Current Andean glacier loss is unprecedented in the Holocene

Andean tropical glaciers are experiencing unprecedented retreat, according to a new study that reveals their current sizes are the smallest in over 11,700 years. “Our finding … identifies this region as a hot spot in our understanding of the changing state of the cryosphere,” say the authors. Glaciers act as important indicators of climate change, with their global retreat accelerating over recent decades. Examining this retreat in the context of the previous 11,700 years of the Holocene interglacial highlights the impact of modern global warming. Although many glaciers worldwide are smaller today compared to their sizes over the Holocene, it’s recognized that glaciers generally retreated to their smallest extents during the early to mid-Holocene. But today tropical glaciers are experiencing unprecedented retreat, possibly due to factors including increasing temperatures at high altitudes. The Andes – home to over 99% of tropical glaciers – are particularly affected, and may be the first to show significant impacts of human-induced climate change on a regional scale. To determine whether the magnitude of tropical glacier retreat in the Andes today exceeds the bounds of Holocene fluctuations, Andrew Gorin and colleagues analyzed paired cosmogenic carbon-14 (¹⁴C) and beryllium-10 (¹⁰Be) measurements in 20 bedrock samples collected near the margins of four tropical Andean glaciers. Cosmogenic nucleotide measurements can be used to estimate how long bedrock has been exposed, such as due to loss of overlying ice. Gorin et al. discovered that most bedrock samples had near-zero ¹⁴C and ¹⁰Be concentrations, suggesting that these locations were covered by ice throughout the Holocene and have only recently been exposed. Thus, the adjacent glaciers are likely smaller now than they have been at any point in the past 11,700 years. “Paired with a similar result in the Arctic, the early emergence of these small, fast-responding glaciers from their Holocene baseline may serve as a ‘canary in the coal mine,’ signaling an approaching contraction of higher-latitude glaciers beyond their interglacial range,” write Gorin and colleagues.