Four North American glaciers advanced past their modern positions thousands of years apart in the Holocene

Abstract. The global retreat of alpine glaciers provides visible evidence of industrial-era warming, but how glacier position today compares to glacier length fluctuations over the Holocene is less clear. Glaciers in North America advanced over the Holocene, occupying their maximum Holocene position in the late 19^th century before rapidly retreating to their sizes today. We assess when four North American glaciers, located between 38–60° N, were larger or smaller than their modern (2018–2020 CE) sizes during the Holocene. We measure 26 paired cosmogenic in situ ¹⁴C and ¹⁰Be concentrations in recently exposed proglacial bedrock and applied a Monte Carlo forward model to reconstruct plausible bedrock exposure-burial histories. We find that these glaciers advanced past their modern sizes thousands of years apart during the Holocene: a glacier in the Juneau Ice Field (BC, Canada) by ~2 ka, Kokanee Glacier (BC, Canada) at ~6 ka, and Mammoth Glacier (WY, USA) at ~1 ka; the fourth glacier, Conness Glacier (CA, USA), was larger than its modern size for the duration of the Holocene until present. The disparate Holocene exposure-burial histories are at odds with expectations of similar glacier histories given the presumed shared climate forcings of decreasing Northern Hemisphere summer insolation through the Holocene followed by global greenhouse gas forcing in the industrial era. We interpret the range in histories to be the result of unequal amounts of modern retreat relative to each glacier’s Holocene length history, rather than asynchronous Holocene advance histories. The intensity and rate of modern warming has exacerbated length differences between glaciers that occur due to hypsometry and response time. We hypothesize that highly varying magnitudes of glacier change in North America today is a departure from similar magnitudes of glacier change over the Holocene.