Ancient weather records provide clues to the present and future

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.”

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 once were 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,” said 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 said. 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 warmest Aprils on record.

“In the last year there was a series of extremely exceptional weather,” he said. “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, said the 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. The range of weather extremes has been smaller in the 40 years Hinder 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 also can be used to obtain a temperature range for most months and years that is more precise than general indicators such 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 recorded the dates of cherry blossom festivals, which allow modern scientists to track the weather of the time.

Early records often are discovered 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 near Vienna, an unidentified writer notes a lack of ice on the Danube in 1343-1344 and calls the winter “mild.” 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.”

Scores of similar clues are pieced together year by year to determine temperature ranges, said Pfister, whose team 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 said. Over the last century, temperatures have gone up an average of 1.3 degrees, which often is attributed to the accumulation of greenhouse gases, primarily carbon dioxide, in the atmosphere.

Global warming is a prominent issue today because of fear of hurricanes and flooding. For most of human history, though, it was the fate of farms and the fear of famine that encouraged careful weather observation.

The Einsiedeln abbots -- princes within the Holy Roman Empire until 1798 -- ruled over large swaths of central Switzerland. Agriculture was the primary source of income and natural disasters such as floods and avalanches posed a threat.

Debts accrued and honored, accidents, local conflicts and business transactions also fill Dietrich’s accounts, “but most days start with the weather,” said Andreas Meyerhans.

The diaries -- written in German sprinkled with old Swiss dialect and margin notes in Latin -- are unique because of the exceptional everyday detail they provide. Pfister said that centuries of weather records make it clear that people need to adapt when extremely hot or cold weather becomes more frequent. The lives of earlier generations were ruled by the weather, but “in the second half of the 20th century people slept and became completely unprepared for natural disasters, because they happened so rarely.”

In Einsiedeln, Hinder reads from a barometer beside the Virgin Mary and worries that humanity is in trouble.

“God still controls the weather,” he said, but people must do their part by taking better care of the planet.