Modelling regional glacier length changes over the last millennium using the Open Global Glacier Model

David Parkes,Hugues Goosse

doi:10.5194/tc-14-3135-2020

Abstract

Abstract. A large majority of the direct observational record for glacier changes falls within the industrial period, from the 19th century onward, associated with global glacier retreat. Given this availability of data and the significant focus in contemporary glacier modelling falling on recent retreat, glacier models are typically calibrated using – and validated with – only observations of glaciers that are considerably out of equilibrium. In order to develop a broader picture of the skill of one glacier model – the Open Global Glacier Model (OGGM) – we model glaciers for extended historical timescales of 850–2004 CE using a selection of six general circulation model (GCM) outputs. We select glaciers for which long-term length observations are available in order to compare these observations with the model results, and we find glaciers with such observations in almost all glacierised regions globally. In many regions, the mean modelled glacier changes are consistent with observations, with recent observed retreat in these regions typically at the steeper end of the range of modelled retreats. However, on the scale of individual glaciers, performance of the model is worse, with overall correlation between observed and modelled retreat weak for all of the GCM datasets used to force the model. We also model the same set of glaciers using modified climate time series from each of the six GCMs that keep temperature or precipitation constant, testing the impact of each individually. Temperature typically explains considerably more variance in glacier lengths than precipitation, but results suggest that the interaction between the two is also significant within OGGM and neither can be seen as a simple proxy for glacier length changes. OGGM proves capable of reproducing recent observational trends on at least a qualitative level in many regions, with a modelling period over a considerably larger timescale than it is calibrated for. Prospects are good for more widespread use of OGGM for timescales extending to the pre-industrial period, where glaciers were typically larger and experience less rapid (and less globally consistent) geometry changes, but additional calibration will be required in order to have confidence in the magnitude of modelled changes, particularly on the scale of individual glaciers.

Highlights

Robust modelling of the evolution of glacier mass and geometry on regional and global scales is of critical importance for understanding the components of historical sea level rise and for predicting one of the potential largest contributors to sea level rise in coming centuries (Church et al, 2013)
The model itself runs using this data from 851 to 2004, but we limit our graphs to 1000 CE onwards in order to limit the impact of a continued adjustment towards equilibrium even after the 300-year spinup for certain model–region combinations, which is most likely a modelling issue and not a response to actual climate trends
In cases where glaciers are still undergoing significant adjustment to a new equilibrium after several hundred years of spin-up and the early part of the main run, this is good evidence that in a 1000-year period responses to trends in the forcing climate variables may not be shown in the Open Global Glacier Model (OGGM) output. This does not invalidate the glacier model output, but the evidence of continuing adjustment leftover from the spin-up being shown in the output rather than being removed with an arbitrarily long spin-up might inform the interpretation of the rest of the time series

Summary

Introduction

Robust modelling of the evolution of glacier mass and geometry on regional and global scales is of critical importance for understanding the components of historical sea level rise and for predicting one of the potential largest contributors to sea level rise in coming centuries (Church et al, 2013). Direct observations of historical glacier geometry (observations of contemporaneous glacier extent, as opposed to secondary sources like moraines or lake sediments) are relatively sparse (Zemp et al, 2015; Cogley, 2009), and it is only through recent aerial (WGMS and NSIDC, 1989) and satellite mapping (RGI Consortium, 2017) that fairly comprehensive inventories of glaciers across all of the world’s glacierised regions have become available, cataloguing upwards of 200 000 glaciers. Even this number is likely a significant underestimate (Parkes and Marzeion, 2018).

Results

Discussion

Conclusion