SIMULATION OF THE THERMAL REGIME OF PERMAFROST IN NORTHEAST CHINA UNDER CLIMATE WARMING

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1 SIMULATION OF THE THERMAL REGIME OF PERMAFROST IN NORTHEAST CHINA UNDER CLIMATE WARMING Li Shxn, Chen Rijie State Key Laboratory of Frozen Soil Engineering, Lanzho Institte of Glaciology and Geocryology, Chinese Academy of Sciences, Lanzho ,China Abstract Nmerical simlation indicates that the geothermal regime of permafrost in northeast China will change as air temperatre continosly rises. Calclated reslts show that, when the initial srface mean annal temperatres, T s, are 0.5, 0.0,-0.5,-1.5,-.5, and -3.5¼C and the annal range of srface temperatre is 5¼C, the mean annal grond temperatres at a depth of 14 m will correspondingly be -0.16, -0.70, -1.17, -.10, -3.03, and -4.05¼C, and the permafrost thicknesses respectively will be 16.9, 8.3, 38.3, 58.9, 81.3 and 11.5 m nder eqilibrim between the climate and permafrost thermal regimes. Fifty years later, the predicted temperatres at a depth of 14 m will rise to 0.0, 0.0, -0.17, -0.94, and -.83¼C nder the conditions assmed. When Ts is lower than -0.5¼C, the depth to the permafrost table will increase by abot 0 cm. If ftre air temperatres rise at a rate of 0.04¼C/a or less, and the grond srface temperatre changes at same rate, the decrease in the area of permafrost in Northeast China will be within the limits of the region where the mean annal srface temperatre in the existing permafrost area is higher than 0¼C. Introdction Changes in the thermal regime of permafrost are generally determined by climatic processes nder natral conditions and climatic or grond srface changes cased by hman activities. One of the latter concerns is global climatic warming cased by increasing concentration of greenhose gases in the atmosphere. Scientists predict that climate warming cased by manõs activities will far exceed those from natral processes dring the next centry. The IPCC impact assessment on climate change (1990) gives a conseqent increase of the global mean annal temperatre in the range of 1.5¼C to 4~5¼C for a dobling of CO in the atmosphere between now and the years 05 to 050 nder a Òbsiness-as-salÓ scenario. The 1995 report predicts a rise in the global mean srface temperatre of abot 1~3.5¼C sing climate models that take into accont greenhose gases and aerosols by 100 (IPCC, 1996). Climate warming cold reslt in an increase in active layer thickness overlying permafrost, a rise in grond temperatres, and thinning of permafrost. Research concerning ftre changes in the geothermal regime of permafrost is therefore important for analyzing present constrction, determining design principles for different types of constrction, and determining its inflence on the geological environment nder a warming climate. The response of permafrost thickness and grond temperatre to climate change nder continos warming is likely to be diseqilibrim. Changes in the permafrost geothermal regime are dependent not only on an increase in air temperatre, bt also on (1) the grond moistre content, inclding ice and nfrozen water, the thermal condctivity and specific heat capacity of frozen and nfrozen grond, () the geothermal gradient below the permafrost base, (3) the annal range of air temperatre changes in the local regions, and (4) the grond srface conditions. Therefore, the response of permafrost to climatic change will differ in different sectors of the same region and in different regions. The following scenario was sed to simlate ftre changes in the thermal regime of permafrost in northeast China. (1) The air temperatre rises at a rate of 0.04¼C per year; the grond srface temperatre rises at same rate. () Based on meteorological observatories in the region, the average annal range of grond srface temperatre is 5¼C. (3) The stdy region contains soil layers that are homogeneos. (4) The geothermal regime of the permafrost is in eqilibrim with the existing environment. Mathematical description Under natral conditions, the permafrost temperatre field, with change, can be described by the following differential eqations and bondary and initial conditions: Li Shxn, Chen Rijie 651

2 T T Cf = f t = 1 1 C T T t = T( 0, t)= f() t Tx (, 0)= gx ( ) ( ) = ( ) = T x, t T x, t T 1 x x= ξ f T T L W W d 1 ξ f x = γ x d( ) dt ( ) = ( ) = T x, t T x, t T 1 xd x= ξd f T1 f x= H = q [1] [] [3] [4] [5] [6] [7] [8] T T L W W d 1 ξ [9] f x d = γ x d d ( ) dt where f, are the thermal condctivities of frozen and thawed grond (W/m K); C f, C are volmetric heat capacities of frozen and thawed grond (kj/m 3 ¼C); γ d is the dry density of the grond (kg/m 3 ); T 1, T are the temperatres of the frozen and thawed grond (¼C); T f is the freezing point (¼C); W, W are the total water content and nfrozen water content of the grond (%); x, t are the spatial and temporal coordinates (m, h); H is the depth in the calclated region (m); ξ is the phase change front (m); and ξ d is the permafrost base. Eqations (1)-(9) were solved by nmerical methods. The thickness, H 1 of the pper layer eqals 5 m. In the layer, W=17%, W =3%, γ d =1600, γ f =1.57, γ =1.8, C f =187, C = 475. In the lower layer, H, W=6%, W =1%, γ d =1600, γ f =1.61, γ =1.8, C f =1411, and C =1673. The thickness of the layer is determined by the permafrost thickness. Let pper and lower bon dary conditions be f(t) and q: Figre 1. Changes in simlated grond temperatre with time at depths of (a) 14 m and (b) 0 m, for different vales of T s. 1: T s =0.5¼C; : T s =0.0¼C; 3: T s =-0.5¼C; 4: T s =-1.5¼C; 5: T s =-.5¼C; 6: T s =-3.5¼C. 65 The 7th International Permafrost Conference

