[ungerik]

Ever, like ever ever? Or ever since we measured?

[leeoniya]

it was and will be higher before and after there is a "we"

[Mountain_Skies]

Probably a combination of observation records, which aren't that old, and computed valued from analysis of sources such as ice cores. Obviously this is not the highest temperature winter in the northern hemisphere over the existence of the planet but may indeed be the warmest out of those periods from which we have records, observed or imputed.