Read here, here and here. The IPCC and Climategate scientists, such as Michael Mann, teamed up to rid the world of the Medieval Warming Period and the Little Ice Age in an effort to convince the world's politicians and mainstream press that the modern warming was "unprecedented." Literally, Mann and his colleges presented a temperature history that had erased the warming and cooling extremes prior to the modern era.
The result of that effort was two fold: one, the infamous 'hockey stick' portrayal of historical temperatures as being a time of little temperature variation was blatantly contrary to known history and evidence; and two, the recognition by other scientists and a minority of the public that climate science had become the complete victim of the governing elites' political agenda.
Subsequent to the 'hockey stick' revelations of politics and fabricated temperature histories, many scientists pushed harder to examine the historical past, in attempts to reconstruct paleo-climate temperatures and climate.
This new study, by Büntgen et al., was able to identify the significant warming periods and the resultant civilization prosperity of a warmer climate, as represented by the known Roman warming and Medieval periods. The tree rings also identified both the Dark ages and Little Ice Age extreme coolings and the associated disease, famine and death of those natural cold periods.
"Climate variations have influenced the agricultural productivity, health risk, and conflict level of preindustrial societies.....Wet and warm summers occurred during periods of Roman and medieval prosperity. Increased climate variability from ~AD 250 to 600 coincided with the demise of the Western Roman Empire and the turmoil of the Migration Period." [Ulf Büntgen, Willy Tegel, Kurt Nicolussi, Michael McCormick, David Frank, Valerie Trouet, Jed O. Kaplan, Franz Herzig, Karl-Uwe Heussner, Heinz Wanner, Jürg Luterbacher and Jan Esper 2010: Science]
"Der Spiegel presents the most important results, which I myself think are not a surprise. The bulk of the Der Spiegel piece focuses on the hunger and misery precipitated by the climatic cold periods throughout the 2500-year period. One really gets a sense of how temperatures in Europe by no means followed the hockey stick shape proposed by Mann, and went from cold to warm, and vice versa. Numerous other proxies show the same applies globally."