Is the hockey stick temperature graph accurate?

In the absence of direct temperature measurements, which began in 1880, climate scientists use proxies such as tree rings, ice cores, stalagmites, corals, and boreholes to reconstruct Earth’s temperature record. Authors of the 1999 hockey stick graph used such methods to estimate Northern Hemisphere temperature anomalies over the past millennium.

The graph has since been replicated by other climate scientists. Its major finding—that Northern Hemisphere temperatures during the late 20th century were the warmest in 1,000 years—was evaluated in a 2006 report commissioned by the U.S. Congress. It concluded that the graph “has subsequently been supported by an array of evidence that includes both additional large-scale surface temperature reconstructions and pronounced changes in a variety of local proxy indicators, such as melting on ice caps and the retreat of glaciers around the world.”