Between the 9th and 12th centuries AD there was a period known as the
Medieval Optimum which was several times more severe than the
global warming that we are experiencing today.
Carbon 14 dating of
tree rings shows that there was significantly more
solar activity than average.
During the Medieval Optimum there were stable Viking settlements in Greenland (with farming land), and most of the Alpine glaciers had receded or completely disappeared. The northern tree limit moved 125 miles to the north, and the sea level was between 1 and 2 feet higher than today. Grapevines grew in Yorkshire, England.
However strong the Medieval Optimum was, it pales in comparison to that of the Post-glacial Optimum:
As the glaciers were receding the world experienced a long warming period (from about 7000 BC to 5000 BC). This period was so warm ("how warm was it?"... sorry) that the shells of animals typical to the Mediterranean were found on the Gulf of Bothnia, while the Mediterranean itself became home to many tropical species.
The important thing to realize about these climate changes is that both were independent from man-made greenhouse gasses and the only correlation is that of the solar activity.