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Why do geologists place the start of the Holocene about 11,700 years ago?

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A detailed geologic time scale chart displaying Earth's history from the Hadean Eon over 4.5 billion years ago to the present, organized by eons, eras, periods, epochs, and ages with color-coded columns and corresponding dates in millions of years. — Holocene Epoch The stratigraphic chart of geologic time.

Geologists define the start of the Holocene Epoch approximately 11,700 years ago because this date marks a significant and rapid global climatic shift.12

This boundary coincides with the end of the Younger Dryas, a period of abrupt cooling that had interrupted the warming trend at the end of the Pleistocene Epoch. Following the Younger Dryas, Earth experienced a dramatic interval of rapid warming, with local temperatures in Greenland increasing by as much as 10 °C (18 °F) within a few decades.3

In 2018, the International Union of Geological Sciences and the International Commission on Stratigraphy formally established this 11,700-year threshold as the beginning of the Greenlandian stage, which serves as the lower boundary of the Holocene. This timing is determined primarily through the analysis of ice cores from Greenland, which provide a precise record of the climatic transition into the modern era.13