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outhold, 9 months ago (edited 9 months ago) to world in This month is the planet's hottest on record by far – and hottest in around 120,000 years, scientists say | CNN

They have various methods. One of the common ones is analyzing ice core samples. The ice sheets are accumulated over the years so each layer on ice sheets is from a certain historical period (much like tree trunks.) By analyzing the chemical status of the ice core on each layer, they can extract data, such as temperature, about a certain period.

outhold, 9 months ago to world in This month is the planet's hottest on record by far – and hottest in around 120,000 years, scientists say | CNN

120k years ago was about the end of the Eemian interglacial period, which was a significantly warm period.