A flurry of recent scientific papers has tried to measure the
warmth of the "Medieval Warm Period" (MWP) of about 1,000 years
ago. Scientists have long debated whether it was cooler or
warmer than today, and whether the warmth was global or
regional. The point for nonscientists: If recent warming has
precedents, some might find it less alarming. Until the late
1990s, researchers generally agreed that the MWP was warmer than
today and that the "Little Ice Age" of 1500-1800 was colder.
Then in 2001 the Intergovernmental Panel on Climate Change
adopted the "hockey stick" graph devised by Michael Mann at the
University of Virginia and colleagues. Using temperature
indicators such as tree rings and lake sediments, the graph
rewrote history by showing little warmth in the 11th century and
little cold in the 17th, but a sharp spike in late-20th-century
temperatures. That graph helped to persuade many people (such as
me) that recent temperature rises were unprecedented in scale
and speed in at least 1,400 years. But