3 Figre. Simlated grond temperatre profiles in the 50th year. (a) T s =0.0¼C; (b) T s =-0.5¼C. ()= + + f t T Gt A s q = G g 1 0 π sin 8760 [10] [11] where G t =0.04¼C/8760 h, the rate of increase in the air temperatre; G g (0.04¼C/m) is the geothermal gradient of the thawed grond below the base of permafrost; A 0 (6¼C) is one-half of the annal range of the grond srface temperatres; T s is the initial mean annal grond srface temperatre. Calclated reslts The permafrost in Northeastern China nderlies abot 39,000 km and occrs north of 45 N and between 131 E and 116 E. Grond temperatres at the depth of zero annal amplitde range from 0 to abot -4.¼C. To determine the ftre permafrost geothermal regime nder a warming climate, eqations (1)-(9) were solved by finite difference methods for T s =0.5, 0.0, -0.5, -.5 and -3.5¼C. In the initial state, the permafrost thicknesses for these vales are, respectively, 16.9, 8.3, 38.3, 58.9, 81.3, 11.5 m. The calclated reslts show that: (1) nder the assmed condition, the T s bondary between permafrost and seasonally frozen grond is 0.5¼C or higher; () in comparison to heat condction withot phase change in the grond, the depth of zero annal amplitde of grond temperatre in permafrost depends on T s in addition to the factors affecting the depth of the zero annal amplitde temperatre change in nfrozen grond (Kydeliaftref, 199); and (3) the amplitde of temperatre change decreases with increasing depth (Figre 1). When a year-rond sprapermafrost talik develops, significant annal temperatre flctations no longer penetrate into the permafrost (Figre 1, T s =0.5 and -0.5¼C) When T s =0.0¼C and -0.5¼C, the changes of grond temperatres with depth in the 50th year at different times are shown in Figre. The profiles of grond temperatres in the figre show that, when T s =0.0¼C, a talik develops between the permafrost and the active layer (Figre a). When T s =-0.5¼C, the frozen grond maintains contact with the active layer in 50th year Li Shxn, Chen Rijie 653

4 Figre 3. The profiles of the mean annal grond temperatres for different times. (Figre b). Therefore, the permafrost deteriorates into seasonally frozen grond, only if the present srface temperatre of the permafrost is higher than -0.5¼C. An analogos simlation (Li et al., 1996) for the permafrost on the Qinghai-Xizang (Tibet) Platea shows that, when T s is higher than -1.1¼C, 50 years later the permafrost degrades and some permafrost taliks develop. There A 0 =13 C, half that of northeastern China. Therefore, a more stable geothermal regime in permafrost will reslt in northeastern China than on the Qinghai-Xizang (Tibet) Platea becase the annal range of air and grond srface temperatres in Northeast China is higher than those on the Qinghai-Xizang Platea. Under a climatic warming scenario, althogh the rate of permafrost degradation in higher-temperatre frozen grond will be more rapid than in lower-temperatre frozen grond, the rate of temperatre rise is slower than in the lower-temperatre frozen grond (Figre 3). For T s =0.5, 0.0, -0.5, -1.5, -.5, -3.5¼C nder the given conditions, the initial mean annal temperatres at the depth of 14 m are correspondingly 0.16, -0.70, -1.17, -.10, and -4.05¼C; and the permafrost thicknesses are respectively, 16.9, 8.3, 38.3, 58.9, 81.3, 11.5 m. The calclated reslts indicate that, 50 years later, the temperatres at a depth of 14 m will rise to 0.0, 0.0, -0.17, , and -.83¼C, and the thicknesses of permafrost will decrease to abot 14.0, 5.7, 36.1, 58.8, 81.3 and 11.5 m. Conclsion Based on analysis of the factors affecting the permafrost geothermal regime and the calclated reslts, the following conclsions are reached. 654 The 7th International Permafrost Conference

5 1. In comparison with heat condction withot phase change in the grond, the depth of zero annal amplitde temperatre change in permafrost depends on T s in addition to other factors affecting the depth of the zero annal amplitde temperatre change in nfrozen grond. The amplitde of the grond temperatre decreases with increasing depth. When the permafrost becomes detached from the seasonally frozen grond (i.e., a spra-permafrost talik is present), the depth of annal temperatre flctation does not penetrate below the top of the permafrost table. When the permafrost is contigos with the active layer, the depth of the annal temperatre flctation normally does not penetrate to the permafrost base. 3. Since the thermal condctivity of the grond is low, changes in permafrost geothermal regime will not eqal those in other climatic environments nder a continosly changing climate where air and grond srface temperatres will increase more rapidly than those in permafrost. 4. Becase the thermophysical properties of frozen grond and nfrozen grond are different, the stability of the permafrost geothermal regime will be less with a smaller annal range of air and grond srface temperatres nder a climatic warming scenario.. Becase of the inflence of the phase change, the sensitivity of the grond temperatre will be lower with a rising temperatre in the frozen grond nder climatic warming. Bt, since the permafrost thickness will thin with the rising temperatre, the depth of the annal temperatre flctation normally does not penetrate to the permafrost base. References Intergovernment Panel on Climate Change (1990). Climatic Change: The IPCC Impact Assessment. Working Grop II, IPCC (1996). Climate Change: Impact, adaptations and mitigation of climate change. Kydeliaftref, B.A. (199). Predicting Principle in Research of Engineering Geology. Lanzho University Press (In Chinese). Li, S., Cheng, G. and Go, D. (1996). The ftre thermal regime of nmerical simlating permafrost on Qinghai- Xizang (Tibet) Platea, China, nder climate warming. Science in China (Series D), 39(4). Li Shxn, Chen Rijie 655

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