Testing the Area-Altitude Balance Ratio (AABR) and Accumulation Area Ratio (AAR) methods of calculating glacier equilibrium-line altitudes

In this study, we compare equilibrium-line altitudes (ELAs) calculated using the Area-Altitude Balance Ratio (AABR) and the Accumulation-Area Ratio (AAR) methods, with measured ELAs derived from direct field observations. We utilise a GIS toolbox to calculate the ELA for 64 extant glaciers by applying the AABR and AAR methods to digital elevation models (DEMs) and polygons of their geometry. The calculated ELAs are then compared to measured zero-net balance ELAs obtained from mass balance time series held by the WGMS for the same glaciers. The correlation between zero-net balance ELAs and AABR (1.56)/AAR (0.58) calculated ELAs is very strong, with an R2 25 = 0.99. The smallest median difference between zero-net balance ELAs and calculated ELAs (i.e., 65.5 m) is obtained when a globally representative AABR of 1.56 is used. When applied to palaeoglacier-climate applications, this difference translates to ~0.42˚C, well within the uncertainty of palaeotemperature proxies used to determine mean summer temperature at the ELA. The more widely used mean AABR of 1.75 is shown to be statistically invalid due to skewness of the dataset. On this basis, when calculating glacier ELAs, we recommend the use of a global AABR value of 1.56.