Norse seafaring and colonization around the North Atlantic at the end of the 9th century indicated that regional North Atlantic climate was warmer during medieval times than during the cooler "Little Ice Age" of the 15th - 19th centuries. As paleoclimatic records have become more numerous, it has become apparent that "Medieval Warm Period" or "Medieval Optimum" temperatures were warmer over the Northern Hemisphere than during the subsequent "Little Ice Age", and also comparable to temperatures during the early 20th century. The regional patterns and the magnitude of this warmth remain an area of active research because the data become sparse going back in time prior to the last four centuries.
The plot below, from the Intergovernmental Panel on Climate Change Fourth Assessment Report (2007), shows numerous Northern Hemisphere paleoclimatic temperature reconstructions. The various studies differ in methodology, and in the underlying paleoclimate proxy data utilized, but all reconstruct the same basic pattern of cool "Little Ice Age", warmer "Medieval Warm Period", and still warmer late 20th and 21st century temperatures. 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. 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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