Title Equilibrium line altitudes of alpine glaciers suggest summers in Alaska were not more than −2 – −5 °C colder than the pre-Industrial during the Last Glacial Maximum

Summary

This study presents a state-wide compilation of equilibrium-line altitudes (ELAs) for Last Glacial Maximum (LGM) glacier extents in Alaska and ELAs for select Little Ice Age (LIA) glacier extents. The authors use these data to discuss paleoclimate implications for the spatial trends in ELAs during the LGM and LIA, and quantify the difference in ELAs between the LGM and ELA to estimate the LGM temperature depression from pre-Industrial. Using past glacier extents or glacial geologic features to reconstruct spatial patterns in glaciers during the LGM across Alaska has been done before, however, previous studies are based on outdated and inconsistent methods, and/or have been applied to smaller regions. This works provides a more comprehensive and consistent approach, and generates what appears to be a comprehensive state-wide LGM dataset.

Another novel aspect of this study, is the determination of LGM ΔELA values by comparing LGM glacier extents to LIA glacier extents. This approach differs from that of previous studies that have different baselines, but often use modern glaciers, which are not in equilibrium with climate. While I was intrigued to see this approach, the mapping of LIA glacier extents is not as comprehensive, and concentrated almost exclusively in the Alaska Range, making statements about state-wide LGM-LIA temperature depressions (including in the manuscript’s title) seem overstated. The poor coverage of LIA glacier extents is a major weakness of the study, and most conclusions about the LGM-LIA temperature depression would not be as believable/justified without references to previous work from these regions (based on different methods).

Overall, this study will be of interest to the CP readership and presents high quality data. Although the ultimate findings and paleoclimate implications are not tremendously different from previous works, this study does provide an updated assessment based on LGM data compiled data from across the state using more recent methods for past ELA reconstructions, and has results that are compared to the most recent proxy and paleoclimate model data. I do not think the limits of the LIA mapping should prevent this from being published, but I do recommend additional mapping if possible or at least that more information be provided about the mapping of both LGM and LIA glacier extents as described below in my two major comments below.

ALso, I commend the authors for an extremely well-written and structured manuscript. It was very clear, for the most part, and I have only a few minor comments. I think the updated assessment of LGM glacier extents and the characteristics of LGM temperature and precipitation changes in Alaska are well documented here and will be well received by the community.

Major Comments

1. More information/justification is needed on the LGM and LIA datasets used for this study.

-LGM glaciers:  In Section 3.1 you state that “…we used the Alaska PaleoGlacier Atlas v2 to guide our mapping of LGM ice extents.” Were LGM glaciers mapped that are not in the Atlas? If so, information should be presented to document how that was done. If not, be clear in the text that ‘all’ individual LGM glaciers mapped are from the Alaska PaleoGlacier Atlas v2.

-LIA glaciers: (i) It is not clear at all how LIA glacier systems were chosen for mapping. Are any of these glaciers systems/moraines chosen based on previous mapping by the authors, or other published information that should be cited (e.g. Sikorski et al., 2009)? (ii) Overall, the LIA data are quite insufficient outside of the Alaska Range. It seems a bit of an overstatement to say that you have calculated ‘state-wide’ LGM-LIA ΔELAs when 90% of the data are from the Alaska Range. Only two of the LIA glacier systems are located outside of the Alaska Range….is this due to a lack of identifiable LIA moraines at other locations in the Brooks Range or the other massifs (which seems hard to believe).  Is it possible to include more LIA glacier extents? At least, more information should be provided on the LIA data used, and possibly to temper statements about state-wide trends (or provide more reference to previous work before making interpretations about state-wide trends).

2. Background.

The authors do a nice job of summarizing previous work on ELA reconstruction methods and how they have been applied in the past to LGM glaciers in Alaska. However there is no information on the LIA in Alaska. Since the LIA is used as a benchmark for calculating DELAs, there should be some introduction to the LIA in Alaska, including previous work mapping LIA glacier extents, how they compare to the extent of modern glaciers, the timing of the LIA, etc.

Specific Comments:

Page 6, line 1: Not clear how your data will be able to say anything about aridity.

