Quantifying topoclimatic control on glacier Equilibrium Line Altitudes at the regional and global scale

Reconstructions of glacier Equilibrium Line Altitudes (ELAs) from geomorphological evidence are often the only source of quantitative palaeoclimatic information in mountainous regions. The ELA is the average altitude of zero net mass balance and divides a glacier into an accumulation and an ablation area. While primarily controlled by summer temperature and winter precipitation, the position of the ELA is also frequently modulated by local topoclimatic factors, such as shading, supraglacial debris cover, avalanching or wind-driven snow redistribution. As a result, there can be substantial differences of several 100 meters between ELAs of neighbouring glaciers within the same climatic region. If such topoclimatic controls are not accounted for, this can introduce notable biases into ELA-based palaeoclimate reconstructions.

To better constrain the effect of topoclimatic control on glacier ELAs at the regional to global scale, we present the results of a comprehensive data analysis based on the Randolph Glacier Inventory (RGI), version 7.0. We compare glacier-specific ELAs calculated through the Accumulation Area Ratio and Area-Altitude Balance Ratio methods to a variety of topographic parameters, such as the amount of received solar radiation, the curvature of the ice surface and the topographic openness of the terrain. We show that there is a strong correlation between local ELA differences and some of these parameters and use a machine-learning tool to predict this ELA offset using only a digital elevation model and a glacier outline as input. This tool can be used to assess the topographic bias related to any calculated ELA and has the potential to lead to more reliable palaeoclimate reconstructions in a variety of settings.