Isotope data indicates that central Greenland was nearly 14 °C (24.5 °F) colder during the Younger Dryas than it is today.
Notes on verification
Confirmed by peer-reviewed gas-isotope paleothermometry (Severinghaus et al. 1998, Nature) and corroborated by subsequent literature (Alley 2000, Buizert et al. 2014). Minor uncertainty remains on exact magnitude but order of ~14-15°C cooling is well supported.