Comment on cp-2023-20

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> The lack of continental ice sheets in Alaska during the Last Glacial Maximum (LGM; 26&ndash;19 ka) has long been attributed to arid and relatively warm summer conditions. Records of this aridity across Alaska are relatively abundant, yet quantitative temperature reconstructions have been comparatively lacking until recently. Climate model outputs, a few isolated paleoclimate studies, and global paleoclimate synthesis products show mild summer temperature depressions in Alaska compared to much of the high northern latitudes. This suggests the importance of summer temperature in controlling the relatively limited glacier growth during the LGM. We present a new statewide map of LGM alpine glacier equilibrium line altitudes (ELAs), LGM ∆ELAs (LGM ELA anomalies relative to the Little Ice Age [LIA]), and ∆ELA-based estimates of temperature depressions across Alaska to assess paleo-precipitation and -temperature conditions. We mapped glacier extents and reconstructed paleoglacier surfaces in ArcGIS to calculate ELAs using an accumulation area ratio (AAR) of 0.58 and an area-altitude balance ratio (AABR) of 1.56. We calculated LGM ELAs (n = 480) across every glaciated massif in the state, excluding areas in southern Alaska that were covered by the Cordilleran Ice Sheet. We see a similar trend of increasing ELAs from the southwest to the northeast during both the LGM and the LIA indicating a consistent southern Bering Sea and northernmost Pacific Ocean precipitation source. Our ∆ELAs from the Alaska and Brooks ranges, and the Kigluaik Mountains, average to &minus;355 &plusmn; 176 m, well above the global LGM average of ca. &minus;1000 m. Using atmospheric lapse rates, we calculate minimum summer cooling of &minus;3.5 &plusmn; 1.7 &ordm;C and maximum summer temperature depressions of &minus;1.9 &plusmn; 0.9 &ordm;C. Our results are consistent with a growing number of local proxy reconstructions and global data assimilation syntheses that indicate mild summer temperature across Beringia. Limited summer temperature depressions could be explained by increased incoming solar radiation across Alaska during the LGM. Limited summer temperature depressions &ndash; and general aridity &ndash; in Alaska during the LGM have been previously hypothesized as resulting from the complex influence of North American ice sheets on atmospheric circulation.