Page 7, line 10: extra “.” before citations

Page 8, line 8: Sentence beginning with “These glaciers deposited….” You should provide references to work that has documented/characterized LIA advances in at least some of these regions in Alaska

Page 14, line 16: The parenthetical statements in the middle of the sentence starting with “In the Alaska Range..” make it difficult to understand the information presented. Consider moving to the end of the sentence or presenting in a different way. Same for how the first sentence of the following paragraph is structured.

Page 14, line 22: “We see no statistical relationship between longitude and ∆ELA.” I don’t think you have enough data from outside of the Alaska Range to suggest that this is significant. If you had better spatial coverage of LIA glacier extents, this would be more of an interesting result.

Page 17, line 14: Sentence starting with “Coversely, in Alaska…” needs a reference. What is the justification that Alaska was drier during the LGM, provide reference.

Page 20, line 19: Use “low” or “small” not both, for describing summer temperature depression

Page 21, line 18: “Our range DELA-derived…” insert ‘of’

Figure 3: A title is not needed since this information is in the caption. Also, the text is too small in the inset with labels for “LGM” and “LIA” points. The x-axis units are also not intuitive and do not make for easy comparison to locations of sites in the other figures… would be easier to understand if they were in degrees longitude.

This study presents an updated assessment of equilibrium line altitudes (ELAs) for Last Glacial Maximum (LGM) and Little Ice Age (LIA) glaciers in Alaska. ELAs are calculated for the LGM and LIA glaciers and are then used to regional ELA trends across Alaska. Additionally, the ELA data are used to quantify differences in ELAs (ΔELAs) between the LGM and LIA, as well as to estimate the temperature differences between the LGM and LIA. Although there have been several previous studies focused on ELAs in Alaska, this study provides a comprehensive, regional assessment using up-to-date methods. An important aspect is the use of the LIA (rather than modern glaciers) as a baseline for LGM ΔELA calculations. Overall, this study will be of interest to the readership of Climate of the Past.

Although this paper has merits and would be of interest to the CP readership, there are some issues that need to be addressed. I have four main comments on the manuscript. 

1.    Approach used to delineate the former glacier limits: based on the manuscript, it is unclear how the authors mapped and established the LGM and LIA glacier limits. For the LGM glaciers, the authors mention that the ‘Alaska PaleoGlacier Atlas v2’ was used to guide the mapping of the glacier limits (p. 10), but it is unclear if the authors undertook any original geomorphological mapping. It is also unclear how the limits of the “well-defined LIA glaciers” (p. 11) were identified. There need to be more detailed and transparent explanations of the mapping methods, with reference to established mapping practices (e.g. Chandler et al., 2018). 

2.    Chronological constraints on the glacier limits: linked to the mapping of the glacier limits, it is unclear how the age of the glacial landforms/limits were determined, as there is no reference to any chronological evidence/constraints. The authors need to provide details on how the ages of the limits were established. Are the ages based on ‘absolute’ chronological data, or are they based on relative methods and/or changes in the glacial landform/landsystem signature? Or is it simply assumed that the limits are either ‘LGM’ or ‘LIA’ glacier limits based on their geographical position?

3.    Spatial distribution of the LIA glaciers: the authors indicate that glacier reconstructions were produced for “selected valleys with well-defined LIA glaciers outlines” (p. 11), but it is unclear how these were chosen. Moreover, the limited spatial distribution of these LIA glaciers presents an issue in terms of the regional ELA assessments, as the LIA glaciers are limited to the Alaska Range, Kigluaik Mountains, and northeastern Brooks Range. Given the limited spatial distribution of the selected LIA glaciers, the ΔELA calculations should be restricted to the aforementioned localities. There is insufficient data to justify the state-wide ΔELA calculations, given that 90% (22 out of 24) of the LIA glaciers are located in the Alaskan Range. 

4.    Uncertainties associated with asynchronous glacier maxima: a key uncertainty in regional ELA assessments is the timing of the ice masses reaching their maxima, and it is generally assumed that all the glacier maxima were reached synchronously. However, this is unrealistic, and the ice masses would almost certainly have reached their maxima diachronously. While it is not possible to quantify and account for this uncertainty, there needs to be some acknowledgement and discussion of this issue in the manuscript. 

