What is Glacial Isostatic Adjustment (GIA)?
Ice ages are periods of long-term reduction in Earth’s temperature, resulting in an expansion of the continental and polar ice sheets and mountain glaciers. They are related to (but not fully explained) by the three Milankovich cycles describing the eccentricity, precession, and tilt of the Earth relative to the ecliptic. The most recent global deglaciation event, which marked the end of the most recent 100 kyr ice age cycle of the late Quaternary period, began only 21,000 calendar years ago (Peltier, 2004), just before the Milankovitch cycle, and was essentially complete about 6000 years ago. However, sea levels relative to land have continued to change to this day due to this cause. How can that be? This continuing variation of land and sea levels exists as a consequence of the solid Earth's delayed viscoelastic adjustment in response to the redistribution of mass on its surface that accompanied deglaciation. The memory effect in the Earth’s mantle is akin to that occurring in cold honey after a spoon creates a depression in its surface, and the honey subsequently returns to a flat surface under gravity driven viscous flow. It may take the honey as much as a half an hour to return to being flat, depending on how cold it is. This is exactly what is happening to the Earth as it seeks to reestablish gravitational equilibrium (like the flat honey surface). Because mantle viscosity is so high, the Earth’s gravity field is still slowly recovering from the load of the past ice and sea-level change.
In regions that were previously ice-covered, such as Canada and Northwestern Europe, relative sea level continues to fall at a rate that is primarily determined by the ongoing glacial isostatic rebound of the crust and which may exceed 10 mm/yr (in the southeast Hudson Bay region of Canada, this rate is near 11 mm/yr). In regions that were previously ice covered, the solid Earth rise actually causes coastal sea levels to fall. However, in regions located in the periphery of the last ice age’s ice masses, the solid earth experiences what is known as ‘forebulge collapse’. For example, the US East coast, and even the Gulf of Mexico, have been sinking due to GIA since the last ice age, and will continue to do so as a consequence of ongoing, viscoelastic glacial isostatic adjustment (e.g., Peltier, 1999; A et al., 2013; Caron et al., 2018). Consequently, along the coast of the US from southern Delaware to Corpus Christi, Texas, relative sea levels are rising due to GIA, compounding contemporary sea level rise from melting glaciers and ocean warming.
GIA affects the Earth's contemporary gravity field
The redistribution of interior Earth mass, still adjusting from the glacial loading of the last ice age, produces long term ('secular') trends in the Earth's gravity field, even today. The GRACE(-FO) measurements record a geopotential change at the satellite altitude, which can be inverted to yield Earth’s geopotential and geoid change map. To compute mass trends from the satellite gravimetry observations of GRACE(-FO) and interpret them as contemporary surface mass changes in the hydrosphere (i.e., trends in water content of hydrologic basins, or ocean bottom pressure, or ice sheet mass), the gravity effects of Glacial Isostatic Adjustment signal must be removed (e.g., A et al., 2013).
The GRACE(-FO)-Tellus mass grids described elsewhere in this website have had a reasonable GIA model of secular geopotential trends removed (see file header for the specific GIA model removed). GIA causes a physical deformation of the lithosphere, mantle and core, and changes Earth’s geoid (expressed as a rate height change in mm/yr). The GIA gravitational effect can also be expressed as changes of an apparent equivalent surface mass distribution that would cause a similar gravity change if the mass were concentrated at the surface, in mm/yr of equivalent water thickness. In this latter case it is important to realize that such a ‘GIA surface mass equivalent’ is not an actual physical mass that exists at the surface – it is merely a hypothetical equivalent mass that would yield the same geopotential effect when measured from space. While not physical, it is useful to assess the magnitude of the GIA trend in this way relative to the residual geophysical quantity of interest (typically water height change in a hydrological basin) to gauge to impact of the GIA correction on geophysical signal of interest.
Is GIA an error in GRACE(-FO) observations?
GIA is not an error in the observations. It is a signal of great scientific interest in itself. GRACE(-FO) observations, in particular when combined with paleo-sea-level records, GPS measurements of vertical surface deformation, have provided new and more accurate estimates of contemporary GIA, and have led to refinements of ice-load histories and GIA models (e.g., Tamisiea et al., 2007; A et al., 2013; Caron et al., 2018). The two main ingredients in GIA models are (1) the ice (deglaciation) history, and (2) the viscosity profile of the mantle. The GIA corrections add uncertainty to inferred surface mass trends over the GRACE(-FO) period; a canonical (and rather heuristic) uncertainty range of 20% has often been assumed for GIA models, based on results for various viscosity values and alternative deglaciation models for Antarctica and Greenland. Near the former center of the Fennoscandian Ice Sheet, uncertainties are considerably smaller due to the spatial and temporal density of terrestrial observations (Wolf, 1993). More recently, Caron et al. (2018) have adapted new GIA modeling and inversion approaches that enable a physical uncertainty quantification of GIA model errors and uncertainty ranges across a range of realistic GIA model parameters. GIA models continue to be refined and improved.
