Abstract
Abstract. It is standard to compare climate model results covering the past millennium and reconstructions based on various archives in order to test the ability of models to reproduce the observed climate variability. Up to now, glacier length fluctuations have not been used systematically in this framework even though they offer information on multi-decadal to centennial variations complementary to other records. One reason is that glacier length depends on several complex factors and so cannot be directly linked to the simulated climate. However, climate model skill can be measured by comparing the glacier length computed by a glacier model driven by simulated temperature and precipitation to observed glacier length variations. This is done here using the version 1.0 of the Open Global Glacier Model (OGGM) forced by fields derived from a range of simulations performed with global climate models over the past millennium. The glacier model is applied to a set of Alpine glaciers for which observations cover at least the 20th century. The observed glacier length fluctuations are generally well within the range of the simulations driven by the various climate model results, showing a general consistency with this ensemble of simulations. Sensitivity experiments indicate that the results are much more sensitive to the simulated climate than to OGGM parameters. This confirms that the simulations of glacier length can be used to evaluate the climate model performance, in particular the simulated summer temperatures that largely control the glacier changes in our region of interest. Simulated glacier length is strongly influenced by the internal variability in the system, putting limitations on the model–data comparison for some variables like the trends over the 20th century in the Alps. Nevertheless, comparison of glacier length fluctuations on longer timescales, for instance between the 18th century and the late 20th century, appear less influenced by the natural variability and indicate clear differences in the behaviour of the various climate models.
Highlights
As it offers a longer perspective compared to the so-called instrumental period, the past millennium is a key period to study decadal to centennial climate variations
Goosse et al.: Simulated Alpine glacier length over the past millennium els tend to underestimate the magnitude of the changes in some regions and to simulate more homogenous changes than in the reconstructions (Goosse et al, 2005; Raibble et al, 2006; Gonzalez-Rouco et al, 2006; Jungclaus et al, 2010; Phipps et al, 2013; Fernández-Donado et al, 2013; Landrum et al, 2013; Neukom et al, 2014; Moberg et al, 2015; PAGES2k-PMIP, 2015; Otto-Bliesner et al, 2016)
A very simple bias correction is applied to climate model results, ensuring that after the adjustment the climate models have the same mean over the reference period as the Climatic Research Unit (CRU) data set used to calibrate Open Global Glacier Model (OGGM) climate sensitivity parameter
Summary
As it offers a longer perspective compared to the so-called instrumental period (from roughly 1850 CE to present), the past millennium is a key period to study decadal to centennial climate variations. Glaciers are complex recorders of past conditions Their fluctuations depend on the surface mass balance, which is influenced by several factors, including temperature, precipitation and incoming radiation changes over the glacier, as well as by the glacier dynamics and local geometry (Oerlemans, 2001; Huss et al, 2008; Roe, 2011). Glacier length fluctuations cannot be directly compared to records with a much faster response or included in multi-proxy reconstructions of past climate changes (Oerlemans, 2005; Roe, 2011; Solomina et al, 2016; Roe et al, 2017) Despite these difficulties, it is possible to estimate the temperature and precipitation variations that were at the origin of glacier length fluctuations (Mackintosh et al, 2017).
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