Abstract

Developing a long‐term understanding of the cryosphere is important in the study of past climatic change. Here we used a nested approach combining diverse instrumental (monthly meteorological data from four weather stations, as well as gridded data) and proxy data (based on blue intensity measurements from local tree ring records) to create a reconstruction of past summer temperature for the central Jotunheimen area in southern Norway. This record was then used to reconstruct annual glacier mass balance from 1962, the start of the yearly measurements, back to 1722, immediately prior to the regional Little Ice Age maximum. Our reconstruction of the ‘average’ Jotunheimen cumulative glacier mass balance is based on three representative glaciers (Storbreen, Hellstugubreen and Gråsubreen) that were synthesized into one composite record which we term ‘Gjennomsnittsbreen’ (‘mean glacier’ in Norwegian) to filter out localized controls on the behaviour of individual glaciers. While not ignoring the role of precipitation on glacier mass balance, our reconstruction demonstrates that glaciers in this region exhibit a strong summer temperature control and appear to have been declining more or less continuously since the mid‐18th century. However, it also shows that this long‐term trend of overall retreat in Jotunheimen is punctuated by relatively short‐lived periods of neutral or occasionally positive glacier mass balance, signifying periods of stillstand or small‐scale glacier advance. These periods or ‘events’ in our reconstruction were compared with an independent record of 12 moraine‐building events developed using lichenometry. A minimum of 10 of the moraine‐building events identifiable in our reconstruction were also identifiable in the lichenometric data which affords confidence in the performance of our interrogative model. A critical implication of this successful glacier mass balance reconstruction based on just summer temperature is that for Jotunheimen – in contrast to Norwegian maritime glaciers further to the west – there is no need (as was proposed in some previous studies) to invoke large, prolonged increases in winter snowfall to explain glacier advances, not even for events such as the Little Ice Age.

Highlights

  • Developing a long-term understanding of the cryosphere is important in the study of past climatic change

  • As instrumental glacier mass balance datawere available for 1962–2017, we were able to test the stability of the model using the six nests by calculating common calibration and verification statistics (R2, reduction of error, Reduction of error (RE), and coefficient of efficiency, Coefficient of efficiency (CE)) for the split periods 2017–1990 and 1989–1962, as well as for the entire period

  • The coefficient of efficiency ‘fails’ (i.e. CE < 0) for the earliest two nests, which possibly reflects the distance of the study site from Uppsala and possible effects of increased precipitation recorded in the early 1990s

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Summary

Introduction

Developing a long-term understanding of the cryosphere is important in the study of past climatic change. It shows that this long-term trend of overall retreat in Jotunheimen is punctuated by relatively short-lived periods of neutral or occasionally positive glacier mass balance, signifying periods of stillstand or small-scale glacier advance These periods or ‘events’ in our reconstruction were compared with an independent record of 12 moraine-building events developed using lichenometry. In recent decades there have been short-term regional variations in the Karakoram region and in New Zealand, where glaciers have shown periods of growth which may be considered somewhat unexpected against the backdrop of a global warming trend Such advances have been attributed to periods of unusually cool summers (Mackintosh et al 2017), positive anomalies in winter precipitation or combinations of the two (Kapnick et al 2014; Forsythe et al 2017).

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