evidence: NARRATOR: But records of one place, like Greenland, during one time, the Middle Ages, may not prove anything. Only by knowing the average global temperature 1,000 years ago can scientists know whether the Earth back then was warmer or colder than today. [...] But what about Greenland and its controversial medieval warm period? According to the combined data, there was no medieval warm period. Specific regions like [...] I started my measurements, it had risen to two and a half. At the present time, it's almost seven.