Denali Ice Core record of Alaska Summertime Temperature over the past Millennium

Abstract:

Surface temperatures in Alaska have increased by 2° C since 1950, causing an acceleration in glacier mass loss and an increasing contribution to global sea level rise. This glacial retreat began at the end of the Little Ice Age in the late 19^th century, and early glacial fluctuations during the Medieval Climate Anomaly are known from AK moraine chronologies. Glacier mass balance is most susceptible to summertime temperatures, but instrumental records only extend to the early 20^th century, and few paleoclimate records preserve a pure summertime record of past temperatures. A high-resolution record of past Alaskan summertime temperatures would improve glaciological model hindcasts of MCA and LIA advances and retreats, and improve predictions of future glacial melt under various warming scenarios. Here we present an ice core melt layer record from a plateau location on Mt. Hunter (63° N, 151° W, 4,000 meters above sea level), providing a robust proxy of central AK summertime temperatures over the past 1000 years. Melt layers in the ice core were identified visually and quantified by brightness contrast with the surrounding ice matrix. Melt layer taphonomy due to densification, thinning and ice core quality was taken into account in developing the record. Our results show that melt layers were present in the ice core between the years 1000-1500 before nearly disappearing between the years 1500 and 1900 (about 0-3 events/century). During the early 20^th century, the study site experienced a rapid 20-fold increase in the frequency of melt events. Since the year 1900, melt events have occurred during 35% percent of summers, with an acceleration of melt frequency in the late 20^th century. Using modern meteorological records from the Alaska Range, we estimate that this increase in melt equates to an increase of 3-4° C in mean summer temperature.