Current theories of the causes and impact of global warming have been thrown into question by a new study which shows that during medieval times areas as far apart as Europe and Antarctica both warmed up.
It then cooled down naturally and there was even a 'mini ice age'.
Read more: http://www.dailymail.co.uk/sciencete...#ixzz1qpIsA4Zo
Then there is this
Prior temperature reconstructions tend to focus on the global average (or sometimes hemispheric average). To answer the question of the Medieval Warm Period, more than 1000 tree-ring, ice core, coral, sediment and other assorted proxy records spanning both hemispheres were used to construct a global map of temperature change over the past 1500 years (Mann 2009). The Medieval Warm Period saw warm conditions over a large part of the North Atlantic, Southern Greenland, the Eurasian Arctic, and parts of North America. In these regions, temperature appears to be warmer than the 1961–1990 baseline. In some areas, temperatures were even as warm as today. However, certain regions such as central Eurasia, northwestern North America, and the tropical Pacific are substantially cooler compared to the 1961 to 1990 average.