Medieval Warm Period - 9th to 14th Centuries
Norse seafaring and colonization around the North Atlantic at the end of the 9th century was generalized as proof that the global climate then was warmer than today. In the early days of paleoclimatology, the sparsely distributed paleoenvironmental records were interpreted to indicate that there was a "Medieval Warm Period" where temperatures were warmer than today. This "Medieval Warm Period" or "Medieval Optimum," was generally believed to extend from the 9th to 13th centuries, prior to the onset of the so-called "Little Ice Age."
In contrast, the evidence for a global (or at least northern hemisphere) "Little Ice Age" from the 15th to 19th centuries as a period when the Earth was generally cooler than in the mid 20th century has more or less stood the test of time as paleoclimatic records have become numerous. The idea of a global or hemispheric "Medieval Warm Period" that was warmer than today however, has turned out to be incorrect.
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For larger viewing version of graph, please click here or on image.
There are not enough records available to reconstruct global or even hemispheric mean temperature prior to about 600 years ago with a high degree of confidence. What records that do exist show is that there was no multi-century periods when global or hemispheric temperatures were the same or warmer than in the 20th century. For example, Mann et al. (1999) generated a 1,000 year Northern Hemisphere temperature reconstruction (shown above) using data from multiple ice cores and tree ring records. This reconstruction suggests that the 1998 annual average temperature was more than two standard deviations warmer than any annual average temperature value since AD 1,000 (shown in yellow). (For complete scientific reference of this study, please click here. Link to Mann 1999 FTP Data.)
In summary, it appears that the 20th century, and in particular the late 20th century, is likely the warmest the Earth has seen in at least 1200 years. To learn more about the so-called "Medieval Warm Period", please read this review published in Climatic Change, written by M.K. Hughes and H.F. Diaz. (For complete review reference click here.)
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