evidence: When climate scientists talk about natural climate swings that came before humans started messing with the system, many invoke two epochs. During the Medieval Warm Period, roughly from 800 to 1200 AD, temperatures rose a few degrees above average. That warming has been connected to improved crop yields in parts of Europe, and the temporary Viking occupation of Greenland. During the following Little Ice Age, which lasted roughly from 1300 to 1850, the Greenland Vikings disappeared, glaciers from California to the European Alps advanced, and New York harbor froze, enabling people to walk from Manhattan to New Jersey without benefit of the George Washington Bridge. For a long time, many took on faith the idea that these phenomena were global. But that assumption has been undermined in the past decade or so by studies from widespread areas (including parts of Greenland) suggesting that in fact temperatures in many places did not line up with one or the other periods. Some regions appear to have been warming when they were supposed to be cooling, and vice versa. The same goes for two lesser-known, more vaguely defined earlier swings, known as the Roman Warm Period (ca. 100-300 AD) and the Dark Ages Cold Period (ca.400-800). [...] We show that conditions during medieval times or during the Little Ice Age are expected to occur naturally. But the large spatial consistency of the present warm phase cannot be explained by natural variability. This result corroborates many existing studies that have shown that humans are causing global temperatures to rise since the beginning of the industrial period.