Pre-industrial Holocene glacier variability in the tropical Andes as context for anthropogenically driven ice retreat

Disentangling the timing and pattern of past glacier change in the tropics provides important perspectives for the future health of the Andean cryosphere. Here we review Holocene paleo-glacial records from the northern and southern tropical Andes to provide context for the loss of glacial ice since the late 20th century. The available archives indicate that glaciers advanced and retreated multiple times during the Holocene with notable shifts during the last millennium. However, the available records of glaciation from the northern and southern Andes depict contrasting climate conditions across the tropics through the early, middle and late Holocene, including during the Medieval Climate Anomaly (MCA). There is clearer evidence of widespread Little Ice Age (LIA) glacier advances throughout the region, however, there were significant, centennial-scale lags in the timing of southern tropical glaciation relative to the onset of cooling in the Northern Hemisphere. Notwithstanding age uncertainty, the combined regional paleoclimate records suggest the MCA in the tropical Andes was somewhat warmer and drier than the LIA, but not warmer than today. In contrast, the vast majority of Andean glaciers appear to be rapidly and uniformly retreating since the late 20th century in response to anthropogenic warming.