Spatial-temporal Variations Of Seasonally Frozen Ground And Its Response To Climate Change In The Northern Hemisphere

Permafrost and seasonally frozen ground regions occupy approximately 24% and 57%,respectively,of the exposed land surface of the Northern Hemisphere.The widely definition of seasonally frozen ground include seasonally frozen layer(seasonally frozen ground regions)and seasonally thaw layer(active layer in permafrost regions).So the area extent of seasonally frozen ground occupied more than 80% land surface over Northern Hemisphere.Soil freeze/thaw cycle is one special character of seasonally frozen ground,which covers area extent,depth,time duration,variation of soil freeze/thaw.These changes in seasonally frozen ground have substantial impacts on energy,water and carbon exchange between the atmosphere and the land surface,surface and sub-surface hydrologic processes,vegetation growth,the ecosystem,carbon dioxide cycle,agriculture,and engineering constructuion,as a whole.Previous studies mostly focus on permafrost degradation.Although a large area extent of seasonally frozen ground exists,it is paid less attention.Thus,we employed statistically method,remote sensing techanically,and numerical simulate method to investigate spatial-temporal variations of seasonally frozen ground and its response to climate change in Northern Hemisphere.The main research studies are as follws:(1)Combine the multi-model ensemble mean of 16 of the coupled model inter-comparison project phase 5(CMIP5)over the historical period(1850-2005)and three representative concentration pathways(RCP 2.6,4.5,and 8.5)for 2006-2100,and Climatic Research Unit(CRU)dataset,available for 1901-2014,we evaluate surface air temperature over Northern Hemisphere,and analyze past and projected surface air temperature changes in different frozen ground regions;(2)evaluate and estimate past and projected freezing/thawing index across Northern Hemisphere,analyze the sensitive of frozen ground in the global warming background through freezing/thawing index;(3)combing soil temperature dataset available 1277 sites and re-analysis dataset,we investigate spatial-temporal variations of soil freeze depth and explore its potential forcing factors across Eurasia Continent;(4)we employ Stefan solution and combine 347 in-situ active layer thickness(ALT)sites to analyze the spatial and temporal variability of ALT in the northern hemisphere under historical and projected climate changes,and to explore the relationship between seasonal air temperature,seasonal precipitation,Arctic sea ice extent and ALT;(5)to explore the influence of frozen ground changes in vegetation growth using remote sensing dataset;(6)Base on the numerical simulate method,we reveal the changes of frozen ground response to land use and land cover changes(LULCC),and make a comparisions between permafrost regions and the whole Northern Hemisphere.The main conclusions are as follows:The evolution result indicates that multiple CMIP5 models can be simulated surface air temperature well over Eurasia continent.In the late 20th-century,there is a slightly better agreement of CMIP5 ensemble averages than it in the early 20th-century.An increasing trend of CRU and CMIP5 ensemble averages mean annual air temperature(MAAT)is remarkable across Northern Hemisphere,and a more rapid increasing trend appears in high-latitude and high-altitude regions.In the historical period(1850-2005),MAAT has a statistically significant increasing trend in permafrost regions(PF)and Non permafrost regions(Non-PF),with a rate of 0.094 ?/decade,and 0.064 ?/decade,a net change of 1.47 ? and 1.0 ?,respectively.In RCP 2.6,RCP 4.5,and RCP 8.5 scenarios,there is a significant increase trend of MAAT,at a rate of 0.096 ?/decade,0.332 ?/decade,and 0.807 ?/decade,respectively;Non-PF with a rate of 0.069 ?/decade,0.248 ?/decade,and 0.613 ?/decade,respectively.It found that the climate warming in PF was more sensitive than in Non-PF.For the spatial climatological of thawing index,it ranges from 0 ?*d to greater than 11,000 ?*d in Northern Hemisphere,and smaller value located in the highlatitude and high-altitude regions,while freezing index ranges from 0 ?*d to more than 10,000 ?*d,and greater value located in the high-latitude and high-altitude regions.Both the spatial pattern and time-series of area-averaged show an increase trend in thawing index,and a decline trend in freezing index.For Northern Hemisphere as a whole,area-averaged thawing index derived from CMIP5 dataset shows an increasing rate of 1.14 ?*d/yr during 1850-2005,while a decline rate of-1.39 ?*d/yr during 1850-2004 in freezing index.Thawing index is generally projected to increase in Northern Hemisphere under all three RCPs.RCP 8.5 exhibits the largest significant trend at a rate of 13.85 ?*d/yr,corresponding to 1315.8 ?*d increase by 2100.The RCP 2.6 indicates the smallest increases,at 1.51 ?*d/yr by 2100.While,there is a decline projected trend in freezing index in three RCPs,greatest decrease by rate of-9.8 ?*d/yr in RCP 8.5,smallest decline by rate of-1.2 ?*d/yr in RCP 2.6.The greatest decrease of freezing index are projected to occur over the high-latitude and high-altitude regions,and the decreasing rate of freezing index is more rapid than thawing index.There is a statistically significant decrease trend of ensemble averages soil freeze depth(SFD)under RCP8.5,RCP4.5,and RCP2.6,by-4.58 cm/decade,-1.85 cm/decade,and-0.45 cm/decade respectively,during 2006-2100 across Eurasia continent.In the historical period(1850-2005),decrease trend is at-0.49 cm/decade.Accounting for the potential forcing variables,there is strongly negative correlation between SFD and air temperature,freezing index,Arctic Oscillation(AO),and sea ice extent.There are negative and positive relationship between SFD and snow depth(SD)in different regions.A long time-series of ensemble area averaged mean ALT is statistically significant,increasing at a rate of 0.57 cm/decade during 1850-2005 in the historical experiment,0.77 cm/decade for RCP 2.6,2.56 cm/decade for RCP 4.5,and 6.51 cm/decade for RCP 8.5 for the period 2006-2100 in the northern hemisphere.Summer air temperature and precipitation have a significant positive effect on ALT increases.However,the declining Arctic sea ice extent trend is strongly negatively correlated with ALT increases,pointing to a common driver of these cryospheric changes.Time series of averaged NDVI in growing season is statistically significant,increasing at a rate of 0.00079 yr-1 during 1982-2015 in PF across Northern Hemisphere.An increasing NDVI is the main trend,taking up approximately 70.39% of the total pixels in the growing season,58.14% in spring,66.02% in summer,and 71.99% in autumn.A higher thawing index,thicker ALT,warmer soil temperature,or more precipitation are most possibly contributing to a greener NDVI.An earlier thawing date and extended thawing days also benefit vegetation growth.The response of LULCC in permafrost regions can be found that there was a statistically significant broad cooling effect.The overall area-averaged annual surface air temperature cools at a rate of-0.032 ?/100 yr,a net change of 0.37 ? during 850-2005,and a rate of-0.0334 ?/100 yr,net change of 0.39 ? of soil temperature at 0.2 m depth.The active layer thickness(ALT)is with a decline rate of-0.54 cm/100 yr ± 0.023 cm,a net decline of 6.24 ± 0.26 cm during 850-2005.Comparisons of LULCC cooling effect between permafrost regions and Northern Hemisphere,it demonstrates that permafrost response to LULCC is more pronounced.In this study,we investigate spatial-temopral variations of seasonally frozen ground and its response to climate change across Northern Hemisphere from surface air temperature,freezing/thawing index,SFD,ALT,vegetation growth,and LULCC.All these results is helpful to further study the physical mechanism between seasonally frozen ground and climate change,eco-hydrology process.