Modelling the future of Nevado Coropuna (Peru), the world’s largest tropical ice cap.

With around 46 km², Nevado Coropuna (NC, 15°32’S, 72°39’W; 6377 m) is the largest tropical icecap in the world. NC is situated on a stratovolcano structure with six peaks over 6000 meters, in the arid border of the Andean plateau, Southern Peru. NC is a vital source of freshwater for the communities within the Majes valley and the vast irrigation plans located in the same valley and on the arid coastal strip. Our MOTICE project will model the evolution of NC until 2100 CE in response to climate change.

We present initial results on the modelling of NC, which will be used to tune the glaciological parameters for the projections under different RCP scenarios. Mass balance was modelled using the COupled Snowpack and Ice surface energy and MAss balance model in Python (COSIPY). This was forced with climate data for the 1950-2020 period from the ERA5-Land reanalysis, which provided surface pressure, cloud cover, incoming shortwave and longwave radiation, wind speed, 2-meter air temperature and relative humidity fields. This was combined with the RAIN4PE gridded product outputs for daily precipitation during the 1981-2015 period. The climate dataset was downscaled and validated with observed temperature and relative humidity from a weather station located on the Cavalca glacier (5800 m above sea level), at the northern part of NC. Glacier mass balance results were validated with measured mass balance in the same glacier for the 2014-2019 period. The mass-balance outputs from COSIPY were used for glacial flow modelling, using the Parallel Ice Sheet Model (PISM).

Subglacial topography was modelled using the Volume and Topography Automation (VOLTA) tool in a DEM that had been previously corrected with 70 differential GPR points measured “in situ”. The subglacial topography was also tuned and validated against in-situ GPR measurements in four glaciers of NC. Both GPR and GPS measurements were conducted during the 2022 fieldwork campaign, in which large areas of debris-covered ice were also located, mapped and measured. However, debris covered ice has not been considered in this initial model run.

Our preliminary results were compared to the actual 1955-2020 glacier surface evolution, which was retrieved from aerial photography and topographic maps for the initial stage in 1955 and from satellite images from 1975 onwards. This work highlighted the difficulty of modelling tropical glaciers, especially accounting for processes important to tropical ice, such as sublimation, and short-lived meteorological events.