Abstract
Abstract. Glaciers are widely recognized as unique demonstration objects for climate change impacts, mostly due to the strong change of glacier length in response to small climatic changes. However, glacier mass balance as the direct response to the annual atmospheric conditions can be better interpreted in meteorological terms. When the climatic signal is deduced from long-term mass balance data, changes in glacier geometry (i.e. surface extent and elevation) must be considered as such adjustments form an essential part of the glacier reaction to new climatic conditions. In this study, a set of modelling experiments is performed to assess the influence of changes in glacier geometry on mass balance for constant climatic conditions. The calculations are based on a simplified distributed energy/mass balance model in combination with information on glacier extent and surface elevation for the years 1850 and 1973/1985 for about 60 glaciers in the Swiss Alps. The results reveal that over this period about 50–70% of the glacier reaction to climate change (here a one degree increase in temperature) is "hidden" in the geometric adjustment, while only 30–50% can be measured as the long-term mean mass balance. For larger glaciers, the effect of the areal change is partly reduced by a lowered surface elevation, which results in a slightly more negative balance despite a potential increase of topographic shading. In view of several additional reinforcement feedbacks that are observed in periods of strong glacier decline, it seems that the climatic interpretation of long-term mass balance data is rather complex.
Highlights
Glacier changes are widely recognized as the best natural indicators of climatic change (e.g. Lemke et al, 2007) which is a result of their systematic and globally coordinated monitoring for more than a century (WGMS, 2008)
When the climatic signal is deduced from long-term mass balance data, changes in glacier geometry must be considered as such adjustments form an essential part of the glacier reaction to new climatic conditions
The modeled accumulation of 3.5 m w.e. near Jungfraujoch (3550 m a.s.l.) is rather high, but only found in small regions. They likely result from unconsidered processes in the mass balance model and too high precipitation values in the input data set for this region
Summary
Glacier changes are widely recognized as the best natural indicators of climatic change (e.g. Lemke et al, 2007) which is a result of their systematic and globally coordinated monitoring for more than a century (WGMS, 2008). Lemke et al, 2007) which is a result of their systematic and globally coordinated monitoring for more than a century (WGMS, 2008) Today, this monitoring follows a tiered strategy within the global terrestrial network for glaciers (GTN-G) as part of the global climate/terrestrial observing systems (GCOS/GTOS) (Haeberli, 2006). The high correlation of mass balance with atmospheric conditions (mainly temperature and precipitation) permits to derive mass balance from meteorologic parameters (cf Oerlemans, 2001) For this and other reasons it was (and still is) quite popular to extend the short record of mass balance time series back in time with data from climate stations Linderholm and Jansson, 2007; Watson and Luckman, 2004) Such extended mass balance time series might no longer be independent proxies of climatic change and have to be treated separately from the measured data (Braithwaite, 2009). Mean values of mass balance over longer time periods can be determined independent of climatic data, e.g. from cumulative length changes (e.g. Haeberli and Hoelzle, 1995; Hoelzle et al, 2003)
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