In the autumn of 1720, a Japanese diarist sat down and recorded what they saw in the sky above Tokyo—clouds, clear days, the subtle dance of light and shadow. Three centuries later, that simple observation has become part of a scientific breakthrough. Researchers from the Research Organization of Information and Systems, Rissho University, and Tokyo Metropolitan University have reconstructed 300 years of solar radiation data for Tokyo by translating qualitative weather descriptions from historical diaries into quantitative measurements, revealing patterns about sunlight, climate, and human survival that instruments couldn't capture until modern times.
The problem scientists faced was fundamental: pyrheliometers, the precision instruments used to measure sunlight, didn't exist until 1838, and automated versions came even later. This left a vast blind spot in climate records. Without reliable measurements of solar radiation stretching back centuries, researchers couldn't understand how changes in sunlight affected crop yields, weather patterns, and the societies that depended on them. But Japan offered a unique archive. Daily weather diaries were a tradition there dating back to the eighth century, kept by aristocrats, officials, and literate farmers. Many have since been digitized in the Historical Weather Database of Japan, providing 60 published daily weather records accessible to modern science.
Mika Ichino, project researcher at the Center for Open Data in the Humanities at ROIS, and her team developed a method to systematize these descriptions. They standardized the qualitative language in the Ishikawa Diaries—one of the most detailed collections—and validated it against Japan Meteorological Agency sunshine duration records dating back to the 1870s. This gave them roughly 140 years of official data to calibrate against, allowing them to work backward through time with confidence. The result was a continuous solar radiation record stretching from 1720 to 1912, a span that encompasses the Little Ice Age.
What emerged from the data told a story written in sunlight and shadow. The researchers identified two dramatic low-insolation episodes—periods of significantly reduced solar radiation—during the 1780s and 1830s. These weren't abstract fluctuations in climate data. Both episodes coincided precisely with severe famines that ravaged Japan. Cool summers followed the drops in solar radiation, agricultural productivity plummeted, and communities suffered. The correlation was striking and clear.
The implications extend far beyond historical curiosity. "We demonstrate that qualitative weather descriptions in historical diaries can be systematically translated into quantitative solar radiation estimates with high monthly-scale reliability," Ichino explained in the study, published in Climatic Change. The team's methodological framework opens new possibilities for understanding how climate shaped pre-industrial societies and how vulnerability to solar variability left populations exposed to hunger and hardship.
Moving forward, the researchers envision a much broader application. They aim to build spatially and temporally extended datasets of reconstructed solar radiation across preindustrial Japan, clarifying how seasonal and interannual climate variability influenced agricultural production and societal change. More ambitiously, they want to apply this framework to historical documents worldwide—diaries, logs, and records from cultures across the globe—to piece together a more complete picture of how sunlight shaped human history. In translating the careful observations of 18th-century diarists into modern science, they're building a bridge between past and future, helping us understand the climate vulnerabilities that have always governed human life.