the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Nov 2025
Fine-scale spatial variability of winter CO2 and CH4 fluxes in Arctic tundra derived from snowpack gradient measurements
Abstract. Winter carbon dioxide (CO2) and methane (CH4) fluxes from soils under seasonal snowpacks make non-negligible, yet poorly constrained contributions to annual carbon budgets across Arctic regions. Quantifying these fluxes and their spatial variance will better constrain uncertainties in simulations of winter carbon fluxes from terrestrial biosphere models. We address this gap by measuring and identifying patterns and spatial variability in CO2 and CH4 soil-atmosphere fluxes through late winter snowpacks at an upland tundra site in the western Canadian Arctic. Instantaneous fluxes were calculated from CO2 and CH4 microsite concentration gradients at 10 cm to 20 cm vertical resolution (n = 119) through the snowpack across five homogeneous surface covers, representing dominant vegetation types. We measured consistent soil-to-atmosphere CO2 fluxes but with significantly different rates across surface covers (0.8 to 100 mgC m-2 day-1), which were strongly influenced by snow depth and soil surface temperature, exhibiting higher emissions under deeper snowpacks and warmer soil surfaces. CH4 fluxes were also coupled to soil surface temperature and varied between −0.04 and 0.08 mgC m-2 day-1. Persistent CH4 uptake was observed in warmer soils (-6.0 to -0.5 °C) in a sparsely populated black spruce and shrub dominated area with deep snow, indicating active methane oxidation during winter. Microsite scale CO2 and CH4 fluxes were statistically independent of vertical snow microstructure, indicating that winter fluxes could be reliably calculated from single gas concentration at the soil-snow interface, negating the need for additional snowpack gas measurements. These results open new avenues for quantifying fine-scale spatial variability of wintertime CO2 and CH4 fluxes in Arctic tundra, which can constrain biogeochemical process representations in terrestrial biosphere models and inform spatial upscaling methodologies.
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Status: open (until 28 Feb 2026)
- CC1: 'Comment on egusphere-2025-5637', Jeff Welker, 28 Nov 2025 reply
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RC1: 'Comment on egusphere-2025-5637', Anonymous Referee #1, 17 Dec 2025
reply
This manuscript presents an extensive and carefully executed field study of wintertime CO₂ and CH₄ fluxes in Arctic tundra using high-resolution snowpack gas concentration gradients. The dataset is unusually rich in spatial coverage for winter conditions, and the combination of snow microstructure measurements (SMP) with gas concentration profiles represents a clear methodological strength. The paper addresses an important and timely topic for Atmospheric Chemistry and Physics, namely the quantification and spatial variability of cold-season greenhouse gas fluxes and their implications for model parameterisation.
The manuscript is generally well written, logically structured, and supported by appropriate statistical analyses. The results provide valuable insights into (i) the role of snow depth and soil temperature in controlling winter CO₂ emissions, (ii) the occurrence of wintertime CH₄ uptake in forested tundra, and (iii) the limited influence of vertical snow microstructure on diffusive flux estimates.
However, some key interpretations require further clarification and, in a few cases, stronger justification. In particular, the discussion of winter CH₄ uptake, the assumption of steady-state diffusion and linear gradients, and the broader implications for process representation in terrestrial biosphere models would benefit from additional nuance. This study represents a valuable contribution to the understanding of winter greenhouse gas fluxes in Arctic tundra and is well suited for Atmospheric Chemistry and Physics. The literature cited is adequate and so are the graphics. I suggest a minor revision of the manuscript according to my specific comments before to consider it for publication on the Journal. After addressing the points above—particularly regarding the interpretation of CH₄ uptake and the limits of methodological generalisation - I recommend its publication.
Specific comments
The observation of consistent net CH₄ uptake in the forested transect (T4-Forest) under frozen conditions is one of the most interesting results of the study, but it also requires a more cautious interpretation. While the authors correctly cite recent evidence of winter CH₄ sinks in Arctic and sub-Arctic environments, CH₄ uptake is more commonly reported during the growing season or shoulder seasons, when liquid water availability and oxygen diffusion are less constrained.
