Boreholes are useful for about 1,000 years into the past, as rock transmits past surface temperatures downward. The University of Michigan’s Shaopeng Huang led a study of 6,000 boreholes in 1997 from all continents. The results showed global average temperatures during the Medieval Warming were warmer than today’s, and during the Little Ice Age they averaged 0.2 to 0.7° C below present.⁶⁰