People have been measuring and recording temperatures since the 17th century, and scientists have been using those data to estimate a mean global temperature since the late 19th century. As weather stations increased in precision and in global coverage, especially in the last half of the 20th century, they facilitated better estimates of the mean global temperature—a key indicator of possible climate change.

For the past 30 years, those estimates have primarily been done by three independent teams: the Climatic Research Unit1 (CRU) of the University of East Anglia in the UK, in collaboration with the Hadley Center of the UK Met Office; NASA’s Goddard Institute for Space Studies2 (GISS); and the National Oceanic and Atmospheric Administration’s National Climatic Data Center3 (NCDC).

Their task is not a simple one. For decades, researchers have compiled databases of historic temperature records from disparate sources in more than 100 countries....

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