As of March 5, there have been 1,051 peer-reviewed studies representing 601 research institutions and involving 44 countries covering the Medieval Warming Period (MWP) ("Slap-shot climate science," Commentary, Wednesday).
These studies show that the MWP was global and as warm or warmer than our current temperatures. These studies arrived at their conclusions using many different proxies for temperature, including coral, caves, pollen grains, ocean floor sediments, ice cores and diatoms.
There is also confirming anecdotal evidence further strengthening these studies. Viking burial locations have been found below the permafrost. Antique vineyards in Europe dating to the period have shown up at latitudes where even today grapes cannot be grown. The Vikings were able to map the entire coastline of Greenland, so it must have been close to ice-free during at least part of their stay.
Michael Mann's hockey-stick graph, on the other hand, dismissed en masse those studies by reducing considerably the significance of the MWP and the subsequent Little Ice Age. The graph also relegates both periods to no more than regional phenomena. According to William Happer, a Princeton University physicist, Mr. Mann's hockey-stick graph seems to have totally left out Europe's climate during these two time periods. Had Mr. Mann treated these two periods as insignificant, it likely would have brought on real problems as Europe's history during that time is well-known.
Disregarding Mr. Mann's study for the moment, an earlier question arises once again: Since the MWP was at least as warm as current temperatures, why couldn't our current warming also be just natural variation? There were many earlier periods during this current interglacial that were also obviously because of natural variation - and they were also likely warmer.
Our current warming would have begun at the coolest point of the Little Ice Age, about 1680. But this is almost 200 years before there was any increase in carbon dioxide levels and long before any industrial revolution.
The only temperature increase we experienced into the mid-1900s appears to be because of natural variation. The 1940s to the 1970s was a cooling period, with warming beginning again in 1975 and extending to about 1998. From then until now, temperature basically has been flat despite carbon-dioxide emissions reportedly at their highest levels and increasing slowly.
It is thus unlikely that Mr. Mann's hockey-stick results are valid. Mr. Mann himself recently said his study is "not that important." The only possible reason for such a comment would be to draw attention away from results that even Mr. Mann views as dubious. His study should be cast aside. It is highly improbable that 1,051 peer-reviewed studies are wrong.
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