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Climate Variations of Summer Half-Years from the 7th to 10th Century in the Kinki Region, West Central Japan Based on Historical Documents

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  • 近畿地方の文献史料から見た7~10世紀の暖候期における気候
  • キンキ チホウ ノ ブンケン シリョウ カラ ミタ 7 10セイキ ノ ダンコウキ ニ オケル キコウ

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Abstract

This paper discuss climate variations in summer half years (May to October) from the 7th to 10th century in the Kinki region of west central Japan based on historical documents. Most previous research used the compilations of meteorological references contained in various unspecified documents. That method of research may be effective when examining climate variations after the 17th century since numerous documents are available. However, before 10th century, few documents are available, and their different features are likely to lead to nonstandardized analysis.<br>Many of the Japanese documentary sources that potentially could have been used to reconstruct the climate from the 7th to 10th century have been lost. This research therefore relied specifically on the Rikkokushi and Nihon-Kiryaku. The former, one of the oldest documents in Japan, was compiled before 887 and consists of six official books. The latter contains records from before 1036, was written in the late 11th century, and was based on the Rikkokushi and many other documents.<br>The terms “a long rain” or “drought” are treated seriously in this analysis because of the large scale they represent. Their occurrence in every 10-day period was counted in this study to discuss changes in the summer climate, because there are numerous comments in ancient texts on the amount of rainfall during the Japanese rainy season and the various stages of production of the staple crop of rice.<br>The results of analysis showed that drought was much more common from the late 7th to 8th century and that long rains were more common in the 9th century, especially from 850 to 887. Mentions of drought in July increased in the late 8th century, which suggests that the occurrence of a northerly polar front connected with warming may have corresponded with the “medieval warm period” defined by other scientific research. The increase in the mentions of long rains in July from 879 to 887 suggests the relation of a southerly polar front associated with cooling. On the other hand, the results could be related not only to polar fronts but also to other climatic and anthropological factors. For example, the increase in long rains in the early 9th century despite the absence of marked temperature changes between the late 8th to the early 9th century could indicate that moving the capital in 794 from Nara to the Kyoto basin with its many rivers shifted the focus of ancient writers to flooding or other phenomena resulting from heavy rainfall.

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