Determination Of Polar Ice Sheet Change From ICESat And GRACE Satellite Observations

Climate change studies require scientific evidence from a network of atmospheric and oceanic sciences as well as solid earth geophysics. One possible way to assess how the Earth changes due to global warming is by studying ice sheet changes and the interaction among ice sheet, ocean and atmosphere. Melting of ice sheet is a key factor which influences the global climate change. Research has showed that the evolution of ice sheet, which now has been making the mean sea level rise and severely threatening the continent where human survive, has major effect on global sea level change. Although major melting is not imminent, the continuous changes of ice sheets impact on the processes of global climate change and greenhouse effect. Since the annual mass exchange between the ice sheets and the ocean is about 8 mm/year of global sea level equivalent, small changes in this rate are significant.1992 IPCC Supplement on Scientific Assessment of Climate Change noted that the largest uncertainty about sea level is "rooted in our inadequate understanding of polar ice sheets whose response to climate change also affects predictions of sea level rise". The National Academy of Sciences(1990) stated "possible changes in the mass balance of the Greenland and Antarctic ice sheets are fundamental gaps in our understanding and are crucial to the quantification and refinement of sea-level forecasts". Quantifying the state of the polar ice sheet is of great importance since uncertainty in ice sheet mass balance directly affects uncertainty of the global sea level change rate. Studies on ice sheet response to climate change require data sets with high accuracy and uniform ice-sheet coverage.The Ice, Cloud and land Elevation Satellite (ICESat), as part of NASA's Earth Observing System program to study the Earth atmosphere and cryosphere, was launched in Jan 2003. The GLAS(Geoscience Laser Altimeter System) on the ICESat can determine the inter-annual and long-term changes in polar ice sheet mass, and quantify the change of polar ice sheet mass and its equilibrium. The aim of this dissertation is to investigate the inter-annual and long-term changes of polar ice sheets and the contribution of the total mass balance of polar ice sheets to global sea level rise using combined ICESat observation data and GRACE gravity data; to obtain key data necessary for predicting the change of total ice sheets mass and the change of sea level; and to investigate the causes of changes in mass balance as well as their impacts on global sea level and global climate.Main contents and achievements of this paper include:(1) Originating from the relationships among global climate change, sea level rise as well as ablation of ice sheets and glaciers, ablation status of ice sheets and glaciers, together with ice sheets mass balance progress in Anactica and Greenland, has been illustrated in detail, and techniques of monitoring surface elevation change and quality change of ice sheet have been studied;(2) The application of ICESat laser altimetry and on-orbit performance of ICESat have been summarized, then the advantage of monitoring ice sheet surface elevation change using laser altimetry has been analyzed, together with waveform and altimetry precision of GLAS; error sources of laser altimetry have been analyzed, and corresponding corrections have been given;(3) GRACE data products and development of GRACE gravity field have been described in detail, and the methods of determining earth gravity model using GRACE data, as well as the theory of determining mass change of earth surface using GRACE time-variable gravity field, have been studied;(4) "Clean"and reliable data set was obtained by studying and realizing the techniques of pre-processing, refining ICESat laser altimetry data with which the cross-over point algorithm was described;(5) The interpolation methods, such as Shephard, continuous curvature splines in tension and Kriging method, have been studied with the conclusion that continuous curvature splines in tension is more suitable for data interpolatation of polar ice sheet;(6) Through the average chang rate of elevation computed by derivatives of elevation difference and time difference obtained from cross-over point algorithm, change tendency of ice sheet mass and its precision were analysized, the factors affect elevation change of ice sheet were revealed;(7) Ice sheet mass changes of Antarctica and Greenland were deduced by using GRACE observations;(8) Through comparing the results of polar ice sheet changes obtained from ICESat data and GRACE data, correlation between polar ice sheet mass balance and sea level change was revealed.In conclusion, the main achievements of this dissertation includes:ICESat laser altimetry data pre-processing and refinement techniques; high-precision digital terrain elevation model interpolation method; a high-precision and high-resolution polar ice DEM was established based on ICESat altimetry data; time series of the polar ice sheets elevation change were obtained via cross-over point algorithm; the changes of polar ice sheet mass were deduced by utilizing GRACE time-varible gravity data; correlation between polar ice sheet mass balance and sea level change was revealed through comparing the results of polar ice sheet changes obtained from ICESat data and GRACE data; the multi-functional software system with independent intellectual property rights was developed for calculating polar ice mass changes and water storage change by combining satellite gravity data and laser altimetry data.