More than a century of direct glacier mass-balance observations on Claridenfirn, Switzerland

Glacier mass-balance observations at seasonal resolution have been performed since 1914 at twosites on Claridenfirn, Switzerland. The measurements are the longest uninterrupted records ofglacier mass balance worldwide. Here, we provide a complete re-analysis of the 106-year series(1914–2020), focusing on both point and glacier-wide mass balance. The approaches to evaluateand homogenize the direct observations are described in detail. Based on conservative assump-tions, average uncertainties of ±0.25 m w.e. are estimated for glacier-wide mass balances at theannual scale. It is demonstrated that long-term variations in mass balance are clearly driven bymelting, whereas decadal changes in accumulation are uncorrelated with mass balance and canonly be relevant in short periods. Mass change of Claridenfirn is impacted by dry calving at afrontal ice cliff. Considerations of ice volume flux at a cross-profile reveal long-term variationsin frontal ice loss accounting for ∼9% of total annual ablation on average. The effect of changesin frontal ablation mostly explains, <10% of the mass-balance difference relative to the period1960–1990, but accounts for ∼20% in 2010–2020. Glacier mass changes are discussed in the con-text of observations throughout the European Alps indicating that Claridenfirn is regionallyrepresentative.