Mountain glaciers are major contributors to sea level rise. In comparison with the Greenland and the Antarctic ice sheets, they respond quickly to climate change, in particular because of high ablation rates and specific accumulation processes (IPCC, 2019). Over the 21st century, glaciers are expected to contribute to one fourth of the sea level rise with contributions of +10 cm (4-16 cm) in a low-emission scenario (RCP2.6) and 16 cm (9-23 cm) in a large-emission scenario (RCP8.5). The uncertainties related to these numbers are large, due to both the uncertainties on future climate forcings and glaciological response to these forcings. Large consequences of glacier retreat on the hydrological cycle are also expected at the local and regional scales, with shifts in amount and seasonality of meltwater (Huss & Hock, 2018). It is in line with the Sustainable Development Goals (SDGs) 2, 6 and 13 (“Zero hunger”, “Clean water and sanitation” and “Climate action”), and with a growing societal demand to accurately predict these changes at local scale, especially in regions such as Central Asia, the Andes or the Indus basin where glaciers play a critical role in the regional hydrology as reliable sources of water (Pritchard, 2019).

Future glacier changes depend largely on climate change through its impact on local atmospheric variables that drive the exchange of energy and mass at the glacier surface. The glacier surface mass balance (SMB) is the sum of snow accumulation and snow/ice ablation year-round. Snow accumulation is the variable with the strongest spatial and temporal variability and the highest uncertainty in glacier SMB modelling (Kumar et al., 2019; Machguth et al., 2006). In highly glacierized mountain catchments, snow accumulation is directly related to solid precipitation rate. Consequently, a large part of the uncertainty about recent and future glacier mass changes is directly inherited from large uncertainties in the spatio-temporal representation of precipitation (amounts and phase) in the observations and in the future climate models (IPCC, 2019). As an example, state-of-the-art glacier SMB models used to simulate future glacier evolution require a correction factor of 0.5 to 5 for the precipitation rate from reanalysis products to match observed glacier mass balance (Rounce et al., 2020; Sakai et al., 2015). They also use an arbitrary phase partition threshold from -1 to +2°C (Marzeion et al., 2020). In order to provide realistic future projections of glacier SMB, it is thus crucial to accurately estimate the spatio-temporal variability of precipitation rates and phase distributions at high elevation, as well as their impact on glacier SMB for both the present and the future.

The phase of precipitation (and thus the rain/snow transition) is a key concern as it affects the surface albedo (snow versus rain) and the heat transfer in the snow layers (Quéno et al., 2018). In different climate settings, different impacts of the rise of the rain/snow transition on glacier SMB are expected (Sakai & Fujita, 2017). At mid-latitudes, with contrasted accumulation and ablation periods (winter and summer), we expect impacts of the rain/snow transition rise to be the largest in spring and fall seasons, linked to the shortening of the snow-covered period (Matiu et al., 2021). At low latitudes, where accumulation and ablation periods overlap, the rise of the rain/snow transition might have an even stronger impact, because the upper reaches of the glacier might remain below the freezing level, and thus starving the glacier and leading to a faster albedo decay of the snow following rain on snow events (Flanner & Zender, 2006).

Accounting for such feedbacks and physical processes implied in the future rain/snow transition rise requires to rely on complex models of SMB, and major improvements in glacier SMB projections can be achieved by focusing on the accumulation processes and modelling. State-of-the-art glacier models are in their vast majority relatively simple (Marzeion et al., 2020). They calculate glacier SMB with empirical relations between monthly or annual precipitation and accumulation on one side, and temperature and ablation on the other side. Consequently, such empirical models are likely to miss important feedbacks related to the strong influence of the snow/ice albedo on glacier SMB (Sicart et al., 2008). More physical approaches that explicitly describe the energy and mass exchanges between the glacier and the atmosphere are required to provide realistic future glaciological projections.

Updated on 10 June 2022