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.
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.
There is actually a lot to like in the article above. It is a rare acknowledgement of hemispheric differences. It is only one small step away from them having to admit that the warming of the 1980s and 1990s was also in the Northern hemisphere only. They won't ever mention that, I imagine, but those of us with no axe to grind can do so.
I also see virtue in their comment that "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". 600 years is only a tiny slice of geological time so how can anyone be sure that recent climate events are anything out of the ordinary?
And even if we accept Mann's hockeystick, look where it ends: in 1998. Since then we have only reached within 0.25 deg of that point once; in Jan 2007, and other than that time, we haven't reached within 0.35 deg C of that 1998 point; but then as of Jan 2008, we are all the way back down to about where we were in the year 1000 AD. (according to Mann). Pesky?
But I think that the Coup de Grace for the bulldust above is this paper, which shows that there WAS a mediaeval warm period in the Southern hemisphere. And note that it relies on tree rings, just as Mann does.
Posted by John Ray. For a daily critique of Leftist activities, see DISSECTING LEFTISM. For a daily survey of Australian politics, see AUSTRALIAN POLITICS Also, don't forget your summary of Obama news and commentary at OBAMA WATCH
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