The IPCC 4th Assessment Report (FAR) is considered in light of Holocene palaeo-records of Arctic and Subarctic land temperatures and the response of the boreal forest–tundra boundary to past and present warming. The palaeo-records appear to support and amplify some of the conclusions in FAR and raise questions about others. Comparison of the FAR estimates for late 21st-century warming with Arctic and Subarctic palaeoclimatic records suggests that anticipated future warming will be unprecedented compared to earlier Holocene warming including the Holocene Thermal Maximum (HTM) and subsequent Medieval Warm Period (MWP). Annual warming and winter warming will likely be greater than during earlier periods in the Holocene, but summer temperatures projected by FAR may approximate or not far exceed the summer warming of the HTM. The geographically synchronous pattern of 21st-century warming projected by FAR will differ from the observed geographic variability in the timing of the HTM, particularly in the western Arctic. Further palaeo-record syntheses are required to fully delineate the similarities and differences between the HTM and the projected conditions of the 21st century. In addition, palaeo-records suggest that the Arctic and Subarctic climate may be sensitive to relatively small changes in solar irradiance and understanding the potential effects of natural variations in irradiance on the future climate of the Arctic and Subarctic remains a question deserving further research. FAR mapped estimates of a significant northward displacement of boreal forest and woodland due to warming imply that boreal forest or woodland will displace tundra as far north as the Arctic coastline by AD 2100. In Eurasia the estimated AD 2100 location of the tree line appears to approximate the HTM location. In North America an advance of forest to the coastline on the north slope of Alaska, central Canada and northern Quebec would far outdistance the relatively limited northward advances of boreal forest during the HTM. However, rates of tree species advance and subsequent forest development based on palaeo-records and currently observed tree line advances raise questions about the likelihood that northward migration and subsequent increases in the population sizes of boreal tree species could occur rapidly enough to establish continuous forest and woodland as far north as the Arctic coastlines of Eurasia and North America by AD 2100. Determining potential rates of northward advance of the northern forests requires further ecological and palaeoecological investigations. Copyright © 2009 John Wiley & Sons, Ltd.