Uncertainties in GIA estimates from forward models contribute significantly to the uncertainties in ice-sheet mass balance and sea level budget studies. These models rely on a past ice loading history and a lower mantle viscosity structure, both of which are, and will remain in the future, poorly constrained. In addition, models that can address lateral variations in mantle viscosity (i.e. 3-D) are in their infancy. An alternative approach is to seek a data driven solution to GIA, with the aid of vertical land movement (VLM) estimates from GPS and gravity data from GRACE. The challenges of obtaining a data-driven GIA estimate include solving a geophysical inversion that does not have a unique solution and often requires a-prior information and several approximations to achieve. For example, VLM is not only due to GIA and present day mass loading (PDML) but also to local processes including tectonics and subsidence, while GRACE measures some, generally, unknown combination of signals due to GIA and PDML.
In this work, a novel geophysical framework is developed to utilise GPS and GRACE data to solve for GIA. Our method relies on geophysical relations between geopotential and vertical land movement (VLM) caused by GIA and PDML. For example, the elastic response of the solid Earth to a positive PDML results in a negative VLM, while a positive GIA mass signal in GRACE leads to a positive VLM. We use these relations to express GPS observed VLM and GRACE observed gravity field anomalies, expressed as VLM, in terms of the sum of GIA and PDML. The method is first tested in a closed-loop synthetic experiment where we impose a uniform synthetic GPS coverage globally. It is then applied to the Nevada Geophysical Lab database of GPS VLM and GRACE spherical harmonic coefficients provided by ITG Graz. Here, we define GIA as any viscous response of the mantle to past loading, irrespective of when that took place. The resulting GIA estimate differs significantly from commonly used GIA models (such as ICE-6G) over, in particular, Alaska and central Greenland. Since the forward models do not consider viscous deformation after the end of the Little Ice Age, they will underestimate this term over Alaska and potentially in Greenland where there is evidence for a monotonic decline in volume since 1900 (Kjeldsen, Korsgaard et al. 2015) although the mantle response is expected to be slower than for Alaska. We also discuss the uncertainties, caveats and limitations of our method and its implications. The GIA product is publicly available at one degree grid resolution.
Kjeldsen, K. K., N. J. Korsgaard, A. A. Bjork, S. A. Khan, J. E. Box, S. Funder, N. K. Larsen, J. L. Bamber, W. Colgan, M. van den Broeke, M. L. Siggaard-Andersen, C. Nuth, A. Schomacker, C. S. Andresen, E. Willerslev and K. H. Kjaer (2015). "Spatial and temporal distribution of mass loss from the Greenland Ice Sheet since AD 1900." Nature 528(7582): 396.