The manuscript would benefit from a clearer discussion of the mechanisms enabling methanotrophic activity at soil temperatures between approximately −6 and 0 °C.
In particular:
- The possible role of zero-curtain conditions should be discussed more explicitly, including whether soil temperature measurements at the snow–soil interface are sufficient to infer liquid water availability in the upper soil layers.
- Alternative explanations, such as net uptake driven by diffusion-limited emissions rather than active oxidation, should be acknowledged and discussed.
- The comparison with summer or shoulder-season CH₄ uptake rates reported in the literature (including sites with similar vegetation and soil types) could be strengthened to contextualise the magnitude of the observed winter sink.
The flux calculations rely on the assumption of steady-state conditions and linear concentration gradients through the snowpack. While the authors demonstrate that most concentration profiles are well approximated by linear fits, some profiles exhibit clear departures from linearity and partial homogenisation in the basal snow layers. In this context, the manuscript would benefit from a clearer justification of the steady-state assumption, particularly considering potential sources of short-term temporal variability such as wind pumping, transient pressure fluctuations, or diurnal temperature changes. In addition, a brief sensitivity analysis, or at least a qualitative discussion, of how deviations from linearity could bias flux estimates would strengthen the robustness of the results, especially in low-flux environments such as the T3-Tussock transect where gradients are often weak. Finally, further clarification is needed on how profiles with weak or non-significant concentration gradients were handled in the flux calculations and in the subsequent upscaling and subsampling analyses.
The finding that reliable flux estimates can be obtained using only basal and near-surface gas concentrations is potentially very impactful and could significantly simplify future field campaigns. However, this conclusion may be partly site- and condition-specific and would benefit from additional clarification. In particular, it would be helpful to specify the environmental conditions under which this simplification is expected to remain valid, such as the absence of ice layers, predominantly dry snowpacks, or limited melt–freeze cycles. Further discussion is also warranted on whether this approach can reasonably be extended to wetter snowpacks, profiles dominated by ice crusts, or sites experiencing mid-winter thaw events, where diffusive transport may deviate from the assumptions applied here.
The manuscript frequently refers to implications for TBMs, but these links remain somewhat conceptual. The paper would be strengthened by a more explicit discussion of:
- Which specific model parameters (e.g. winter soil respiration temperature sensitivity, snow thermal conductivity, CH₄ oxidation schemes) are most directly informed by the presented data.
- How the observed fine-scale spatial variability could realistically be represented or parameterised at the grid-cell scale used in Earth system models.
Citation: https://doi.org/10.5194/egusphere-2025-5637-RC1 -
RC2: 'Reply on RC1', Anonymous Referee #1, 17 Dec 2025
reply
in the previous post, Atmospheric Chemistry and Physics was incorrectly indicated instead of the correct journal, Biogeosciences.
We apologize for the confusion.Citation: https://doi.org/10.5194/egusphere-2025-5637-RC2
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Gabriel , great job on this and thanks so much for continuing to focus on winter emissions so so important.
However, you have missed my program's beginning of this modern day (past ~ 3 decades) field of research which was initially criticized when I presented our initial NSF funded data from Arctic Alaska in CPH at the 1997 ITEX meeting. I showed the flux data, measurable and argued that when winter emissions (~240 days of snow cover in N AK) were added to summer net fluxes, the tundra was a net C source: "Jeff you are crazy". how can all that SOM, permafrost be present if the system is a net C source ?came the audience challenges
my reply was today is different than the past and we now have real emission data from what makes the Arctic, the Arctic, winter.
I have been delighted see Webb, Natalie, and others, including your team, join my winter emission crusade.
These are some of the absolute foundational papers that you should be including in your introduction of the "issue" and in the discussion as to how this data compares
to what the Welker program discovered decades ago and continue today.
