Read here and here. A peer-reviewed study confirming that Medieval Warming temperatures were considerably warmer than those experienced during modern era. The research reveals that modern temperatures are not "unprecedented" as claimed, and that significant natural warming does happen without the influence of human CO2 emissions. (click on image to enlarge)
"The Jämtland multi-millennial tree-ring width chronology is comprised of living and subfossil Scots pines (Pinus sylvestris L.) sampled close to the present tree-line in the central Scandinavian Mountains.....Several periods of anomalously warm and cold summers were noted throughout the record: (1) 550 to 450 BC (Roman Warm Period), when summer temperatures were the warmest of the entire record, exceeding the 1961-1990 mean by more than 6°C, (2) AD 300 to 400 (Dark Ages Cold Period), which was "the longest period of consecutive cold summers," averaging 1.5°C less than the 1961-1990 mean, (3) AD 900 to 1000, a warm era corresponding to the Medieval Warm Period, and (4) AD 1550 to 1900, a cold period known as the Little Ice Age..... With respect to the latter portion of the tree-ring record, which encompasses the period of modern global warming, Linderholm and Gunnarson declare that this phenomenon "does not stand out as an anomalous feature in the 3600-year record, in fact other periods show more rapid warming and also higher summer temperatures." What should be even more embarrassing to the world's climate alarmists is the fact that the last half of the 20th century actually experienced cooling."