Climate change will have a destabilizing impact on society in the coming decades and therefore reliable climate change predictions are of utmost importance. Reconstructions of climate, that extend instrumental records to pre-industrial times, are key for the development of reliable climate models. Climate reconstructions using ancient tree rings as a climate proxy can address this issue. We will create a temperature reconstruction extending more than 5000 years back in time from the oldest temperature-sensitive trees in the world (bristlecone pine, Pinus longaeva). We will use high-resolution X-ray CT scanning (XµCT) to retrieve the maximum latewood density (MXD), an excellent proxy for summer temperature, from these ancient tree rings. We will use this record to explore past climatic changes and their potential forcing mechanisms. In particular, we will investigate decadal to centennial climate change periods (Medieval Climate Anomaly, Little Ice Age), as well as the impact of abrupt climate change events due to volcanic forcing over a long time period. A state-of-the-art XµCT workflow will produce a new, multi-millennial reconstruction to inform future climate projections, provide context for current climate change, and to better understand climatic forcing of both gradual and abrupt climate changes.