Glacial isostasy, mass balance, and ice dynamics

Different geophysical processes, like the glacial isostatic adjustment (GIA) of the solid Earth or recent changes in the ice and ocean surface loads, are superimposed in the area of the Greenland Ice Sheet (GrIS) and its peripheral glacier and ice caps. Subproject 6 is dedicated to the investigation of interactions between these subsystems by means of geodetic data. These data describe changes in the geometry of both the Earth’s crust (in-situ GNSS observations) and the ice sheet (satellite altimetry), the ice mass (satellite gravimetry) as well as glacier frontal positions and ice-flow velocities (satellite remote sensing). Due to the different sensitivity of these data with respect to the involved processes, a combination of data sets allows for a separation of the various effects, in particular ice dynamics and surface processes of the GrIS. Uncertainties of GIA effects still play a key role. In-situ observations of GIA-induced crustal deformations may be used to validate available GIA models and to reduce their uncertainties. This lays the foundation for a more precise determination of the GrIS ice-mass balance.

Results from the combination of satellite gravimetry and altimetry based on the GIA model by Caron et al. (2018). Mass-change rates for the Greenland Ice Sheet (a) and the study area (b) including their uncertainties (c, d). The locations of TU Dresden and GNET GNSS sites are indicated by triangle and circle, respectively. (Kappelsberger et al., 2021)

Predicted vertical deformation rates from the sum of the elastic response derived from the combined mass-change estimates and the respective GIA model: (a) A13 (A et al., 2013); (b) ICE-6G_D (Peltier et al., 2015, 2018); (c) Caron (Caron et al., 2018); (d) Huy3 (Lecavalier et al., 2014); (e) GNET-GIA (Khan et al., 2016). Predictions are displayed against results from GNSS at TU Dresden sites (triangles) and GNET sites (circles) using the same color scale. (Kappelsberger et al., 2021)

What was the main question you aimed to answer during the first project phase?

The main objective of the first project phase was to determine of GIA-induced crustal deformations by means of GNSS measurements in north-east Greenland. This allows for a validation of available GIA models and to identify the model most suitable for the correction of satellite gravimetry data.

Which methods did you use to answer your research question?

  • Deformations of the Earth’s crust were derived from ground-based GNSS observation at bedrock sites. Conducted in 2016 and 2017, the observational campaigns realize repeated observations on sites established and first observed in 2008 and 2009. The differential GNSS analysis also incorporated additional observations at permanent GNSS sites of the GNET project.
  • Elastic deformations were modeled by convoluting Green’s functions with present-day mass changes of high spatial resolution. The latter was derived for the period 2010 - 2017 by combining mass-change estimates inferred from satellite gravimetry data acquired by the GRACE mission and from radar altimetry data provided by CryoSat-2. The combined mass-change data set reveals a spatial resolution of 1.5 x 1.5km² in the study area (Figure 1b).

What were your main results?

  • Vertical crustal deformation rates between 2.8 and 8.9 mm/yr were derived at the GNSS sites in the study area. At these sites, the contribution of the elastic deformation varies between 2.4 and 7.0 mm/yr. Our results were published here: Kappelsberger et al. (2021).
  • The comparison of observed uplift rates and the sum of modeled elastic and GIA-induced deformation rates revealed that the global GIA models ICE-6G_D and A13 overestimate the GIA effect in the study area (Figure 2). All other considered models provide GIA predictions consistent with the observations. In case of the model by Caron et al. (2018) the predictions at the GNSS sites vary between 1.3 and 4.4 mm/yr.
  • By using this model, which was verified over the study area, the mass-change rate of the entire GrIS was estimated to be -233±43 Gt/yr, while -31±19 Gt/yr are attributed to the peripheral glaciers and ice caps.
  • For the drainage basin of the 79°N glacier the mass-change rate is -0.9±0.4 Gt/yr, only.
  • The mass changes derived from the combination of satellite gravimetry and altimetry data (Figure 1) are available under https://doi.pangaea.de/10.1594/PANGAEA.922884.

What are your goals for GROCE-2?

During GROCE-2, subproject 6 focuses on the separation of the processes contributing to ice-mass changes, especially ice dynamics and surface processes. Different models will be utilized to characterize the signal and error content in temporal variations of the surface-mass balance and firn structure. Satellite data products used in the combination approach will be continued and extended by means of data acquired by new satellite missions (gravimetry: GRACE Follow-on, altimetry: ICESat-2, Sentinel-3). We will also continue the GNSS observations, although at a lesser extent.