EINSIEDELN, Switzerland - A librarian at this 10th-century monastery leads a visitor beneath the vaulted ceilings of the archive past the skulls of two former abbots.
He pushes aside medieval ledgers of indulgences and absolutions, pulls out one of 13 bound diaries inscribed from 1671 to 1704 and starts to read about the weather.
"January 11 was so frightfully cold that all of the communion wine froze," said an entry from 1684 by Brother Josef Dietrich, governor and "weatherman" of the once-powerful Einsiedeln Monastery. "Since I've been an ordained priest, the sacrament has never frozen in the chalice."
"But on Jan. 13 it got even worse and one could say it has never been so cold in human memory," he added.
Diaries of day-to-day weather details from the age before 19th-century standardized thermometers are proving of great value to scientists who study today's climate. Historical accounts were once largely ignored, as they were thought to be fraught with inaccuracy or were simply inaccessible or illegible. But the booming interest in climate change has transformed the study of ancient weather records from what was once a "wallflower science," says Christian Pfister, a climate historian at the University of Bern.
The accounts dispel any lingering doubts that Earth is heating up more dramatically than ever before, he says. Last winter - when spring blossoms popped up all over the Austrian Alps, Geneva's official chestnut tree sprouted leaves and flowers, and Swedes were still picking mushrooms well into December - was Europe's warmest in 500 years, Pfister says. It came after the hottest autumn in a millennium and was followed by one of the balmiest Aprils on record.
"In the last year, there was a series of extremely exceptional weather," he says. "The probability of this is very low."
The records also provide a context for judging shifts in the weather. Brother Konrad Hinder, the current weatherman at Einsiedeln and an avid reader of Dietrich's diaries, says his predecessor's precise accounts of everything from yellow fog to avalanches provide historical context.
"We know from Josef Dietrich that the extremes were very big during his time. There were very cold winters and very mild winters, very wet summers and very dry summers," he said, adding that the range of weather extremes has been smaller in the 40 years he has recorded data for the Swiss national weather service.
"That's why I'm always cautious when people say the weather extremes now are at their greatest. Without historical context, you lose control, and you rush to proclaim every latest weather phenomenon as extreme or unprecedented," Hinder said.
Most historians and scientists delving deep into archives seek accounts of disasters and extreme weather events. But the records can also be used to obtain a more precise temperature range for most months and years that goes beyond such general indicators as tree rings, corals, ice cores or glaciers.
Such weather sources include the thrice-daily temperature and pressure measurements by 17th-century Paris physician Louis Morin, a short-lived international meteorological network created by the Grand Duke of Tuscany in 1653, and 33 "weather diaries" surviving from the 16th century. In Japan, court officers kept records of the dates of cherry blossom festivals, which allow modern scientists to track the weather of the time.
Early records often are discovered only by chance in documents that have survived in centuries-old European monasteries like Einsiedeln, or in the annals of rulers, military campaigns, famines, natural hazards and meteorological anomalies.
In Klosterneuberg, Austria, near Vienna, an unidentified writer noted a lack of ice on the Danube in 1343-1344 and called the winter "mild," while the abbot of Switzerland's Fischingen Monastery laments the late harvest of hay and corn in the summer of 1639 when "there was hardly ever a really warm day."