Mountain glaciers are excellent indicators of climate variability and change because their evolution depends on the balance between snow accumulation during the cold period and ice and snow ablation during the warmest season. The Aneto glacier is one of the southernmost glaciers in Europe and, despite being a very small one (<0.5 km2), is the largest in the Pyrenees.

A recent study led by the Pyrenean Institute of Ecology (IPE-CSIC) and published in the journal ‘The Cryosphere’ confirms that for the 1981-2022 period, the surface area of the Aneto glacier decreased by 64.7 %, from 135.7 ha (1.36 km2) to 48.1 ha (0.48 km2), and its front has retreated from 2,828 to 3,026 metres.

By using the longest temporal dataset of glacier thickness loss in the Pyrenees, the study set out to analyse how the largest and highest glacier in the Pyrenees has evolved recently. The work also enabled the impact of a single, extremely warm, dry season (2022) on the glacier’s evolution to be assessed. “We carried out the most detailed study of the Aneto glacier (very high-resolution, three-dimensional models of the glacier) and the longest one so far (41 years). To do this, we reconstructed the glacier’s surfaces between 1981 and 2022 using very high-resolution aerial images obtained by the National Geographic Institute for 1981 and by LIDAR flights for 2011, and by unmanned aerial vehicles (drones) for 2020, 2021 and 2022,” explained Eñaut Izagirre-Estibaritz, lecturer in the Department of Geography, Prehistory and Archaeology at the University of the Basque Country (UPV/EHU) and a participant in the research. All this was complemented by an extensive geo-radar campaign in 2020, during which we were able to determine the thickness of the glacier in that year by means of various transects (more than 7 km travelled on the glacier)".

Disappearing glaciers

The loss of glacier area in the Pyrenees is significant: there were more than 100 glaciers in 1850, 39 in 1984, 21 in 2020 and 18 in 2022, corresponding to an area of 2,060 ha (20.6 km2) in 1850, 810 ha (8.1 km2) in 1984, 232 ha (2.3 km2) in 2020 and 170 ha (1.7 km2) in 2022, representing a loss of 92 % of the glacier area since the end of the Little Ice Age.

Specifically, on the Aneto over the last 41 years, the glacier surface has decreased by 64.7 % and the ice thickness by, on average, 30.5 m. “The average remaining ice thickness in the autumn of 2022 was 11.9 m, compared with the average thickness of 32.9, 19.2 and 15.0 m reconstructed for 1981 and 2011 and observed in 2020, respectively. The results indicate the critical situation of the glacier, with imminent segmentation into three smaller ice bodies and no evidence of an accumulation zone. We also found that the influence of an extremely hot, dry year, as observed in the 2021-2022 season, led to a drastic degradation of the glacier, which constitutes a high risk for the survival of the Aneto glacier, a situation that could extend to the rest of the Pyrenean glaciers in a relatively short time,” the researchers pointed out.

The research data indicate that the ice thickness distribution shows areas around the glaciers with very little thickness (<2 m), so these areas are very close to deglaciation over the next few summers. The loss of surface area and thickness of the Aneto glacier indicate the critical situation of this ice mass. It is in its terminal stage, displaying fragmentation into smaller ice bodies and the presence of debris cover in some areas. “The glacier thinning rates, which were around 1 m per year, tripled in the last year of the study (between 2021 and 2022), which shows the effect of a fairly dry and, above all, extremely warm year like 2022 on the ablation of snow and ice on the Aneto glacier,” concluded the UPV/EHU lecturer.