- The goal of dissertation research was to use geochemical, statistical and geological methods to constrain and understand climate variability over several different time scales. Specifically, I have addressed three questions regarding past climate change: (1) how does the record of Irish cirque glaciers constrain the dimensions of the Irish Ice Sheet during and since the Last Glacial Maximum (LGM);(2) what is the record of millennial-scale glacier variability in Ireland during the last glaciation; and (3) how did variability of various components of the climate system interact to contribute to the evolution of climate over the last 800,000 years.The first chapter involves constraining the vertical and spatial extent of the Irish Ice Sheet (IIS). Reconstructions of the LGM IIS are widely debated, in large part due to limited age constraints on former ice margins and due to uncertainties in the origin of the trimlines used to identify vertical ice limits. The greatest differences exist in southwestern Ireland where reconstructions either have complete coverage by a contiguous IIS that extends onto the continental shelf or a separate, southern sourced Kerry-Cork Ice Cap (KCIC) with more limited spatial and vertical extent. New ¹⁰Be surface exposure ages from two moraines in a cirque basin in this region provide a unique constraint on ice thickness for this region insofar as the presence of a cirque glacier at a given time clearly indicates that the site was not covered by the IIS. My new ¹⁰Be ages from these two moraines show that the central mountains in southwestern Ireland were not covered by the IIS or a KCIC since at least 24.5±1.4ka, thus supporting the more-limited reconstructions of the IIS at the LGM, indicating a reduced contribution to sea-level change and a smaller loading of the solid Earth,which is consistent with models of glacial isostatic adjustment to the IIS.The second chapter presents research that has developed a record of millennial-scale variability in former Irish cirque glaciers between ~25 ka and 10 ka.Small alpine glaciers are sensitive to climate, and the paleo record of past small glacier fluctuations offers an outstanding opportunity to use this glacier sensitivity for developing centennial- to millennial-scale records of climate variability. Because Ireland is immediately adjacent to, and downwind of, the North Atlantic, glacial records there are ideally located to record past climate changes associated with changes in North Atlantic Deep Water (NADW) formation and attendant feedbacks. Ihave developed a high-precision ¹⁰Be surface-exposure chronology of multiple moraines deposited by glaciers in eight cirque basins across Ireland to constrain this variability. The data show a remarkable record of persistent millennial-scale variability between 24.5±1.4 ka and 10.8±0.7 ka. Several of these events are associated with known climatic events during the last deglaciation such as onset of the Bølling-Allerød and end of the Younger Dryas. However, this persistent signal extends back to the Last Glacial Maximum (LGM), suggesting a previously unidentified mode of climate variability unrelated to large changes in the NADW. Multi-decadal to multi-centennial variability identified in Greenland ice cores present a mechanism for the variability recorded in the Irish glaciers.The third chapter of this research involves characterizing and explaining climate variability at orbital timescales across the mid-Brunhes Transition (MBT;~430 ka). The MBT involved a change in the amplitude of variability associated with cooler interglacials prior to 430 ka and warmer interglacials after. The key questions Iaddress include determining whether other components of the climate system changed at this time, and identifying the mechanism for the MBT. Statistical tests of multiple proxies (sea-surface temperature, ∂¹⁸O, ∂¹³C, CO₂, CH₄, and dust) indicate that theM BT was largely a reorganization of the global climate system perhaps driven by an increase in interglacial CO2 concentrations. Changes in marine ∂¹³C may provide insights into this change in the carbon cycle, perhaps associated with changes in global ocean circulation. In particular, there is a large positive ∂¹³C excursion during the interglacial immediately prior to the MBT, suggesting an enrichment of the Atlantic basin at this time relative to other interglacials of the past 800 kyr.Variability in the depth gradient of ∂¹³C from the North Atlantic show increased correlation with South Atlantic ∂¹³C records after the MBT, indicating more homogenous mixing of the northern- and southern-component water masses. This is accompanied by a greater difference in the ∂¹³C latitudinal gradient in the Atlantic basin prior to the MBT that is reduced afterwards.