Read here. The large list of studies that prove Earth's climate is naturally variable and prone to extremes, while confirming that periods prior to modern warming were warmer, just keeps on growing.
Latest peer reviewed research from Ljungqvist et al. confirms both - see adjacent chart.
"...authors write that "a number of Northern Hemispheric (NH) temperature reconstructions covering the last 1-2 millennia, using temperature-sensitive proxy data, have been made to place the observed 20th century warming into a long-term perspective."..."retrieved from a wide range of archives including, but not limited to, ice-cores, pollen, marine sediments, lake sediments, tree-rings, speleothems and historical documentary data."..."they present a list of antecedent analyses where the number of proxy records used ranged from only 3 to 46 (compared to their 120), and where the number of records with annual resolution ranged from only 3 to 30, whereas their study included 49 such annual-resolution records"...And they say that "these studies generally agree on the occurrence of warmer conditions ca. 800-1300 AD and colder conditions ca. 1300-1900 AD, followed by a strong warming trend in the 20th century,"..."scientists report that "during the 9th to 11th centuries there was widespread NH warmth comparable in both geographic extent and level to that of the 20th century,"" [F. C. Ljungqvist, P. J. Krusic, G. Brattström, H. S. Sundqvist 2012: Climate of The Past]
Global warming science facts: The Medieval Warming was warmer than our modern warming; modern warming represents a significant, natural rebound response to the extreme cold of the Little Ice Age; and, extreme climate changes are primarily natural, not solely the result of human activity.