Active Layer Thickness in the Northern Hemisphere: Changes From 2000 to 2018 and Future Simulations

Abstract

Permafrost is a product of cold climates and is mainly distributed in cold climatic high latitudes and high elevations in the Northern Hemisphere (Zhang et al., 1999). The permafrost regions account for approximately 22% of the land area in the Northern Hemisphere (Obu et al., 2019). Permafrost degradation may lead to greenhouse gas emissions from the decomposition of organic carbon stored in the permafrost regions, further contributing to global warming and accelerating permafrost thaw (Schuur et al., 2015).The active layer is the soil layer where water and heat are exchanged between the surface and air in permafrost areas. The freeze-thaw process of the active layer can greatly affect the hydrothermal physical properties of the soil, surface evaporation, vegetation growth, and surface albedo Zhao et al., 2019).These changes regulate exchange processes such as sensible and latent heat fluxes between the soil and the atmosphere, which significantly influence regional circulation (Zhang et al., 2014). The hydrothermal conditions within the active layer are the main factors controlling the water and energy exchange between permafrost and the atmosphere (Cheng et al., 2019), which can directly affect the ecological environment, hydrological processes, and carbon cycle in permafrost regions (Jorgenson & Osterkamp, 2005).There is great spatial heterogeneity in the active layer thickness (ALT) across the Northern Hemisphere. The ALT varied from approximately 30 cm in the Arctic and circumpolar regions to greater than 10 m in the midlatitude mountainous permafrost zone during 1990(Luo et al., 2016). The regional average ALT values were 48 cm in Alaska, 93 cm in Canada, 164 cm in the Nordic countries (including Greenland and Svalbard) and Switzerland, 330 cm in Mongolia, 476 cm in Kazakhstan, and 230 cm on the Qinghai-Tibet Plateau (Luo et al., 2016).During the past decades, the rate of warming in the permafrost zones in the Northern Hemisphere has been 2-3 times the global average (Hu et al., 2021;Mu et al., 2020). Global warming has led to an increase in permafrost temperature, a reduction in the extent of permafrost, and an increase in the ALT. From 1990 to 2012,

Widespread deepening of the active layer in northern permafrost regions from 2003 to 2020

“…The dataset therefore provides a comprehensive view of landscape level spatial and temporal patterns of ALT over the NPR that corresponds closely with in situ ground measurements of ALT represented from the sparse station network. Our results suggest that there are widespread deepening trends of ALT, similar to previous analyses (Park et al 2016, Peng et al 2018, Li et al 2022a, 2022b. Furthermore, the deepening ALT is broadly aligned with increases in permafrost temperature (Biskaborn et al 2019).…”

“…Li et al (2022b) used the Kudryavtsev method and integrated various forcing data (such as temperature, snow, vegetation and soil data) to simulate changes in ALT in the NPR and found ALT increased at a linear rate of 0.86 cm yr −1 from 1969 to 2018, with the highest thickening rate in Alaska and the Mongolian Plateau (∼2.4 cm yr −1 ) and the lowest thickening rate in Greenland (∼0.3 cm yr −1 ). Li et al (2022a) reported an overall significant linear increase in ALT of 0.65 cm yr −1 , with approximately 71% of the NPR showing an increase from 2000 to 2018. These studies provide useful understanding of broad regional patterns and climate impacts on ALT trends, but are generally too coarse to resolve finer landscape heterogeneity, where local terrain, soil, and vegetation features may have a larger influence on permafrost stability.…”

Increasing Alaskan river discharge during the cold season is driven by recent warming

“…Rapid climatic warming has increased the mean annual air temperature (MAAT) in Alaska by 2.1 • C over the last 70 years (Walsh and Brettschneider 2019), which has altered hydrologic conditions via reduced snowpack, permafrost degradation (Li et al 2022), increased subsurface flow and soil moisture (Walvoord et al 2012), and altered precipitation (Bieniek et al 2014). These hydrologic responses to anthropogenic climate change are often reflected by the magnitude and seasonality of river discharge (Van Vliet et al 2013).…”

Distribution and Recurrence of Warming‐Induced Retrogressive Thaw Slumps on the Central Qinghai‐Tibet Plateau

“…Permafrost has been considered to be an important indicator of moderate climate change because it is sensitive to the air temperatures and precipitation (Farquharson et al., 2019; Mekonnen et al., 2021). Recently, the degradation of permafrost has received considerable attention (Li et al., 2022; Ran et al., 2018; Xia et al., 2022). Previous studies indicate that modern climate change has triggered numerous retrogressive thaw slumps (RTSs) in permafrost landscapes, especially during the thaw season in years with higher air temperatures and precipitation (Lewkowicz & Way, 2019; Luo et al., 2022b).…”