The northern hemispheric glacial-interglacial climate states during the late Quaternary period drove Caspian Sea level (CSL) changes of up to 150 m and can be used as an analogue for assessing present and future climate impacts. Geologic reconstructions of these paleo-lake levels and potential links with different climate events remain complex while future declines in modelled lake levels vary widely and are uncertain. The key drivers for such CSL include variations in the water budget balance between precipitation and evaporation (P-E). This thesis employs a climate modeling approach to investigate long-term changes in the Caspian Sea (CS) hydroclimate during different climate states from the late Quaternary to the end of the 21st century. The new results from the Community Earth System Model (CESM1.2.2), contribute to an improved interpretation of reconstructed paleo-lake levels with respect to changing P-E patterns and identify key drivers for future CSL changes. This study produced new modeling results for the late Quaternary period, that constrain the timings and identify the climate conditions favourable for major CS transgressions and regressions, in comparison with selected geological reconstructions; under three glacial (Marine Isotope 3 (MIS3), Last Glacial Maximum (LGM), Heinrich event 1 (H1)) and two interglacial (last interglacial (LIG) and early Holocene (EH)) climate states. The two interglacial climate states suggest favourable climate conditions (higher temperature and precipitation) for the CS that result in a positive water budget (LIG-P-E anomalies of 14.6 meter/1000 years and the EH-P-E anomalies 5 meter/1000yr). The results propose a transgression was initiated by the summer large-scale and convective precipitation, triggered by enhanced summer insolation and the associated wind anomalies. The warmer and wetter MIS3 interstadial climate is identified as being responsible for a transgression with P-E anomalies of 16 meter/1000yr. The colder and drier LGM climate favours a regression with P-E of -12 meter/ 1000yr. These P-E anomalies and climate conditions agree with the reconstructions. However, our simulated P-E anomalies (for the H1); do not capture the magnitude of the reconstructed highstands during the deglaciation, and it is clear that meltwater into the CS is responsible for this highstand; and as our model does not include a sophisticated meltwater routing into the CS; hence comparisons with selected reconstructions remain complicated. This study also assessed different CESM horizontal resolutions and model setups to identify the best version that can represent the CS climate and climate modes of variability such as North Atlantic Oscillation (NAO) for the period 1850-2000 CE, as well as presenting new CSL under two new emission scenarios by the end of the 21st century. CESM1.2.2 with 1AAAdegree CAM5 is identified as the best skill in simulating the NAO and its effects on CS catchment hydrology. Projections under the Representative Concentration Pathways RCP4.5 and RCP8.5 confirm the winter NAO remains the major winter variability with a significant impact on the Caspian catchment hydroclimate. However, under global warming, the evaporation over the sea is the key driver for a CSL decrease of about 9 m and 18 m between 2020 and 2100 for the RCP4.5 and RCP8.5 scenarios, respectively. The new CSL values are larger than previous projections of CSL, and include an overestimated total evaporation due to a larger CS surface area in CESM. Despite the clear potential for this, current global climate models neglect to include accurate representations of CS area. This study generated new results to notably aid in evaluating the impacts of different CS surface areas on the regional and large-scale climate. Regionally, the presence of a larger CS area has a clear impact as higher evaporation over the sea and higher precipitation over the south-west catchment, while reducing (summer) and increasing (winter) surface air temperatures and vice versa for smaller CS area. Most importantly, this summer temperature disturbs the upper atmospheric circulation (at the 200 hPa and the 500 hPa level) with a southward shift and increase in speed of the summer jet stream. This leads to enhanced summer precipitation over central Asia and increased winter warming over the north-western Pacific. An accurate Caspian Sea area representation is thus vital in global climate models for paleo and future scenarios and share serious implications for expanding coastal communities, agricultural activities, fisheries and the ecosystem
