A frequent shibboleth of climate change denialism is that the Medieval Warm Period — a period of apparent global climate change in medieval times — indicates that the Earth's climate rises and falls all the time, and that therefore, human beings don't cause global warming. In this paper, scientists from the National Oceanic and Atmospheric Administration investigate the best scientific findings on the Medieval Warm Period hypothesis, with special emphasis on the massive, wide-ranging, independent UN Intergovernmental Panel on Climate Change. They conclude that "the late 20th and early 21st centuries are likely the warmest period the Earth has seen in at least 1200 years."
In summary, it appears that the late 20th and early 21st centuries are likely the warmest period the Earth has seen in at least 1200 years. For a summary of the latest available research on the nature of climate during the "Medieval Warm Period", please see Box 6.4 of the IPCC 2007 Palaeoclimate chapter. To learn more about the "Medieval Warm Period", please read this review published in Climatic Change, written by M.K. Hughes and H.F. Diaz. (Click here for complete review reference). Discussion of the last 2,000 years, including the Medieval Warm Period, and regional patterns and uncertainties, appears in the National Research Council Report titled "Surface Temperature Reconstructions for the Last 2,000 Years", available from the National Academy Press.
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