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. "Jan. 11 was so frightfully cold that all of the communion wine froze," says 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 adds.
Ancient 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 the Earth is heating up more dramatically than ever before, he says. Last winter - when spring blossoms popped up all over the Austrian Alps 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 says, 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 says.
Most historians and scientists delving deep into archives seek accounts of disasters and extreme weather. 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.
Pfister's team of four uses old "weather reports" to work back as far as the 10th century.
Pfister has found that from 1900 to 1990, there was an average of five months of extreme warmth per decade. In the 1990s, that number jumped to an unprecedented 22 months. The same decade also had no months of extreme cold, in contrast to the half-millennium before.
Even in the last major global warming period from 900 to 1300, severe winters were only "somewhat less frequent and less extreme," Pfister says.