Read here. Scientists confirm that Northern Hemisphere temperatures during the Medieval Period exceeded those of recent years of the 20th and 21st centuries. The Greenland ice sheet scientific evidence is clear about this, as the chart depicts. (click on image to enlarge)
"The authors say their data "show clear evidence of the Medieval Warm Period and Little Ice Age in agreement with documentary evidence," and those data clearly show that the Medieval Warm Period was at times considerably warmer than the Current Warm Period has been to date, and that even the Little Medieval Warm Period was considerably warmer than it was over the last decades of the twentieth century, as well as the first decade of the 21st century. Thus, there is no compelling reason to believe that the 20th-century increase in the air's CO2 content (a 100-ppm rise above what it was during the warmer Medieval Warm Period) was the cause of 20th-century global warming, especially when climate alarmists claim that such warming, when it occurs, should be most evident and earliest expressed in high northern latitudes..."
Additional historical temperature charts. Climate history postings. Modern temperature charts.