Reconstruction Of The Past Ground Surface Temperature Changes Using Borehole Paleothermometry

Characteristics of past climate variation of Qinghai-Tibetan Plateau are the basis for understanding present climate and predicting future climate of Tibetan Plateau. In this study, we investigated changes in ground surface temperature of several boreholes on Qinghai-Tibetan Plateau using Borehole Paleothermometry. Borehole temperature data are direct measurements of temperature from boreholes drilled into the Earth’s crust. Departures from the expected increase in temperature with depth(the geothermal gradient) can be interpreted in terms of changes in temperature at the ground surface in the past, which have slowly diffused downward, warming or cooling layers meters below the surface. Temperature perturbations under the ground surface are direct thermal response to ground surface temperature changes and preserve past climatic signal due to the heat diffusion process in permafrost-affected soils. To track past surface temperature variation we employ Borehole Paleothermometry that based on a physical connection between ground surface temperature and measured borehole temperature profiles. Thus ground surface temperature variation can be reconstructed from borehole temperature measurements using borehole paleothermometry inversion method. The annual temperature variation significantly attenuated permafrost with depth, the temperature variation signal of decade or longer time will spread to deeper ground. Ground temperature profiles in permafrost(deeper than 10 m) are sensitive indicator of surface climate change. The Earth as a natural low-pass filter making the temperature-depth profiles in permafrost area can be used to reconstruct past ground surface temperature changes.We need to understand the forward problem before dealing with surface temperature reconstruction problem, i.e. the heat transfer problem of ground temperature variation. Based on the one-dimensional heat conduction model there are analysis solutions of ground temperature with different types of ground surface temperature changes. According to the complex heat transfer process of permafrost area with phase change we can only numerically simulate ground temperature. This study use Control Volume method to simulate ground temperature variation under surface temperature variation. The simulation considers the effect of unfrozen water especially its effect on thermal properties and computes thermal properties of soil and then ground temperature at every time point. This study presents the simulation steps of ground temperature through a numerical example of surface temperature variation with cycles and the effect of unfrozen water on thermal properties and ground temperature.This study presents a numerical Tikhonov method that can be used to reconstruct past ground surface temperature records from measured borehole temperatures. We validate this method by applying it to two synthetic surface temperature cases: linear increase and synthetic climate history. These two cases represent climatic events such as recent global warming and climatic events respectively. Since measured borehole data include uncertainty we add random noise on synthetic borehole temperature profiles to simulate the process of noise suppression. Ground surface temperature recovered with corresponding uncertainty shows a close match with synthetic surface temperature for both cases. We show that this method can successfully suppress the noise disturbance and achieve smoother solutions. We illustrate the best criterion for choosing regularization parameters for these two synthetic cases. This study includes necessary theoretical and practical framework required for further method implementation with measured borehole temperatures. The ability of borehole temperature data to resolve past climatic events is investigated using Tikhonov method. The results can be used to help interpret existing surface temperature histories derived from borehole temperature data.Deep borehole temperature profiles have successfully been used to reconstruct past ground surface temperature history and the results are dependent on the inversion methods. These methods are tedious and sometimes unstable in iterative computation. In this study, we propose a new fundamental solution method to reconstruct the past ground surface temperature variation, which depends on the assumption that ground temperature field in a homogeneous region is governed by a one-dimensional heat conductive equation. To regularize the resultant ill-conditioned linear system of equations, we apply successfully both the Tikhonov regularization technique and the generalized cross validation parameter choice rule to obtain a stable approximation solution of the ill-posed inverse problem. Our new method is stable and meshless, and it does not require iteration. We conducted idealized simulations with good results.The problem of inferring ground surface temperature history(GSTH) from borehole temperature-depth data, like virtually every other geophysical inverse problem, is characterized by instability due to presence of noise. Due to the different ways in which the problem may be parameterized and optimized the solution is method-dependent. In this work we attempt to analysis the results obtained by four methods, including currently widely used Functional Space Inversion(FSI) and Singular Value Decomposition(SVD), and also new developed Method of Fundamental Solutions(MFS), and Tikhonov method. All of four methods are based on the theory of 1-D heat conduction. To assess the effectiveness of various methods, synthetic ground temperature profile data with noise were prepared and used to compare different methods. We analyze five mathematical models describing the GSTH:(1) one-step signal,(2) single-ramp signal,(3) smooth single-ramp signal,(4) sinusoidal signal, and(5) mixed sinusoidal signal. We use the same forward solver and spatial and temporal discretization in the four methods in order to eliminate possible differences arising from these sources. The four inverse methods yield similar results of the variation trends of the GSTH that are concerned. However, the estimated GSTHs differ in details of the timing and the magnitude of changes. The effectiveness of four methods becomes signal dependent that sinusoidal signal can be inverted robust by MFS method, other types of signal are reconstructed exactly by Tikhonov method when adding small level of noise, and FSI is good at suppressing the noise.This study use Borehole Paleothermometry method to reconstruct past ground surface temperature of several sites on Qinghai-Tibet Plateau: 1) 100 m PT1 borehole on the upstream of Heihe, the ground surface temperature has increased linearly from-2.7 ℃ to-2.05 ℃; 2) We reconstructed past ground surface temperature history at Heihe PT9 borehole using SVD, FSI, Tikhonov and MFS methods. Inversion results indicate the increasing trend of 0.7 ℃ in the past 120 years; 3) Ground surface temperature variation of Wudaoliang station is inverted using Temperature variation inverted from 120 m borehole at Wudaoliang Station using both SVD method and Tikhonov method. The reconstructed ground surface temperature shows the increasing trend of 1.8 ℃ of Wudaoliang during the past 83 years.; 4) Temperature reconstruction using 220 m borehole temperature-depth profile at Kunlun Mountains indicates the surface temperature increased 3.7 ℃ of Kunlun Mountains during in the past 308 years. Both Wudaoliang and Kunlun borehole sites show more exacerbated warming starting from 1980 s. Tikhonov method induced GST history is more robust and dependable. These GST trends fit the air temperature observation trend from the nearest Wudaoliang Meteorological Station but have small value deviation caused by local topography and surface energy budgets of ground surface. 5) Borehole transient temperature measurement from seven sites in northwestern Qaidam basin were separated from geothermal gradients and analyzed by functional space inversion method to determine past ground surface temperature variations in this region. All temperature profiles show the effects of recent climatic disturbances. Inversion shows an overall increase in ground surface temperature by an averaged 1.2℃(-0.11~2.21℃) during the last 500 years. Clear signs of a cold period between 1500 and 1900 A.D., corresponding to the Little Ice Age, have been found. Its coldest period was between 1780~1790 A.D. with the ground surface temperature of 5.4 ℃. During the 19 th and the 20 th century, reconstructed ground surface temperature shows a rising trend, and in the late 20 th century, the temperature started to decrease. However, the highest temperature in 1990 s broke the record of the past 500 years. This reconstructed past ground surface temperature variation is verified by the simulated annual surface air temperature computed by EdGCM and the cooling trend is also confirmed by other reconstruction of winter half year minimum temperatures using tree rings on the northeastern Tibetan Plateau.Based on the research of Borehle Paleothermometry this thesis prerents an improved method and novative mthod and comparison of them. In this paper, the reconstruction of the past ground surface temperature changes in different regions of central Tibetan Plateau is studied.