Causes of pre-collapse changes of the Larsen B ice shelf: Numerical modelling and assimilation of satellite observations

Satellite observations revealed that beside a rapid thinning, the Larsen B ice shelf (LBIS) was undergoing a significant acceleration before its collapse in 2002. This paper investigates the ice shelf acceleration between 1995 and 1999 using a combination of data assimilation and numerical modelling. Based on a flow model adjusted to the 1995 InSAR velocities, perturbation experiments are performed, such as ice front retreat, thinning, increase in tributary flow and rheological weakening. Furthermore, an inversion for ice shelf rheology and tributary flow velocity is performed for both the 1995 and the 1999 InSAR velocities. The perturbation experiments together with the inversion strongly suggest that the acceleration cannot solely be explained by the retreat of the ice shelf front but relies on a further significant rheological weakening of the already weak shear zones within the LBIS. Minor tributary acceleration is found to be an effect rather than a cause of the ice shelf acceleration. Furthermore, the observed acceleration cannot be explained by the observed recent thinning. We conclude that for smaller ice shelves such as the LBIS, such weak shear margins play a crucial role in controlling their dynamics and are the key to understand changes in the future. Finally, we compare the dynamic thinning likely to be associated with the observed acceleration with the observed thinning. For the ice shelf as a whole, this thinning accounts for 20% of the observed value, which implies that factors such as enhanced basal melt were the primary cause of the observed thinning.