The summer of 1540 was burning hot in the vine-covered hills of Burgundy, France—so hot as to be “almost unbearable,” according to one written account from the time.

In fact, it was hot all across Europe that year. In the Alps, glaciers melted, their snouts retreating up steep-sided valleys. Fires burned from France to Poland. And in the wine country of central France, the grapes withered to raisins on the vine, so sugary the wine made from them was syrupy and extra-alcoholic.

Normally, winemakers harvested grapes at the very end of September or early October. But that year, they had to scurry to get overripe grapes off the vine weeks earlier than usual.

Now, a nearly 700-year-long record of harvest dates from