Synchronization of summer peak temperatures in the Medieval Climate Anomaly and Little Ice Age across the Northern Hemisphere varies with space and time scales

Previous studies have suggested that preindustrial climate epochs such as the Medieval Climate Anomaly (MCA) and Little Ice Age (LIA) did not occur synchronously across large areas of the globe. However, it is still unclear whether and to what extent this conception is independent on the space and time scales considered. Here, we assess the agreement of MCA and LIA peak temperatures at various timescales (i.e., from 51 to 351 years) by analyzing proxy-based temperature reconstructions at local, regional, and continental scales across the Northern Hemisphere. We show that the synchronization of the MCA and LIA peak temperatures tends to be more coherent at longer time and larger space scales. At local and regional scales, the timing of MCA and LIA peak temperatures differs substantially, in line with previous findings. At continental scale, there is still no rigorous synchronization of these climate epochs, but the differences of the timing of the MCA/LIA peak temperatures are mostly smaller than five decades at multi-centennial timescales. The time and space dependent synchronization is related to the relative contribution of variations in external forcing and internal variability at these different scales. These results on the synchronization are robust across several different proxy-based reconstructions used in this study, but the precise timing of the MCA maximum and LIA minimum temperatures detected is largely influenced by the uncertainties in temperature reconstructions. Our results support the relevance of the terms MCA and LIA at long time and large space scales, and suggest that both external forcing and internal variability played roles in driving these climatic epochs.