamically Constrained 3D Earth Structures. Geochemistry, Geophysics, Geosystems, 22(11), e2021GC009853. https://doi.org/10.1029/2021GC009853
- Eicker, A., Schawohl, L., Middendorf, K., Bagge, M., Jensen, L., Dobslaw, H.: Influence of GIA Uncertainty on ClimateIn contrast to weather, which refers to daily or very short-term events, climate refers to an average condition in the atmosphere over a longer period of 30 to 40 years. All processes such as average temperature, precipitation, wind direction, wind s... Model Evaluation with GRACE/GRACE-FO Satellite Gravimetry Data, JGR: Solid Earth, in revision.