GIA corrections in GRACE(-FO) Tellus Level-3 data
The GRACE-Tellus gridded data (Level-3) have already been corrected with a GIA forward model (see header information in Level-3 data files for GIA model details) – no additional GIA correction is necessary. These corrections conveniently apply to problems of hydrology, oceanographic and/or cryospheric science. Nonetheless, as noted above, GIA corrections have a range of uncertainty. To explore the typical impact of different GIA models on the inferred surface mass rates, we provide the ‘apparent GIA surface mass equivalent’ in gridded format, with Gaussian smoothing radii that are commensurate with the Tellus Level-3 post-processing parameters (i.e., GIA in mascon bins, and with 300 km / 500 km Gaussian smoothing). A typical use case could be to difference two GIA models’ apparent GIA surface mass equivalent grids to assess the sensitivity of the inferred hydrosphere surface mass rates in the GRACE-(FO) data. The data and browse images can be accessed here. It is also advised that there is currently no accommodation for GIA corrections that may be applied to areas sustaining post-glacial uplift due to the collapse of the Little Ice Age ice mass (Grove 1988) that once covered areas such as in Patagonia or Alaska.
While it is algebraically possible to ‘swap’ two different GIA models in the GRACE(-FO) Tellus Level-3 grids with the gridded data provided here, care must be taken that consistent postprocessing filters have been applied to all GIA models and the GRACE data (i.e., same spatial smoothing filter). Even then, small inconsistencies can arise due to nonlinearities in the typical GRACE(-FO) postprocessing (i.e., destriping, CRI filter, geocenter corrections) and we strongly encourage users to explore GIA model differences, uncertainties and sensitivities in the geopotential domain (Level-2) rather than in the gridded surface mass domain (Level-3).
Acknowledgement and Citation
When using these data, please acknowledge receiving the data from "http://grace.jpl.nasa.gov", and reference the corresponding GIA model papers as follows:
Peltier, W. R., R. Drummond, and K. Roy (2012), Comment on “Ocean mass from GRACE GRACE and glacial isostatic adjustment” by D. P. Chambers et al., J. Geophys. Res., 117, B11403, doi:10.1029/2011JB008967.
Peltier, W.R., 2004. Global Glacial Isostasy and the Surface of the Ice-Age Earth: The ICE-5G(VM2) model and GRACE, Ann. Rev. Earth Planet. Sci., 32, 111-149.
A, G., J. Wahr, and S. Zhong (2013) "Computations of the viscoelastic response of a 3-D compressible Earth to surface loading: an application to Glacial Isostatic Adjustment in Antarctica and Canada", Geophys. J. Int., 192, 557–572, doi: 10.1093/gji/ggs030.
Peltier, W.R., Argus, D.F. and Drummond, R. (2018) Comment on "An Assessment of the ICE-6G_C (VM5a) Glacial Isostatic Adjustment Model" by Purcell et al. J. Geophys. Res. Solid Earth, 123, 2019-2018, doi:10.1002/2016JB013844.
Caron, L., Ivins, E. R., Larour, E., Adhikari, S., Nilsson, J., & Blewitt, G. (2018). GIA model statistics for GRACE hydrology, cryosphere, and ocean science. Geophysical Research Letters, 45, 2203–2212. https://doi.org/10.1002/2017GL076644
Additional References & Resources:
Ivins, E. R., and T. S. James (2005), Antarctic glacial isostatic adjustment: A new assessment, Antarct. Sci., 17, 541–553.
Grove, J.M. 1988: The Little Ice Age, London: Routledge, 498 pp.
Mitrovica, J.X., J. Wahr, I. Matsuyama, and A. Paulson. The rotational stability of an Ice Age Earth, Geophys. J. Int., 161, 491-506, 2005.
Peltier, W.R., Ice-Age paleotopographie, Science 265 (5169): 195-201, 1994.
Tamisiea, ME; Mitrovica, JX; Davis, JL , 2007. GRACE gravity data constrain ancient ice geometries and continental dynamics over Laurentia. Science, 316, 881 - 883.
Wolf, D., (1993) The changing role of the lithosphere in models of glacial isostasy: a historical review, Global and Planetary Change, 8, (3), 95-106.