1. Glacial Isostatic Adjustment Contributions to Tide Gauge, Altimetry and GRACE Observations Glenn Milne Dept of Earth Sciences University of Durham, UK Contributors: Mark Tamisiea, Konstantin Latychev, Erik Ivins, Philippe Huybrechts, Jerry Mitrovica, Bert Vermeersen. Edited version: original slides 12-19 removed

2. http://www.ngdc.noaa.gov/paleo/slides/slideset/index11.htm

3. G LACIAL I SOSTATIC A DJUSTMENT Surface Mass Redistribution EarthEarth Response Relative sea level Geopotential Rotation vector 3D solid surface deformation Model Surface load + Rotational potential Rheological Earth model Better understanding of GIA process Constraints on Earth rheology Constraints on surface mass redistribution

4. Earth ForcingEarth Model Rotational potential Euler equations Surface loading Ice Model Multidisciplinary approach Ocean Model Sea-level equation GeometryRheology Spherical/Flat Internal structure: 1D & 3D Viscoelastic Linear and non- linear viscous deformation Key Elements of a GIA Model

5. Tamisiea et al., 2003 GIA Response: Driven by Contemporary and Past Mass Flux

6. How Can the GIA Community Contribute to a Better Understanding of Recent Sea- Level Changes? Climate change SEA-LEVEL FORCINGS Anthropogenic effects Ocean dynamics Solid Earth motion Ocean- atmosphere interaction SEA-LEVEL OBSERVATIONS Proxy recordsTide Gauges Satellite Gravity Satellite Altimetry Ice-ocean mass exchange Ocean warming

7. Viscous “Memory” of Solid Earth to Past Ice-Ocean Mass Flux

8. A Mitrovica and Milne (2002) B

9. The Influence of Variations in Earth Model Viscosity Structure on Observations of Sea-Level Change How sensitive is the GIA signal associated with past ice-ocean mass flux to changes in Earth model viscosity structure? Consider the “correction” to be applied to tide gauges, satellite altimetry and GRACE. Is the uncertainty in the correction significant compared to errors in the observations? Note: results based on a single global ice model.

10. Douglas, 1997 Employ Careful Selection Criteria to Minimise Influence of Solid Earth Motion

11. IIII SLR=1.5±0.1 mm/yr SLR GIA =1.8±0.1 mm/yr

12. Influence of Radial Mantle Viscosity Variations on GIA-Correction at Tide Gauge Sites LT: 70-120 km UMV: 0.1-1x10 21 Pas LMV: 2-50x10 21 Pas

13. GIA Contribution to Observations of Recent Cryosphere Changes? Satellite Gravity ICE SHEET OBSERVATIONS Satellite Altimetry Airborne Altimetry Synthetic Aperture Radar (InSAR) SEA-LEVEL OBSERVATIONS Proxy recordsTide Gauges Satellite Gravity Satellite Altimetry

14. Adopt a few different ice models for Antarctic and Greenland and predict present-day crustal uplift and geoid rate signals. Consider only the on-going viscous Earth response to past variations of these ice sheets. Influence of Earth model uncertainty is not considered. How Sensitive are GIA Contributions to Altimetry and GRACE Observations to Differences in Current Ice Models?

21. Summary Solid Earth motion associated with past ice- ocean mass flux is a significant contaminant signal in observations related to sea-level changes (GRACE, tide gauges and proxy records, Satellite Altimetry) and cryosphere changes (Altimetry and GRACE). The accuracy of the climate signal inferred from these observations therefore depends on the accuracy of the GIA model correction. The correction applied is sensitive to the adopted ice history and Earth viscosity model.

22. Recommendations GIA community: - Make predictions available (data correction and site selection) - Continue to improve and refine Earth and ice components of model User community: - Employ well-calibrated regional models if possible or… - Use a suite of model predictions - Use measurements of crustal motion