Change of weather condition and its effect on rice production during the past 40 years in Japan

In recent Japan, the increasing summer temperature and its influence on rice production have been recognized. The change in management and cultivar to adapt to this warming trend is necessary to maintain the yield and quality of rice. In this study we quantified the changes of weather condition and their effects on the production of rice in Japan during the period from 1964 to 2003. The average CO₂ concentration in the atmosphere from June to August increased from 322 ppm in 1964 to 367 ppm in 2003. The average daily maximum temperature from June to August increased by about 0.8°C and minimum temperature about 1.1°C during the 40 years. The rice heading season in 1994-2003 was one to two weeks earlier than that in 1964-1973. The shift of heading season resulted in lowering of air temperature before heading and rise in air temperature and increase in solar radiation after heading. The change in CO₂ concentration was estimated to increase the rice productivity by 2.5%, the climate change to increase it by -6.0 to 3.1%, and the shift of heading season to increase it by -0.5 to 6.8%. The analysis indicated that the positive effect of increased solar radiation on rice productivity was offset by the negative effect of temperature rise. By analysis of variance and multiple regression analysis we estimated that a 1°C increase in average daily minimum temperature during 10 to 30 days after heading reduced the ratio of first-class rice by 3.57%, and that a 1 Mj increase in average daily solar radiation increased the ratio by 2.59%. Although late transplanting is sometimes recommended to maintain grain quality of rice, the results of this study suggest that the late transplanting slightly improves the quality but definitely decreases the yield.