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1. Canopy cover and microtopography control precipitation-enhanced thaw of ecosystem-protected permafrost

   https://doi.org/10.1088/1748-9326/ad31d7  

   Eklof, Joel F.; Jones, Benjamin M.; Dafflon, Baptiste; Devoie, Élise G.; Ring, Katie M.; English, Marie E.; Waldrop, Mark P.; Neumann, Rebecca B. (March 2024, Environmental Research Letters)

   Abstract Northern high-latitudes are projected to get warmer and wetter, which will affect rates of permafrost thaw and mechanisms by which thaw occurs. To better understand the impact of rain, as well as other factors such as snow depth, canopy cover, and microtopography, we instrumented a degrading permafrost plateau in south-central Alaska with high-resolution soil temperature sensors. The site contains ecosystem-protected permafrost, which persists in unfavorable climates due to favorable ecologic conditions. Our study (2020–2022) captured three of the snowiest years and three of the four wettest years since the site was first studied in 2015. Average thaw rates along an across-site transect increased nine-fold from 6 ± 5 cm yr⁻¹(2015–2020) to 56 ± 12 cm yr⁻¹(2020–2022). This thaw was not uniform. Hummock locations, residing on topographic high points with relatively dense canopy, experienced only 8 ± 9 cm yr⁻¹of thaw, on average. Hollows, topographic low points with low canopy cover, and transition locations, which had canopy cover and elevation between hummocks and hollows, thawed 44 ± 6 cm yr⁻¹and 39 ± 13 cm yr⁻¹, respectively. Mechanisms of thaw differed between these locations. Hollows had high warm-season soil moisture, which increased thermal conductivity, and deep cold-season snow coverage, which insulated soil. Transition locations thawed primarily due to thermal energy transported through subsurface taliks during individual rain events. Most increases in depth to permafrost occurred below the ∼45 cm thickness seasonally frozen layer, and therefore, expanded existing site taliks. Results highlight the importance of canopy cover and microtopography in controlling soil thermal inputs, the ability of subsurface runoff from individual rain events to trigger warming and thaw, and the acceleration of thaw caused by consecutive wet and snowy years. As northern high-latitudes become warmer and wetter, and weather events become more extreme, the importance of these controls on soil warming and thaw is likely to increase. 

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2. Active Layer Dynamics Near Norilsk, Taimyr Peninsula, Russia

   https://doi.org/10.24057/2071-9388-2021-073  

   Grebenets, Valery I.; Tolmanov, Vasily A.; Streletskiy, Dmitry A. (December 2021, GEOGRAPHY, ENVIRONMENT, SUSTAINABILITY)

   This paper provides information on active layer thickness (ALT) dynamics, or seasonal thawing above permafrost, from a Circumpolar Active Layer Monitoring (CALM) site near the city of Norilsk on the Taimyr Peninsula (north-central Siberia) and the influences of meteorological and landscape properties on these dynamics under a warming climate, from 2005 to 2020. The average ALT in loamy soils at this 1 ha CALM site over the past 16 years was 96 cm, higher than previous studies from 1980s conducted at the same location, which estimated ALT to be 80 cm. Increasing mean annual air temperatures in Norilsk correspond with the average ALT increasing trend of 1 cm/year for the observation period. Active layer development depends on summer thermal and precipitation regimes, time of snowmelt, micro-landscape conditions, the cryogenic structure (ice content) of soils, soil water content leading up to the freezing period, drainage, and other factors. Differences in ALT, within various micro landscape conditions can reach 200% in each of the observation periods. 

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3. Vegetation green‐up date is more sensitive to permafrost degradation than climate change in spring across the northern permafrost region

   https://doi.org/10.1111/gcb.16011  

   Wang, Jian; Liu, Desheng (December 2021, Global Change Biology)

