How can we tell how glaciers vary over time?
Mountain glaciers are still abundant in high-latitude, high-altitude areas on Earth. Their presence in the landscape, and dynamic behaviour, are intimately linked to the local climate. From observations we know that relatively minor shifts in temperature and precipitation are quickly manifested in the mass balance budget of the glacier, which can result in a glacier advance or retreat. Glaciers are consequently among the most trusted indicators of global climate change and therefore a valued proxy. Their sensitivity to climate change is captured by the widespread systematic retreat of glaciers regardless of their geographic location, orientation, thermal regime and so on and so forth. Reconstructing how glaciers have varied in the past can therefore afford insights about natural climate variability across thousands of years, on decadal time scales, adding data that is comparable across regions worldwide. Here I will visit some of the methods employed to reconstruct past glacier variability, associated uncertainties and challenges. Emphasis will be put on the last 10 000 years with examples from both hemispheres.