Reduced Black Carbon Deposition Weakens Arctic Snow Darkening and Snowmelt Acceleration during 2046&ndash;2050

Zhang, Zilu; Zhou, Libo; Zhang, Meigen

doi:10.5194/egusphere-2026-4792

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https://doi.org/10.5194/egusphere-2026-4792

Preprints

04 Sep 2026

| 04 Sep 2026

Reduced Black Carbon Deposition Weakens Arctic Snow Darkening and Snowmelt Acceleration during 2046–2050

Zilu Zhang, Libo Zhou, and Meigen Zhang

Abstract. Air-pollution mitigation can reduce Arctic black carbon (BC) deposition and thereby slow Arctic warming by weakening snow darkening and associated radiative effects. Yet the regional land-surface benefits of future BC-deposition reductions across contrasting mitigation pathways remain poorly quantified. We use Polar-WRF coupled online with the SNICAR snow-albedo scheme to isolate the effects of spatially heterogeneous BC deposition associated with low-emission (SSP1-2.6), intermediate-emission (SSP2-4.5), and high-emission (SSP5-8.5) pathways during 2046–2050. All experiments share an SSP2-4.5 large-scale meteorological background and differ only in prescribed BC deposition; they therefore quantify the BC-deposition pathway rather than the full climate response to each SSP. Relative to simulations without BC in snow, Arctic-mean spring snow albedo decreases by 16.9 %, 18.1 %, and 19.3 % under SSP1-2.6, SSP2-4.5, and SSP5-8.5, respectively. BC deposition increases surface net radiation by 1.0–1.4 W m⁻², spring snowmelt by 4.1–6.1 mm d⁻¹, advances snow disappearance by 6.7–7.6 days, and increases Arctic land-mean near-surface air temperature by 0.09–0.12 °C. Compared with SSP5-8.5, the 39.6 % lower BC deposition under SSP1-2.6 produces 12.4 %, 28.6 %, and 32.8 % smaller BC-induced changes in snow albedo, net radiation, and snowmelt, respectively. Responses are regionally heterogeneous: Greenland shows the strongest albedo and snowmelt responses despite relatively low deposition, whereas the European Arctic and West Siberia show stronger radiative responses, where substantial snow darkening coincides with greater incoming solar radiation. These results provide a process-based, regional evaluation of the deposition-related Arctic climate benefits of mid-century mitigation pathways.

Received: 08 Aug 2026 – Discussion started: 04 Sep 2026

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Zilu Zhang, Libo Zhou, and Meigen Zhang

Status: final response (author comments only)

RC1:
'Comment on egusphere-2026-4792', Anonymous Referee #1, 29 Sep 2026
This manuscript investigates how future reductions in BC deposition may affect Arctic snow albedo and near surface climate using Polar-WRF coupled with the SNICAR snow-albedo scheme. The study finds that higher BC deposition enhances snow darkening, increases surface net radiation, and accelerates snowmelt, with substantial regional heterogeneity across the Arctic. The topic is intrinsically interesting. However, the current analysis has substantial methodological concerns, as noted in detail below. The manuscript formatting also needs improvement, particularly several figures that contain non-English characters. I therefore recommend the manuscript for publication following major revision.
Major Comments
L172. The reported Arctic BC deposition fluxes of 15 to 26 mg m^-2 d^-1 across the SSP scenarios seem very large to me, corresponding to more than 5 g BC m^-2 yr^-1. These values appear substantially higher than those reported in previous studies and therefore require clarification. I recommend that the authors provide a quantitative comparison with published Arctic BC deposition estimates. In addition, as noted in Line 507, the prescribed BC deposition is derived from a single model. Given the importance of these deposition fields to the subsequent analysis, I recommend that the authors evaluate the sensitivity of their results to the prescribed BC deposition.

The reported snowmelt enhancement of 4 mm d^-1 also appears inconsistent with the simulated surface energy perturbation of 1 W m^-2. If all of the added energy were used for melting, 1 W m^-2 would correspond to only ~ 0.26 mm d^-1 of snowmelt. This discrepancy requires clarification.

