The evolution of the Greenland Ice Sheet from the Last Glacial Maximum to present-day: An assessment using glaciological and Glacial Isostatic Adjustment modelling

Abstract

In this thesis we constrain a three-dimensional thermomechanical model of Greenland ice sheet (GrIS) evolution from the Last Glacial Maximum (LGM, 21 ka BP) to the present-day using, primarily, observations of relative sea level (RSL) as well as field data on past ice extent. The new model (Huy2) fits a majority of the observations and is characterised by a number of key features: (i) the ice sheet had an excess volume (relative to present) of 4.1 m ice-equivalent sea level at the LGM, which increased to reach a maximum value of 4.6 m at 16.5 ka BP; (ii) retreat from the continental shelf was not continuous around the entire margin, as there was a Younger Dryas readvance in some areas. The final episode of marine retreat was rapid and relatively late (c. 12 ka BP), leaving the ice sheet land based by 10 ka BP; (iii) in response to the Holocene Thermal Maximum (HTM) the ice margin retreated behind its present-day position by up to 80 km in the southwest, 20 km in the south and 80 km in a small area of the northeast. As a result of this retreat the modelled ice sheet reaches a minimum extent between 5 and 4 ka BP, which corresponds to a deficit volume (relative to present) of 0.17 m ice-equivalent sea level. The results suggest that remaining discrepancies between the model and the observations are likely associated with non-Greenland ice load, differences between modelled and observed present-day ice elevation around the margin, lateral variations in Earth structure and/or the pattern of ice margin retreat.
Predictions of present-day vertical land motion generated using the new Huy2 model are highly sensitive to variations of upper mantle viscosity. Depending on the Earth model adopted, different periods of post-LGM ice loading change dominate the present-day response in particular regions of Greenland. These results will be a useful resource when interpreting existing and future observations of vertical land motion in Greenland. In comparison to the sparse number of GPS observations available, predictions from the Huy2 model are in good agreement to the absolute measurements from south and southwest Greenland. This suggests that the response of the ice sheet to the HTM is reasonably well produced by the Huy2 model and, thus, corroborates our earlier findings. Uplift predictions generated using a surface mass balance reconstruction of the GrIS (Wake et al., 2009), which covers the period 1866-2005, show that decadal-scale ice mass variability over the last c. 140 years plays a small role in determining the present-day viscous response (it is as large as ±0.2 mm/yr). Results from the same reconstruction show that high rates of peripheral thinning in west and southwest Greenland from 1995 to 2005 (due to surface mass balance changes) generate elastic uplift rates over 6 mm/yr.
In the final part of the thesis, we examine how non-Greenland ice mass loss influenced vertical land motion and sea-level change around Greenland over the last deglaciation and consider the implications for GrIS evolution. Results from this analysis suggest non-Greenland solid Earth deformation had little impact on the evolution ice sheet. Sea-level change around Greenland which is driven by non-Greenland ice mass loss departs from the associated eustatic signal; largely because of the close proximity of the late North American ice sheets (NAIS). Non-Greenland RSL change is also spatially non-uniform and is characterised by a distinct east-west gradient. For example, we find that from 16 to 14 ka BP rates of sea-level rise remained relatively low in the west (0-2 m/ka), whereas, those in the east reach values between 6 and 8 m/ka (although these results are sensitive to the source of meltwater, in particular, the relative partitioning of meltwater pulse 1A (mwp-1A, 14.2 ka BP) between the North American and Antarctic ice sheets). If the marine breakup of the GrIS was forced by non-Greenland RSL change, then we would expect the retreat of the ice sheet to reflect the sea-level changes described. A preliminary modelling study suggests that, assuming a conventional ice sheet model calving treatment, a more realistic sea-level forcing results in a pattern of ice margin retreat which is at least partly due to spatial variations in non-Greenland RSL change.
Thus, the modelled marine retreat is generally earlier in east Greenland and later in the west than for when non-Greenland RSL change is not accounted for - this pattern of ice margin retreat is generally consistent with observations from the continental shelf.