How a single graph of past temperatures became a lightning rod for climate controversy.

Nature · Published 3 October 2026 Swipe more like this →

In short: In 1999, climatologist Michael E. Mann helped produce a reconstruction of Northern Hemisphere temperatures over the past millennium. Its distinct shape earned it the nickname the hockey stick graph. The study became a focal point for intense public debate, legal challenges, and investigations into the integrity of climate science research.

Michael E. Mann, a climatologist and geophysicist, pioneered techniques to isolate climate signals from noisy data. In 1998, he collaborated with Raymond S. Bradley and Malcolm K. Hughes to publish a reconstruction of temperature patterns over the past 600 years. By 1999, the team extended this work to cover the past 1,000 years. This reconstruction, known as MBH99, used principal component analysis to integrate sparse proxy data—such as tree rings, ice cores, and corals—with instrumental records. The resulting graph, which showed a sharp rise in 20th-century temperatures compared to the preceding 900 years, was featured in the 2001 IPCC Third Assessment Report.

The graph's prominence sparked significant controversy. Critics questioned the statistical methodology, particularly the use of principal component analysis. However, a 2006 report by the U.S. National Academy of Sciences endorsed the study's broad conclusions, noting that while the specific statistical method had a slight bias, it did not significantly influence the final results. Today, more than two dozen independent reconstructions support the general consensus presented in the original hockey stick graph, despite variations in how the pre-20th-century data appears.

Mann’s work also became entangled in broader political and legal battles. Following the 2009 Climategate email leak, Mann was the subject of multiple investigations, including reviews by Pennsylvania State University, the EPA, and the National Science Foundation. All these inquiries cleared him of scientific misconduct. Additionally, he successfully defended his academic freedom against a 2010 Civil Investigative Demand from the Virginia Attorney General, which a court ultimately ruled lacked sufficient cause. While Mann has faced defamation lawsuits and public scrutiny, his research remains a foundational element in the scientific understanding of historic climate variability and the human influence on modern warming.

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