In 1981, NASA’s James Hansen showed a Medieval Warm Period – based on temperatures in Greenland, California and England.
Global temperature trend is based on temperatures in central England, the tree limit in the White Mountains of California, and oxygen isotope measurements in the Greenland ice (W. Dansgaard of the Geophysical Isotope Laboratory, University of Copenhagen, pers. comm.), with the temperature scale set by the variations in the last 100 years
The 1990 IPCC report showed the same thing.
Overlaying Hansen’s 1981 graph on the 1990 IPCC graph, it can be seen that they are almost identical.
This research was reported in Nature in 1975.
The Medieval Warm Period has been known about for over 80 years.
The existence of the Medieval Warm Period completely wrecked the global warming scam, so professional climate fraudsters like Michael E Mann decided to get rid of it.
And by 2001, the Medieval Warm Period had been officially erased by the criminals known as “climate scientists.”