Thank you for all that you and your team are doing but knowing and recognizing the origins of this research theme, is essential to educating the readers on the foudational science and
how your new science continues to strengthen our understanding of 2/3 of the Arctic's calendar :)
The first 4 are in many ways the "beginning of the modern winter C emission studies initiated by my team/s", W Ochel's group also published a 1997 paper as well.
And, our continued contributions with packages like Natali et al. 2019
Fahnestock, J. T., Jones, M. H., Brooks, P. D., Walker, D. A., & Welker, J. M. (1998) Winter and early spring CO₂ flux from tundra communities of northern Alaska. Journal of Geophysical Research 102(D22):29925-29931.
Fahnestock, J. T., Jones, M. H., Brooks, P. D., & Welker, J. M. (1999) Significant CO₂ emissions from tundra soils during winter: Implications for annual carbon budgets of arctic communities. Global Biogeochemical Cycles 13:775-779.
Jones, M. H., Fahnestock, J. T., & Welker, J. M. (1999) Early and late winter CO₂ efflux from Arctic tundra in the Kuparuk River watershed, Alaska. Arctic, Antarctic and Alpine Research 31:187-190.
Welker, J. M., Fahnestock, J. T., & Jones, M. H. (2000) Annual CO₂ flux from dry and moist arctic tundra: Field responses to increases in summer temperature and winter snow depth. Climatic Change 44:139-150.
Schimel, J., Fahnestock, J., Michaelson, G., Mikan, C., Ping, C., Romanovsky, V., & Welker, J. M. (2006) Cold-season production of CO₂ in Arctic soils: Can laboratory and field estimates be reconciled through a simple modeling approach? Arctic, Antarctic and Alpine Research 38(2):249-255.
Sullivan, P. F., Welker, J. M., Arens, S. J. T., & Sveinbjörnsson, B. (2008) Continuous estimates of CO₂ efflux from arctic and boreal soils during the snow-covered season in Alaska. JGR Biogeosciences 113:G04009.
Nowinski, N., Taneva, L., Trumbore, S., & Welker, J. M. (2010) Decomposition of old organic matter as a result of deeper active layers in a snow depth manipulation experiment. Oecologia 163(3):785-792.
Lupascu, M., Welker, J. M., Xu, X., & Czimczik, C. I. (2014) Rates and radiocarbon content of summer ecosystem respiration in response to long-term deeper snow in the High Arctic of NW Greenland. JGR Biogeosciences. doi.org/10.1002/2013JG002494
Lupascu, M., Czimczik, C. I., Welker, M., Cooper, L., & Welker, J. M. (2018) Winter ecosystem respiration and sources of CO₂ from the High Arctic tundra of Svalbard: Response to a deeper snow experiment. JGR Biogeosciences. doi.org/10.1029/2018JG004396.
Ala-aho, P., Welker, J. M., Bailey, H., Pedersen, S., Kopec, B., Klein, E., Mellat, M., Mustonen, K., Noor, K., & Marttila, H. (2021) Arctic snow isotope hydrology: A comparative snow-water vapor study. Atmosphere. doi.org/10.3390/atmos12020150
Rixen, C., Hoye, T., Welker, J. M., et al. (2022) Winters are changing: snow effects on Arctic and alpine tundra ecosystems. Arctic Science. doi.org/10.1139/as-2020-0041
Pedron, S., Jespersen, R. G., Xu, X., Khazindar, Y., Welker, J. M., & Czimczik, C. (2023) More snow accelerates legacy carbon emissions from Arctic permafrost. AGU Advances. doi.org/10.1029/2023AV000942
Kantola, N., Welker, J. M., Leffler, A. J., et al. (2025) Impacts of winter climate change on northern forest understory carbon dioxide exchange determined by reindeer grazing. Science of the Total Environment. doi.org/10.1016/j.scitotenv.2025.180089.