|General Comments |
The authors present a careful analysis of the timing of refractory black carbon (rBC) deposition in the Alps of western Europe based on duplicate ice cores. The record of rBC from this core in the late 19th Century is significantly different from that obtained in an earlier ice core (Thevenon et al., 2009) and indicates the deposition of rBC did not significantly increase beyond the ‘natural’ pre-industrial variability until at earliest 1868. The authors use this observation to attempt to refute the hypothesis that industrial emissions of black carbon may have been responsible for forcing the synchronous retreat of glaciers at the end of the little Ice Age in the European Alps (Painter et al. 2013). The authors compare the new record to similar records from ice cores elsewhere in the Alps and in Greenland. They also compare the records of rBC deposition with bottom-up emission estimates.
While the authors clearly demonstrate that statistically significant increases in rBC deposition occurred after the generally accepted timing of the onset of glacier retreat in the Alps, they do not present conclusive evidence that rBC did not play a role in this retreat. Specifically, they have not used methods that can clearly demonstrate the role (or lack of role) of rBC in forcing glacier fluctuations during this period. To do this would require modelling the often-complex relationships between glacier fluctuations and meteorological forcing, considering glacier feedbacks (e.g. albedo feedback) and the effect of rBC on albedo, to demonstrate the mechanisms responsible for retreat in the second half of the 20th Century (e.g. Zekollari 2017, or Goosse et al., 2018). The authors seem to be looking for a mono-causal relationship between glacier fluctuations and rBC forcing over the past centuries, rather than accounting for the dynamic relationships between various meteorological forcing and glacier feedbacks and demonstrating the effect of each on forcing glacier fluctuations. This point was raised in the initial review by Reviewer 2, and the authors have not made significant revisions in response. The addition of longer records of glacier fluctuations and volcanic aerosols provided (in new Fig. 9), do not provide any quantification of the influence of different factors on glacier length fluctuations, as requested by the reviewer. There are no significant changes to the discussion or conclusions (other than discussion of longer aerosol and glacier records), and the title reports a result that has not been thoroughly tested in the paper. Either much greater revisions to the text are needed or additional analyses are needed to support the arguments.
The authors certainly cast doubt on the hypothesis of Painter et al, 2013, but cannot take their interpretation as far as disproving it. It appears that rBC may not have played such a large role in the initial retreat of glaciers in the Alps before 1875 but it cannot be ruled out that they played a role or that they did not force the continued retreat in the 20th Century. The results presented here certainly highlight the need to reconcile differences in the air temperature estimates in the 19th Century from observational records, reconstructions and tree ring analysis, to enable confident attribution of observed glacier changes to different forcing.
The discussion of the validity of gradually increasing emissions during the 19th Century is a useful outcome of the manuscript. However, the authors need couch their discussion more careful in the context of the effect of meteorological variability on the relationship between emissions and the deposition records. This should include an evaluation of the influence of disconnection of the study site from the valley planetary boundary layer as suggested by Painter et al. 2013. To demonstrate that high frequency fluctuations (annual) in rBC can be interpreted as commensurate variations in emissions, an analysis of the relationship between contemporary emissions rates (which are more well constrained) and deposition rates at the field site is needed.
The paper will certainly be an important addition to the literature but needs to be reframed and the discussion and conclusions changed to reflect the interpretations that can reasonably be made from the data presented. In this light, the title is misleading and emphasises a more speculative implication of the research that has not been clearly demonstrated. I would suggest the title needs to be changed to align with its key finding e.g. “Increase in high-altitude industrial black carbon deposition occurred after initiation of 19th century glacier retreat in the Alps.”
Specific Comments (page-line):
p1 ln28 – “study reveals that in 1875 AD, the time when European rBC emission rates started to significantly increase” – but this has not been demonstrated – deposition at high altitudes increased at this time, but the question of emissions-deposition has not been addressed, so this sentence needs rewording.
p2 ln7 – “because it absorbs - in minute amounts - solar radiation” - this statement is ambiguous. Do you mean "even at very low concentrations, rBC absorbs solar radiation..." (as is, the sentence read that rBC on absorbs only small amounts of solar radiation).
p2 ln 29 – “involving snow albedo feedbacks that significantly enhance initial radiative forcing” – the use of ‘radiative forcing’ here is confusing as it is used earlier when referring to atmospheric forcing – suggest rewording to “that are significantly enhanced by snow albedo feedbacks”
p3 ln28 – this description of the BC records is very different from the description in Painter et al. 2013, who show a step change at Colle Gnifetti in 1850 – please explain the differences.
p3 ln32 – this discussion of the hypothesis of Painter et al. 2013 should be in the discussion section. Also, these limitations are not borne out by the data presented here (1) the new records show “the overall EC trend from Fiescherhorn ice core is closely reproduced by the new rBC record” – which would support the results of Painter et al. 2013, (2) – the paper does not demonstrate that the new records of glacier retreat differ significantly from those used by Painter et al. 2013, (3) – no data is presented to show that contributions from dust were important in the late 19th Century retreat.
p4 ln2 - “Here we set out to rigorously re-evaluate the hypothesis of industrial BC forcing the “End of the Little Ice Age in the Alps” (Painter et al., 2013)”. As discussed earlier, the manuscript does not use the appropriate methods to address this aim. I would suggest altering this aim and keeping the current analyses, rather than adding additional analyses that would address the current aim.
p6 ln6 – this uncertainty needs to be addressed I the discussion as it is rather large compared to the short time between the time of emergence of rBC and the onset of glacier retreat.
p8 ln8 – these correlation coefficients (0.35 and 0.63) are not high, especially for Pearson’s correlation coefficients. Please reword.
p9 ln3 – here and in the discussion, the authors need to comment directly on how and why their record differs from the that in Thevenon et al. 2009. This is a key aspect of the manuscript that is lacking, which is important given it is central to the re-interpretation of the results of Painter et al. 2013. In this context, it may be worth pointing out that in Figure 2 of Painter et al. 2013, the lines for Fiescherhorn and Colle Gnifetti ice cores seem to have been mislabelled as the opposite core.
p6 ln17 – “both, our” – remove comma.
p15 ln4 – Data availability details are missing
Supplementary Fig. S5: Is the correlation shown in panel c a Pearson’s coefficient? If so, please state.
References (not already cited in the manuscript)
Goosse, H., Barriat, P.-Y., Dalaiden, Q., Klein, F., Marzeion, B., Maussion, F., Pelucchi, P., and Vlug, A.: Testing the consistency between changes in simulated climate and Alpine glacier length over the past millennium, Climate of the Past Discussions, doi: 10.5194/cp-2018-48, 2018. 1-27, 2018.
Zekollari, H., Fürst, J. J., and Huybrechts, P.: Modelling the evolution of Vadret da Morteratsch, Switzerland, since the Little Ice Age and into the future, Journal of Glaciology, 60, 1155-1168, 2017.