The manuscript describes how deposition flux is converted into BC mass concentration, but provides limited information about the resulting snow BC concentration. I recommend adding maps and regional statistics of snow BC concentration in ng g^-1 for all SSPs. These values can then be directly compared with Arctic snow observations and previous SNICAR studies.

The treatment of BC within the evolving snowpack also needs substantially more detail, including its vertical redistribution within the snowpack and any changes in optical properties during snow accumulation and melt.

As acknowledged, the interpretation of atmospheric and cloud responses from single realizations is not sufficiently robust. Ensemble simulations would be needed to more accurately represent these complicated processes rather than making relatively speculative inferences from a single realization. I therefore recommend moving Section 3.5 to the Supplement and moderating the discussion.

Minor Comments
Please clarify whether the reported spatial and temporal means are weighted by grid-cell area and by year.

L273 to L287. The manuscript seems to repeat essentially the same explanation twice.

L364. The non-monotonic Greenland snowmelt response across the SSP scenarios deserves more discussion. Why does SSP2-4.5 show the largest snowmelt increase?

L441. Please define the cloud liquid water fraction.

Technical Corrections
Please check the axis labels in Fig. 2d, Fig. 3d, Fig. 6b, and Fig. 9.

L295. “W W m-2” should be corrected.

L375. This should be Figure 9b instead of Figure 8b.

L388. Duplicated citation.
Citation: https://doi.org/10.5194/egusphere-2026-4792-RC1
RC2:
'Comment on egusphere-2026-4792', Anonymous Referee #2, 10 Oct 2026
Review of “Reduced Black Carbon Deposition Weakens Arctic Snow Darkening and Snowmelt Acceleration during 2046-2050” by Zhang et al.
This paper presents a unique sensitivity study looking at the effects of changes in black carbon emissions on snow darkening effect/albedo reduction into the future. Through a variety of controls in their model setup, they isolate the impacts of varied black carbon deposition on snow darkening effect for the SSP2-4.5 pathway. They find that weaker fossil fuel reduction pathways lead to more snow darkening effect and present a quantification of these impacts. Their analysis is focused on the Arctic during peak melt season and is an important consideration in global radiative balance estimates going forward.
While this work is an important and worthwhile contribution to the literature, there were major issues that I would like to see addressed before recommending this paper is published. First and foremost was the lack of acknowledgement of snow fraction in the manuscript. In order to clarify the impacts of BC on a future projection scenario, the authors set the background model climatology to the SSP2-4.5 pathway. Barring a misunderstanding on my part, this background climatology will then set the background snow surfaces. While this is a reasonable control, the paper uses language to suggest that the results are representative of different SSP mitigation pathways, but the feedbacks of the background climate on snow surfaces mean their results are representative of different black carbon deposition schemes in the context of the SSP2-4.5 pathway and its resultant snow cover. I think this is an important to identify when setting up the message of the paper, and I didn’t think the impacts of snow fraction on the spatial pattern and effects were sufficiently addressed. I also think that if this paper wants to speak to the future SSP mitigation pathway impacts, there needs to be an update to the methodology to include different background climates in each of the mitigation pathway calculations (see more below). And the last major comment was that the figures had significant issues that made them hard to interpret and compare to one another, and these need to be carefully reviewed.
Please see the major comments and minor comments below. Given the above comments and the following comments, I recommend this paper for acceptance after major revisions.

Major comments:
The snow fraction is going to be a driver of your spatial pattern in albedo reduction, and this in turn depends a lot on the background climate state of your simulations. It seems that one side effect to using the same climate state in all simulations is that you may introduce bias in your low (SSP1-2.6) and high (SSP5-8.5) pathways, which will have different effects given these pathway’s background snow fraction. As a hypothesized example: SSP1-2.6, while having a lower deposition rate, may also have higher snow fraction which could lead to enhancement in the albedo reduction potential of said BC deposition; on the other hand, SSP5-8.5, with lower snow fraction due to higher temperatures, may have a weaker albedo reduction potential over the Arctic. I would like to see these concerns address and justified more strongly by the authors. I see potential options as a snow fraction weighted formulation for the albedo reduction or more careful wording of the results to indicate their pertinence only to the SSP2-4.5 pathway. I see the most thorough option to be the inclusion of the different background climate states in each pathway calculation and included as additional variance in the snow albedo reduction for each pathway, but this is likely much more work than major revisions suggests and one of the above options may be better suited.

