Reconstruction of solar radiation in Tokyo since 1720 based on historical weather records

The impact of climate change on human society has been a significant issue in historical studies and is also vital for future adaptation to climate change. To understand climate change and its devastating impacts on societies in the past, before the start of meteorological observations, the spatial patterns of climate variation must be reconstructed with a higher temporal resolution than those provided by the annual data. Japan has a large volume of records, including those related to daily weather conditions (e.g., "fine", "cloudy", and "rainy"), from the 17th to 19th century.

This study has developed a method for estimating solar radiation using daily weather descriptions recorded. Using this method and the daily weather records in historical diaries of Tokyo, Japan, we reconstructed solar radiation fluctuations for the period 1720–1912, pertaining to the Little Ice Age. We compared our estimations with the observed sunshine duration records of the Japan Meteorological Agency to validate our findings. This method is effective in reconstructing solar radiation for all seasons and allows for the reconstruction of historical climate fluctuations with higher temporal resolution compared with that reconstructed using other methods or proxy data often used previously. In addition, solar radiation is a fundamental factor for not only the energy balance of the Earth but also the hydrological cycle and agricultural productivity.

Here, we created a long-time series of solar radiation for the period 1720–2022 in Tokyo, thereby providing insights into low and high solar radiation for this practical 300-year period. Furthermore, the reconstruction of historical solar radiation fluctuations could enable the exploration of the correlation between climate variations and social changes.