Two methods-multiproxy and geothermal-are commonly used to reconstruct Northern Hemisphere climate of the last 500 to 1000 years. Both show warming in the 20th century, but in earlier centuries the temperature curves diverge strongly. In his Perspective, Beltrami investigates the reasons for these discrepancies. He explains the difficulties that arise in trying to compare the two types of records and calls for integrated analyses in which all models are interpreted jointly.