The figures had many typos and the axes and order were not consistent. Please review these carefully. The maps would benefit from having the regions as overlays to aid in interpretation of regional means.

In-line comments:
Line 153: Can you include these regions as overlays on fig. 1?
Line 171: Consider renaming SSP1-2.6, SSP2-4.5, … to something other than the pathways to differentiate from actual climatological differences. Maybe something like BC1-2.6, BC2-4.5, BC5-8.5
Line 176: Is this unexpected given the derivation from NorESM? Does the prescription of bc emissions in the SSP frameworks mirror the deposition pattern?
Table 1: What is SEN? Please define.
Figure 2: y-axis label needs to be translated (double check other plots as well); consider overlaying the regions detailed in 2d on the arctic maps; it seems the colors in 2d are reversed and green is actually showing SSP-5.85, not SSP-1.26; 25-75th pct boxes are missing in this and other box plot figures.
Line 194: correct regiona to regional
Line 213: In part, this consistency is likely due to the presence of snow, which will be determined by the SSP2.45 climatology. Please add a line mentioning the effect of snow fraction on the signals shown.
Line 248: I would imagine this is related to the background snow fraction which is set by your climate state. Please show a plot of snow fraction to see if it is comparable to the reduction pattern.
Fig. 6: Can you explain why the WR net sw reduction is constant across emission pathways, but the net longwave radiation varies the most across these pathways?
Line 325: how is snowmelt energy consumption estimate? Is this latent heat flux?
Line 327: I'm not sure I agree from looking at 7b - it seems that ground heat flux is dominating in these regions.
Fig. 7: Sensible heat acronym is switched from SH to HS; also, please keep the axis the same as it makes comparison across the regions very difficult - check all figures for consistency. Do the same for the ssp bars - please keep the order uniform across all plots
Line 364: Fig. 8a?
Line 375: It seems that fig 8 is showing melt rate. I do not see the induced shift in snow free days
Line 388: repeated citation
Fig. 9: The acronyms for your regions are different here than in previous plots.
Line 510: I think this is a good control, but given the feedbacks in snow darkening effect should be included in this study. One issue is that the background climate state will set your snow fraction, and your snow fraction will determine what kind of albedo reduction you have. If we assume the SSP-1.26 scenario has more snow, then the smaller deposition may have a larger effect. On the other hand, if you have the SSP-5.85 scenario climatology, there is likely less snow and the increased deposition may have less of an albedo effect due to there being less snow. Unless I am misunderstanding something in the work, I think snow fraction needs to be considered in these analyses, perhaps through an extrapolation of the different climate states for each emission scenario, which could add additional uncertainty bars to the final estimates. This is especially important since the two boundary scenarios (SSP-1.26, SSP-5.85) are the most compared in this work. Either that, or rephrase your message to clarify that these effects are in the context of SSP2-4.5.
Line 512: I agree with this interpretation, but I think more care needs to be taken in discussion within the paper that treat spatial pattern as a deposition effect, and not a set climatological snow fraction effect. So this is not just a BC-deposition pathway, it is an analysis of changing BC-deposition within the SSP2-4.5 meteorology.
Line 540: This may be true. See previous mention of potential controls by snow fraction on these results. The climatological background snow fraction should be addressed more thoroughly in the paper.
Citation: https://doi.org/10.5194/egusphere-2026-4792-RC2

Zilu Zhang, Libo Zhou, and Meigen Zhang

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Short summary

Black carbon, a soot-like particle from burning fuels, can travel to the Arctic and settle on snow, making it darker. Darker snow absorbs more sunlight, warms the surface, and melts sooner. We used computer simulations to compare future pollution-control pathways. Stronger controls reduced this pollution, weakening snow darkening and spring melting. The benefits differed across the Arctic, showing that local snow and sunlight conditions matter when assessing cleaner-air policies.


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