A 400‐Year Ice Core Melt Layer Record of Summertime Warming in the Alaska Range

Abstract

Warming in high‐elevation regions has societally important impacts on glacier mass balance, water resources, and sensitive alpine ecosystems, yet very few high‐elevation temperature records exist from the middle or high latitudes. While a variety of paleoproxy records provide critical temperature records from low elevations over recent centuries, melt layers preserved in alpine glaciers present an opportunity to develop calibrated, annually resolved temperature records from high elevations. Here we present a 400‐year temperature proxy record based on the melt layer stratigraphy of two ice cores collected from Mt. Hunter in Denali National Park in the central Alaska Range. The ice core record shows a sixtyfold increase in water equivalent total annual melt between the preindustrial period (before 1850 Common Era) and present day. We calibrate the melt record to summer temperatures based on weather station data from the ice core drill site and find that the increase in melt production represents a summer warming rate of at least 1.92 ± 0.31°C per century during the last 100 years, exceeding rates of temperature increase at most low‐elevation sites in Alaska. The Mt. Hunter melt layer record is significantly (p < 0.05) correlated with surface temperatures in the central tropical Pacific through a Rossby wave‐like pattern that enhances high temperatures over Alaska. Our results show that rapid alpine warming has taken place in the Alaska Range for at least a century and that conditions in the tropical oceans contribute to this warming.

Evidence for Elevation-Dependent Warming in the St. Elias Mountains, Yukon, Canada

“…Using air temperature reanalysis and satellite infrared data, Williamson et al (2018) showed that in the St. Elias Mountains of southwest Yukon (608-628N, 1378-1428W), springtime (May) surface temperatures rose at rates from 0.148C a 21 at 1000 m to 0.198C a 21 at 5000 m during the years 2000-14. Farther west, analysis of melt layers (a proxy for summer warmth) in ice cores from Mount Hunter, Alaska (638N, 1518W), revealed an average warming trend at 3900 m of 0.02288 6 0.00448C a 21 between 1950 and 2011 (Winski et al 2018). Comparison to air temperature data from 15 lowelevation stations in Alaska and the Yukon suggests that summertime EDW is occurring on Mount Hunter.…”

Precipitation and ice core δD-δ&lt;sup&gt;18&lt;/sup&gt;O line slopes and their climatological significance

“…If diffusion lengths are sufficiently low relative to the annual layer thickness such that the annual cycle is preserved to the depth where vapor diffusion ceases to take place (e.g. at high accumulation locations in Alaska; see Winski et al, 2018), the annual cycle could be measureable for much greater timescales because solid diffusion is about four orders of magnitude slower than vapor diffusion (Johnsen et al, 2000). At this point, the limit likely becomes measurement resolution, or other physical factors, rather than diffusion.…”

History and Data Records of the Automatic Weather Station on Denali Pass (5715 m), 1990–2007

“…In more recent times, sporadic meteorological measurements on or near Denali have been carried out in the context of glaciological studies at lower elevations, for example, on the Kahiltna Glacier (Young et al 2018), or for brief periods of time during scientific expeditions, for example, on the summit plateau of Mount Hunter during the drilling of glacial ice cores (Winski et al 2018;Osterberg et al 2017;Saylor et al 2014). Winski et al (2018) use temperature data collected in 2013 and 2015 on Mount Hunter to determine the distribution of summer temperatures at the drill site, which informs the analysis of melt layers in the ice core and the development of a long-term temperature record, providing valuable insights into Holocene climate in the Denali region and beyond. The ice core analysis suggests a summertime warming rate of 1.928 6 0.318C in the last century, which exceeds typical warming rates at lowerelevation sites of similar latitude and is in the same range as a temperature increase of 0.028C yr 21 found by Hartl et al (2020a) from NCEP-NCAR reanalysis data (Kalnay et al 1996).…”

“…Existing data are all the more valuable and serve to improve understanding of important atmospheric and climatic processes (e.g., Marty and Meister 2012;Ohmura 2012;Rangwala and Miller 2012), as well as ground-atmosphere interactions, for example, in the context of local catchments, regional hydrology, and the high-mountain cryosphere (e.g., Immerzeel et al 2014;Shea et al 2015;Immerzeel et al 2020;Litt et al 2019). Data records from high-elevation mountain AWS have proven essential for delineating drivers of local glaciological processes (Salerno et al 2015) and are important for the calibration of proxy data such as ice cores from mountain glaciers to atmospheric variability (Hardy et al 1998;Osterberg et al 2017;Winski et al 2018;Bohleber 2019). Under ongoing climate change, mountain environments are changing rapidly, with potentially dramatic consequences for local populations affected by related natural hazards or downstream communities reliant on mountain-fed river systems (Hock et al 2019;Immerzeel et al 2020).…”