Stable isotopes of water recorded in polar ice cores are used to reconstruct past temperatures and the fractionation during phase changes make them a useful tracer of the hydrological cycle. This study focuses on the global and regional variations in the distribution of water isotopes with changes in the climate. Sensitivity experiments and time-slice simulations for the Last Glacial Maximum (LGM), Heinrich Stadial-1 and mid-Holocene climates were carried out both to understand the boundary conditions that exert the maximum influences on the isotopic composition of precipitation, and to reproduce the isotopic distribution of precipitation during these time periods. The numerical climate model, the National Center for Atmospheric Research (NCAR) Community Atmosphere Model CAM3.0 fitted with an oxygen-isotope module (Iso- CAM), is used to carry out the experiments. The first part of this study focuses on understanding the distribution of oxygen isotopes in precipitation during the LGM and to associate the anomalies from the control climate with the influence of different boundary condition constraints. Results from a pre-industrial control simulation are compared against experiments in which the influence of individual boundary conditions (greenhouse gases, ice-sheet albedo and topography, sea-surface temperature (SST), and orbital parameters) were changed each at a time to the LGM values to assess their individual impact. The results show that the SST and ice-sheet topography changes during the LGM are responsible for most of the modeled variations in the climate and hence the 18Oprecip distribution. In this study a detailed analysis of the seasonal and annual variations of 18Oprecip for the control and a combined LGM simulation is carried out. tion, the spatial and temporal slopes between the 18Oprecip and surface temperature are calculated for the combined LGM and control simulations over Greenland and Antarctica, which are compared with the reconstructions from the ice-cores and those simulated with other isotope models. Secondly, four different time slice experiments - preindustrial, mid-Holocene, LGM, and Heinrich Stadial-1 - were carried out to analyze the water isotope distribution over the African continent during these time periods. The local and non-local climate influences on the hydrogen isotope composition of precipitation ( Dprecip) during these different climates are investigated. The study highlights the strong impact of convection and rainout on the Dprecip over the tropics, along with the changes in large-scale circulation. In addition, model results for Dprecip for these time periods are compared with Dwax data obtained from the stable hydrogen isotope composition of plant leaf-wax n-alkanes, and show a qualitative agreement between the proxy and the model data. In a third part of the thesis, the present-day distribution of the isotopes in precipitation and water vapor were compared with the observations. The measurements of isotopes in water vapor have the advantage over the isotopes in precipitation that the observations are available around the year and also over arid regions where the precipitation events are very few. The study highlights the robustness of the results as well as some of the drawbacks of the model due to deficiencies in reproducing the hydrology over the land and because of the simplistic cloud isotope scheme
