Read here and here. Although the IPCC and its Climategate scientists tried mightily to rid the universe of the Medieval Warming Period, the continuing stream of paleo-climate research just confirms how idiotic that strategy was. (click on image to enlarge)
The six scientists report that "evidence was found of a cold phase at Lake Anterne between AD 400 and 680, a warm episode between AD 680 and 1350, and another cold phase between AD 1350 and 1900," and they say that these events were "correlated to the so-called 'Dark Age Cold Period' (DACP), the 'Medieval Warm Period' and the 'Little Ice Age'." In addition, they say that "many other climate reconstructions across western Europe confirm the existence of several significant climatic changes during the last 1800 years in Central Europe and more specifically the DACP, the MWP and the LIA.""