Abstract
It has long been believed that the 20th warming was preceded by the "Little Ice Age" and the "Medieval Warm Period" as a result of the natural variability of climate. But as Bradley explains in this Perspective, recent research has shown that these climate patterns may not have been global, and are much more variable in time and space than previously assumed. However, one fact remains indisputable, namely that end-20th century temperatures were higher than at any time during the last millenium.