(May 26, 2011) A linchpin in the global warming theory is the claim that we now live in a time of unprecedented warmth. Unfortunately for those who assert this theory, the history books all said it was hot during the centuries immediately before and after 1000 AD – the time known as the Medieval Warm Period.

MCA in the graph refers to the Medieval Climate Anomaly (the authors decline to use the term Medieval Warm Period), CWP refers to the Current Warm Period, and LIA refers to the Little Ice Age between these two periods. The chart shows current temperatures in Peru to be no warmer than those experienced during the Medieval period.

“We have to get rid of the Medieval Warm Period” read one email from the asserters, circa 1995, unimpressed that England grew grapes and the Vikings settled Greenland during the centuries they viewed as cooler than today’s. They promoted the theory that, although Europe might have been hot, the rest of the world was not.

Except that Chinese experts disagreed. According to the Chinese Academy of Meteorological Sciences, China was one degree warmer during the Medieval Warm Period. The Chinese Academy determined this by looking at records of where citrus groves and subtropical herbs were cultivated at that time – at far more northerly regions than today. As with Europe, the advanced Chinese civilization had a wealth of historical evidence to rebut the claims of the asserters. Other evidence – reported by the United Nations Intergovernmental Panel on Climate Change, no less — shows that Japan was also hot during the Medieval Warm Period.

Now comes more evidence still, this time from South America. In a study published this month in the Proceedings of the National Academy of Sciences, researchers at the University of Pennsylvania found that Peru was warm during the Medieval Warm Period. Although South American civilization of 1000 years ago didn’t leave behind a rich historical record for experts to assess, sediment deposits in a Peruvian lake had a story of their own to tell. Based on the researchers’ analysis of the lake bed’s composition over the last 2300 years, they were able to unearth data showing Peru’s temperature to have been in line with that elsewhere during the Medieval Warm Period.

Lawrence Solomon is executive director of Energy Probe and author of The Deniers. LawrenceSolomon@nextcity.com.