Bradley S. Klapper, Associated Press
"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 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 only 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," 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."
Scores of similar clues are pieced together year by year to determine temperature ranges, says Pfister, whose 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. Over the past century, temperatures have gone up an average of 1.3 degrees Fahrenheit, which is often attributed to the accumulation of greenhouse gases, primarily carbon dioxide, in the atmosphere.
Global warming is one of the world's top issues today because of fears of massive hurricanes and flooding. For most of 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 — were powerful leaders who ruled over large swaths of central Switzerland's mountainous terrain. Agriculture was the primary source of income for the region and natural disasters such as floods and avalanches posed an omnipresent threat.
Debts accrued and honored, accidents, local conflicts and business transactions also fill Dietrich's accounts, "but most days start with the weather," says Andreas Meyerhans, who cares for the monastery's precious documents.
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 says. He adds that centuries of weather records make it clear that people need to adapt when extremely hot or cold weather becomes more frequent. While the lives of earlier generations were ruled by the weather, "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 flanked by the Virgin Mary, and worries that humanity is in trouble.
"God still controls the weather," he says. But, he adds, people must do their part by taking better care of the planet.
