Research On The Processes And Simulation Of The Active Layer Change In Permafrost Zone Over The Northern Alaska

The active layer is a key feature of permafrost zone and is the primary places for heat and mass exchange between the permafrost and the atmosphere.The active layer and its changes have important impacts on inter-Earth system water exchange,surface hydrologic processes,geomorphological processes,cold region ecosystem,and cold zone engineering construction.Permafrost is widespread over 80%of the surface in Northern Alaska,and the hydrothermal state of the active layer has significant implications for the climate and underlying permafrost changes in the Arctic region.In particular,the melting of underground ice in rich-ice permafrost has made the permafrost thermal state and the active layer freeze-thaw cycle processes more sensitive to climate change.Since the 1990s,the rapid warming of the climate in Northern Alaska has led to a rapid increase in permafrost temperatures and consequent changes in the active layer,with significant impacts on the ecology,hydrology,carbon cycle,and infrastructure of the Arctic.The spatiotemporal change in the active layer is a key indicator of Arctic and global climate change.However,the extreme climatic conditions and harsh environmental conditions make it difficult to carry out long-term and large-scale permafrost monitoring in the region,and it is difficul to obtain long-term monitoring data records.Therefore,in the context of rapid Arctic warming,important scientific questions need to be addressed,namely,what are the processes of the active layer change in the permafrost zone of the Northern Alaska and its response to climate warming?To address these issues,this study comprehensively integrates site observations,remote sensing products,reanalysis data,and climate model output data on a regional scale to clarify the pattern of climate change in different periods in Northern Alaska.The improved Control Volume Permafrost Model(CVPM)is used to numerical simulations and predictive analyses of the active layer change and its response to climate change in Northern Alaska,revealing the active layer changes and its response to climate change and physical mechanisms on a regional scale.The main conclusions are as follows:(1)The air temperature in Northern Alaska has shown a significant warming trend.Over the past 70 years(1950-2021),the overall warming rate is about 0.5℃/10a.By the end of the 21st century,it is expected that the air temperature will be on a significant warming trend,with an increase at 0.19℃/10a,0.41℃/10a,0.81℃/10a,and0.89℃/10a under different climate scenarios.The condition of snowpack is relatively stable(with no significant statistical trend).During the last decade or so,the snowpack started on average in late August and ending in mid-June,with an average snow extent of 0.16×10~6 km~2.There is an early trend in snow onset day and snow off day,and the snow cover duration shows an extended trend,but the extension trend is not significant,with an average extension of only 1 day.On the entire region,the air temperature variation is controlled by a combination of latitude,longitude,topography,altitude,ocean and atmospheric circulation,and snow cover variation is strongly influenced by altitude and ocean.(2)Soil temperatures at all depths within the active layer in Northern Alaska has shown a significant but modest warming trend in inter-annual variability from 1998-2019.The average soil moisture content at 15 cm is about 0.4 m~3/m~3.The variation in soil water content gradually increases as soil temperature continues to decrease and the moisture phase change intensifies.The exponential relationship between unfrozen water content and ground temperature is closely related to soil texture and soil moisture content.The Active Layer Thickness(ALT)shows a slowly fluctuating increase of 3.6cm/10a,and the average ALT is 50 cm.The Active Layer Index(ALI)shows a significant increase of 59.04 cm-day/a.The active layer usually starts to thaw in early June and reaches its maximum melt depth by early October,with the freezing process taking less than one month and the thawing process takes nearly four months.There is no obvious trend in the near surface freeze-thaw time of the active layer,and the overall change is relatively smooth.The active layer changes are weakly influenced by summer temperatures and the ALI better reflects the response of the active layer to air temperature changes compared to ALT.In addition,snowpack has an effect on ALT changes,but its effect is very limited,as the heat-insulating effect of snowpack is diminished by the longer period of snow cover on the ground.(3)During the historical period(1950-2021),the ALT in Northern Alaska showed a significant increasing trend,with a rate of increase of 0.005 m/10a and a mean ALT of 0.18 m.The mean annual ground temperature(MAGT)variation also showed a significant increasing trend,with a rate of increase of 0.16℃/10a and a mean MAGT of-6.71℃.The SSPs scenario(2022-2100)show a slight increasing trend in ALT variation.The linear growth rates of ALT are 0.003 m/10a,0.007 m/10a,0.009 m/10a and 0.01 m/10a for the SSP126,SSP245,SSP370,and SSP585 scenarios,respectively,The mean ALT is 0.2 m,0.21 m,0.22 m,and 0.22 m,respectively.The SSPs scenarios all show a significant increase in MAGT variation,with linear growth rates of0.099℃/10a,0.17℃/10a,0.23℃/10a,and 0.28℃/10a,respectively.Using the historical period as a baseline,MAGT would increase by 1.19±1.13℃,1.83±1.08℃,2.14±1.03℃ and 2.37±1.01℃ by the end of the 21st century(2091-2100)under these four scenarios,respectively.The MAGT is more sensitive to climate change than the ALT,with a sensitivity of 0.63℃ for MAGT to climate change in the historical period.The sensitivity of MAGT to climate change in the SSPs scenario is 0.57℃,0.55℃,0.52℃ and 0.53℃,respectively.Both in the past and in the future,the air temperature has been the main factor influencing changes in the thermal indicators of permafrost.At the regional scale,ALT and MAGT vary most significantly in the Arctic coastal plain region and in the Brooks Ridge Mountains.This study investigates the changes in hydrothermal processes in the active layer of the permafrost over the Northern Alaska and their response to climate change in terms of climate change characteristics,hydrothermal processes in the active layer,and spatial-temporal changes of the active layer in historical and future periods.This study helps to explore the physical mechanisms between the active layer changes and climate change in the permafrost region in the context of climate warming,and provides important scientific information and references for the study of permafrost and climate change in the Arctic and globally.