   Abstract Global climate change substantially influences vegetation spring phenology, that is, green‐up date (GUD), in the northern permafrost region. Changes in GUD regulate ecosystem carbon uptake, further feeding back to local and regional climate systems. Extant studies mainly focused on the direct effects of climate factors, such as temperature, precipitation, and insolation; however, the responses of GUD to permafrost degradation caused by warming (i.e., indirect effects) remain elusive yet. In this study, we examined the impacts of permafrost degradation on GUD by analyzing the long‐term trend of satellite‐based GUD in relation to permafrost degradation measured by the start of thaw (SOT) and active layer thickness (ALT). We found significant trends of advancing GUD, SOT, and thickening ALT (p < 0.05), with a spatially averaged slope of −2.1 days decade⁻¹, −4.1 days decade⁻¹, and +1.1 cm decade⁻¹, respectively. Using partial correlation analyses, we found more than half of the regions with significantly negative correlations between spring temperature and GUD became nonsignificant after considering permafrost degradation. GUD exhibits dominant‐positive (37.6% vs. 0.6%) and dominant‐negative (1.8% vs. 35.1%) responses to SOT and ALT, respectively. Earlier SOT and thicker ALT would enhance soil water availability, thus alleviating water stress for vegetation green‐up. Based on sensitivity analyses, permafrost degradation was the dominant factor controlling GUD variations in 41.7% of the regions, whereas only 19.6% of the regions were dominated by other climatic factors (i.e., temperature, precipitation, and insolation). Our results indicate that GUDs were more sensitive to permafrost degradation than direct climate change in spring among different vegetation types, especially in high latitudes. This study reveals the significant impacts of permafrost degradation on vegetation GUD and highlights the importance of permafrost status in better understanding spring phenological responses to future climate change. 

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4. Post-fire stabilization of thaw-affected permafrost terrain in northern Alaska

   https://doi.org/10.1038/s41598-024-58998-5  

   Jones, Benjamin M.; Kanevskiy, Mikhail Z.; Shur, Yuri; Gaglioti, Benjamin V.; Jorgenson, M. Torre; Ward Jones, Melissa K.; Veremeeva, Alexandra; Miller, Eric A.; Jandt, Randi (April 2024, Scientific Reports)

   Abstract In 2007, the Anaktuvuk River fire burned more than 1000 km²of arctic tundra in northern Alaska, ~ 50% of which occurred in an area with ice-rich syngenetic permafrost (Yedoma). By 2014, widespread degradation of ice wedges was apparent in the Yedoma region. In a 50 km²area, thaw subsidence was detected across 15% of the land area in repeat airborne LiDAR data acquired in 2009 and 2014. Updating observations with a 2021 airborne LiDAR dataset show that additional thaw subsidence was detected in < 1% of the study area, indicating stabilization of the thaw-affected permafrost terrain. Ground temperature measurements between 2010 and 2015 indicated that the number of near-surface soil thawing-degree-days at the burn site were 3 × greater than at an unburned control site, but by 2022 the number was reduced to 1.3 × greater. Mean annual ground temperature of the near-surface permafrost increased by 0.33 °C/yr in the burn site up to 7-years post-fire, but then cooled by 0.15 °C/yr in the subsequent eight years, while temperatures at the control site remained relatively stable. Permafrost cores collected from ice-wedge troughs (n = 41) and polygon centers (n = 8) revealed the presence of a thaw unconformity, that in most cases was overlain by a recovered permafrost layer that averaged 14.2 cm and 18.3 cm, respectively. Taken together, our observations highlight that the initial degradation of ice-rich permafrost following the Anaktuvuk River tundra fire has been followed by a period of thaw cessation, permafrost aggradation, and terrain stabilization. 

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5. Wildfires offset the increasing but spatially heterogeneous Arctic–boreal CO2 uptake

   https://doi.org/10.1038/s41558-024-02234-5  

   Virkkala, Anna-Maria; Rogers, Brendan M; Watts, Jennifer D; Arndt, Kyle A; Potter, Stefano; Wargowsky, Isabel; Schuur, Edward_A G; See, Craig R; Mauritz, Marguerite; Boike, Julia; et al (February 2025, Nature Climate Change)

   Abstract The Arctic–Boreal Zone is rapidly warming, impacting its large soil carbon stocks. Here we use a new compilation of terrestrial ecosystem CO₂fluxes, geospatial datasets and random forest models to show that although the Arctic–Boreal Zone was overall an increasing terrestrial CO₂sink from 2001 to 2020 (mean ± standard deviation in net ecosystem exchange, −548 ± 140 Tg C yr⁻¹; trend, −14 Tg C yr⁻¹;P < 0.001), more than 30% of the region was a net CO₂source. Tundra regions may have already started to function on average as CO₂sources, demonstrating a shift in carbon dynamics. When fire emissions are factored in, the increasing Arctic–Boreal Zone sink is no longer statistically significant (budget, −319 ± 140 Tg C yr⁻¹; trend, −9 Tg C yr⁻¹), and the permafrost region becomes CO₂neutral (budget, −24 ± 123 Tg C yr⁻¹; trend, −3 Tg C yr⁻¹), underscoring the importance of fire in this region. 

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