The debate over the characteristics and impact of the Medieval Warm Period (MWP), roughly dated from 950 to 1250 AD, lies at the heart of discussions on historical climate variability and its implications for understanding current climate change.

Following the publication of the Intergovernmental Panel on Climate Change (IPCC) Third Assessment Report in 2001, the MWP was essentially erased from the paleoclimatological record in favor of the ‘hockey stick’ graph. This disappearance remains a point of contention that has been particularly prominent in public and scientific debate.

The ‘hockey stick’ graph, first published by Michael Mann, Raymond Bradley, and Malcolm Hughes in 1999, depicted temperature anomalies over the past millennium. It showed relatively minor temperature fluctuations for most of the last millennium (the "shaft" of the hockey stick) and a sharp rise in temperatures in the 20th century (the "blade" of the hockey stick). This presentation suggested that the modern warming period has been unprecedented over the last millennium. This finding has been at the core of calls for robust measures aimed at addressing climate change.