Specific comments

p. 7, l. 13: I disagree with the statement that ELA reconstructions are a “labor efficient” approach to assessing regional palaeoclimate and that they are “more easily applied to a large region”. While the ELA calculations themselves may not be time consuming, the process of glacier reconstruction is time consuming, especially when dealing with multiple ice masses across a large study area. 

p. 8, l. 11: “if” should be of.

p. 10, l. 23: did you apply a uniform basal shear stress value along the whole glacier flowline? Were variations in basal topography accounted for in the basal shear stress values?

p. 11, l. 8: the AABR method is not one of the simplest methods of calculating ELAs, it is one of the more robust and sophisticated methods.

p. 14, l. 5: why are there data for the LIA glacier systems (total numbers, distribution), but not for the LGM glaciers?

p. 14, l. 8: why have you not plotted the LIA ELA data on a map, as for the LGM glaciers?

p. 14, l. 15: a general point regarding the use of the term “temperature depression(s)”. I think it would be better to state “temperature differences” or similar, as the use of temperature depressions does not make any logical sense in this context. The LGM temperature cannot be depressed from the LIA because it occurred before the LIA (assuming that time is linear!). 

p. 14, l. 16 ff.: the text in the parentheses make this sentence difficult to read. Explanations of the error values reported would be better placed in the Methods section.

p. 14, l. 21: the ΔELA calculations should be restricted to the localities where there are LIA glaciers, as the limited spatial distribution of the LIA glaciers could skew the state-wide assessment.

p. 15, l. 2: as above, explain the errors in the Methods section.

P, 15, l. 10 ff.: Section 5.1 is excessively long and could be streamlined substantially. Could you present some of the data in a table format and then synthesise the previous work? There is no need to describe each previous study individually.

p. 15, l. 14–15: this compares ELA values at different scales. Why not directly compare the ELA values for the Brooks Range calculated in your study with those from Balascio et al. (2005)?

p. 16, l. 2: it would be better to indicate that the THAR method has been superseded by more robust methods of ELA calculation.

p. 17, l. 19–21: the independent clause after the colon is incomplete. 

p. 19, l. 1 ff.: Section 5.3 is excessively detailed and could be streamlined.

p. 22, l. 2: it is an overstatement to suggest that the ELA gradients are similar, given the very limited data for LIA glaciers outside of the Alaskan Range.

The paper presents an interesting synthesis of data from the ELA of the LGM and the LIA of Alaska, associated with the original ones reconstructed by the authors in some specific mountainous sectors.

The regionalization of the ELA is a delicate job because, differences in the method used for the calculation, uncertainties in the position of the frontal moraines, and subjectivity in the reconstruction of the surface of the past glacier, can introduce important biases in the final values, significantly invalidating the key interpretation paleoclimate.

In this paper the authors calculated the ELA on the basis of reconstructions based on numerical modeling anchored to glacial mechanics (GlaRe), and calculated the ELA in a systematically constant manner, arriving at discussing a dataset as robust as possible. The data density and spatial distribution over the Alaskan territory have therefore opened the door to insight palaeoclimatic discussions, although honestly acknowledging the points where uncertainties remain present (e.g. the lacking of a definitive mechanism for explaining the relatively warm summer temperatures).

The discussion of the data is skilfully proposed with a continuous comparison with results from different climate proxies (terrestrial and marine), which in the end gives the idea of a coherent palaeoenvironmental picture.

The figures are clear, I just suggest including some other geographical terms mentioned in the text (Gulf of Alaska, Bering Sea among others), for those who are not confident with the Alaska region. 

I agree with other reviewers who indicate that more effort could be made to show how glacier extent data extracted from datasets (e.g. Alaska PaleoGlacier Atlas v2) were handled. From the text, it can be understood that they have been taken as they are, but perhaps some geomorphological checks have been made. I believe that some examples (accompanied by a figure for both LIA and LGM glaciers) could be added. Even how the chronological data was associated with glacial extensions could be further commented on. On these two points (glacial extents and their ages) the authors could also evaluate what kind of uncertainty they might have introduced in their dataset, and if this could ultimately be relevant.

Specific comments

I have no further specific comments that have not already been reported by other reviewers