**2. Study site**

*Glaciers and the Polar Environment*

Caucasus [11].

by global warming [1]. The response of a glacier to climate imbalance occurs initially through mass loss; eventually, the glacier adjusts to mass changes by changing its geometry, including area and length [2]. These geometric changes are also often accompanied by a shift in the glacier geomorphology, including an increase in the glacier debris cover, which can then decouple the glacier response from the temperature signal, and the appearance of thermokarst features such as kettles and ice-contact lakes [3], with possible impacts on glacier hazards downstream [4]. In fact, debris-covered glaciers and the expansion of supraglacial debris cover on debris-free glaciers are increasingly prominent features of the world's glaciated catchments in mountain regions including Himalaya [5], Karakoram [6, 7], Andes [8], Alps [9], Southern Alps of New Zealand [10], and

Over the past century, Italian glaciers have been shrinking at high rates [12–14].

Among these, Lys Glacier, located in the Monte Rosa massif (**Figure 1**), can be considered paradigmatic of the changes affecting alpine glaciers. Its response to increasing temperature and reduced accumulation is in fact similar to that of most glaciers in the Alps and elsewhere [14–17]: terminus retreat, area reduction and decreasing ice thickness. In addition, Lys Glacier has also recently shown other climate-related changes, including detachment of the main debris-covered tongue from the rest of the glacier body and possible separation of the two branches of the glacier tongue (**Figure 1a**); variations in supraglacial debris cover (both surface and thickness) on the glacier tongue, thermokarst features (e.g. kettle ponds) and processes (**Figure 1b**); and calving processes at the terminus where

*Location of Lys glacier within Italy (red star) and of the weather station used in this study (red dot). The bottom pictures show some typical features found on the glacier tongue in recent years, including (a) separation of two tributaries from the main debris-covered tongue; (b) thermokarst features and processes; (c) calving* 

an ice-contact lake developed in the late 1990s (**Figure 1c**).

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**Figure 1.**

*processes at the ice-contact lake.*

Lys Glacier drains the southern flank of the Mont Rosa Group (45°54' N, 07°50′ E) (**Figure 1**). The most recent Italian glacier inventory ([14], data from 2005) reports a glacier area of 9.58 km<sup>2</sup> , a south westerly aspect, an elevation range between 2392 and 4323 m a.s.l. and a length of 5.71 km. While Lys Glacier is presently the fourth largest Italian glacier, comparatively few studies have been conducted to investigate its evolution over the past century and recent decades. Strada [25] described terminus fluctuations until the early 1980s; Pelfini et al. [26] estimated the glacier response time using dendrochronology; Rota et al. [27] used DEMs from cartographic sources to calculate volume changes between 1925 and 1994. None of these studies or iconographic sources show evidence of continuous supraglacial debris cover until the late 1980s. Since then, debris cover has been present on the lower sector of the ablation tongue, initially as the continuation of medial moraines at an elevation below 2550 m a.s.l. The medial moraines formed below the glacier icefalls (**Figure 2**), thanks to the debris supplied by the surrounding deglaciated rock walls (owing to macrogelivation processes, permafrost degradation and structural rock falls). In more recent years, debris has come to cover the entire glacier tongue below 2550 m a.s.l., until its detachment from the upper sector of the glacier, which occurred in 2009 [28]. The occurrence of debris cover is thus a recent phenomenon and probably a consequence of the present deglaciation, which has affected Lys Glacier as well as the other alpine glaciers [1, 15]. Although Lys Glacier cannot be considered a debris-covered glacier even in recent times (as only a small sector of the glacier surface is continuously debris-covered, see [29]), the supraglacial morphologies and the processes affecting the debris-covered glacier tongue are comparable to those of larger debris-covered glaciers in Asia and in the Alps, including thermokarst processes and the formation of cavities and ice-contact lakes [29].

#### **Figure 2.**

*Comparison of historical photographs of Lys glacier. (a) 1989. The glacier exhibits medial moraines on the distal part of its tongue, which is well developed. (b) 2019. The glacier has retreated above a rock wall in the western sector, leaving a dead ice tongue underneath, with a proglacial lake developing in the depression left by the disappearing tongue. The moraine ridges from the 1980s advance phase are also evident. (photo credits: (a) Willy Monterin-© Archivio Monterin. (b) Fabiano Ventura-© F. Ventura-sulletraccedeighiacciai.com).*
