Reconstruction of equilibrium-line altitudes for tropical and sub-tropical glaciers

Abstract

Past fluctuations of tropical and sub-tropical glaciers provide important palaeoclimate proxies for regions where other forms of evidence are rare. However, published equilibrium-line altitude (ELA) estimates for tropical and sub-tropical glaciers at the LGM vary widely, reflecting the diversity of methods and approaches employed by different research groups. This complicates regional and global comparisons of ELA estimates, and emphasises the need for standardised methods. The distinctive character of tropical and sub-tropical glaciers, however, means that standard methods for reconstructing former glacier limits, ELAs, and palaeoclimate need to be adapted for local conditions. Many methods of ELA reconstruction explicitly or implicitly make assumptions about glacier mass balance gradients, and care needs to be taken that the choice of accumulation area ratios (AARs), balance ratios (BRs) and terminus-to-head ratios (THARs) is appropriate, as such indices are influenced by climatic regime, debris cover and other factors. ELA reconstructions should employ multiple methods, and should be cross-checked and fully reported, to allow assessment of the accuracy of ELA estimates. Reliable glacier chronologies are equally important. Dating should be based on multiple radiometric techniques wherever possible, and method of dating, the type of material dated, and the context of the date must all be reported.