National Agriculture and Food Research Organization (NARO) has updated a rice growth model that now incorporates the combined effects of high temperature and high CO2 on yield and appearance of rice, reflecting the recent findings obtained from open-field experiments. The improved model was used to estimate the impact of climate change on rice yield and appearance quality in Japan in the 21st century. The model projected that negative impacts on both rice yield and appearance quality would be more pervasive and greater than previously anticipated. These results call for immediate action to introduce crop management technology and new varieties that strengthen climate resilience. The site-specific and time-dependent projections can assist in formulating and updating climate change adaptation plans by the national and local governments.
Overview
The Climate Change Adaptation Act has come into effect since 2018, which stipulates that the national and local governments formulate climate change adaptation plans, calling for a need for reliable scientific projections of the impacts of climate change. Rice is the most important crop in Japan, and various research groups, including NARO, are periodically updating the impact assessment on key aspects of rice production such as grain yield and appearance quality under changing climate, using rice growth models that account for major rice growth process and rice yield.
Recently, open-field experiments using the Free air CO2 enrichment (FACE) technology1 conducted in Iwate and Ibaraki prefectures revealed that the positive effects of an increase in CO2 concentration on yield decrease as air temperatures increase. It also showed that a high CO2 concentration exacerbated negative effects of high temperature on the appearance quality. These negative interactions of CO2 and temperature on yield and quality have not been accounted for by the previous growth model. Researchers at NARO, therefore, have incorporated the interactive effects of high temperature and CO2 into the existing rice growth model, which considered these effects independently. The model was used to project the impact of climate change on domestic rice yield and appearance quality for the period between 1981 and 2100 in Japan using multiple climate scenarios. The previous impact assessment model showed that national average grain yield would increase from the baseline (1981-2000) period until the middle of this century, then decreased to the same level as the end of the 20th century, under a high emission scenario (RCP8.5). On the other hand, the improved model projected average yields consistently lower than those calculated by the previous model, and the difference is expected to widen as the period progresses. At the end of this century, the projected yield would be about 80% of the baseline yield, indicating that the yield losses would override the positive effects of increasing CO2 fertilization.
Percentage of chalky grains, a major indicator of appearance quality deterioration, was also projected using multiple climate scenarios until the end of the century. The previous model that only considers temperature projected that the chalky grain rate would be about 15% and 30% at the mid and end century, respectively. In contrast, the new model, accounting for temperature and CO2 concentration, projected that the chalky grain ratio would reach about 20% at the middle of this century and about 40% at the end of this century, suggesting appearance quality degradation would worsen than previously anticipated.
The modelled impacts of climate change obtained by the new model are expected to serve as important background information when formulating and updating climate change adaptation plans by local governments and other organizations in the future. A caveat to the current results is that the negative impacts projected here are based on conventional cultivation methods and cultivars. They could be mitigated by adaptation measures such as adopting heat-tolerant varieties, shifting planting times, using appropriate fertilization management tailored to environmental conditions in each production region.
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1  Free air CO2 enrichment (FACE) technology
FACE is an experimental method that simulates high CO2 concentration projected in the future in open fields. In the Rice FACE experiment, CO2 concentration in the treated area is maintained at about 580ppm, which is 200ppm higher than the present concentration.
Reference Information
Yasushi ISHIGOOKA, Toshihiro HASEGAWA, Tsuneo KUWAGATA, Motoki NISHIMORI, Hitomi WAKATSUKI (2021) Revision of estimates of climate change impacts on rice yield and quality in Japan by considering the combined effects of temperature and CO2 concentration. Journal of Agricultural Meteorology, 77 (2), 139-149, doi:10.2480/agrmet.D-20-00038