Read here. Using multiple ice cores with highly resolved stable isotope records, in combination with air temperature data from 13 locations, researchers were able to develop a temperature history going back 1,400 years. They confirmed temperatures were warmer during Medieval Period when CO2 levels were significantly lower, versus the late 20th and early 21st centuries temperatures. (click image to enlarge)
"In describing this record, the seven scientists said that "temperatures during the warmest intervals of the Medieval Warm Period," which they defined as occurring "some 900 to 1300 years ago, "were as warm as or slightly warmer than present day Greenland temperatures." Hence, we conclude that for southwest Greenland, various portions of the MWP (AD 700-1100) were warmer than they have been over the last several years."