The Written Climate

Long before anyone measured the atmosphere with mercury or satellites, humanity was inadvertently building its longest continuous environmental monitoring system. Monasteries, naval fleets, and agricultural estates kept meticulous logs for entirely mundane reasons. They tracked taxes, religious tithes, and grain yields, capturing the weather along the way.

These historical documents—farmer journals, traveler diaries, and ship logs—now serve as high-resolution climate proxies. They bridge the resolution gap between coarse-grained geological records like ice cores and modern meteorological tracking, offering daily or seasonal granularity that tree rings often lack. Royal Navy ship logs from the seventeenth and eighteenth centuries, for instance, record daily wind directions and weather descriptions that allow scientists to reconstruct the North Atlantic Oscillation.

Even a glass of wine can tell us how hot it was centuries ago. Written observations of phenological events, such as annual grape harvest dates, let scientists accurately reconstruct European summer temperatures centuries prior to instrumental records. Because grape ripening relies heavily on spring and summer warmth, a harvest date becomes a remarkably reliable temperature proxy.

Of course, turning prose into data requires looking past human subjectivity. Researchers must navigate the tricky translation of historical shorthand, calibrating what different cultures and centuries actually meant by phrases like "very cold" or "heavy rain." But once decoded, these accidental archives reveal that our ancestors were keeping track of the climate long before they realized what they were measuring.