Paddy fields have the effect of mitigating daytime temperature rises in paddy fields and surrounding areas. In order to estimate the magnitude of meteorological mitigation effect of paddy field, Researchers have succeeded in developing a numerical model by reflecting the stomatal response of the rice paddy. It was revealed from this model that when the concentration of atmospheric carbon dioxide (CO2) doubled from the current level (400ppm), transpiration from the leaves of rice paddy was suppressed. Also the daytime temperature of paddy fields in the vicinity of Kanto on a sunny day in summer rises by 0.44°C on average, in urban areas by 0.07°C on average, and by up to 0.3°C in the vicinity of paddy fields. It has found that the expected increase in atmospheric CO2 concentration by the end of this century may not only bring about a rise in temperature due to the greenhouse effect, but also weaken the meteorological mitigation effect of paddy fields.
Paddy fields, which are popular as part of the typical landscapes of Asian countries including Japan, are known to have the effect of alleviating temperature rises in the paddy fields and surrounding areas. In general, actively evaporating plants have a lower body temperature due to vaporization cooling, which suppresses the temperature rise in the surrounding area, especially during sunny days. When the CO2 concentration in the atmosphere rises, the degree of stomatal opening of the plants becomes smaller. Also, since the transpiration is reduced, the effect of mitigating the temperature rise due to plants is also reduced. This raises a question that the "heat mitigation effect of paddy fields" may decline in the future.
Therefore, researchers from National Agriculture and Food Research Organization(NARO) and the Institute of Low Temperature Science, Hokkaido University have developed an atmospheric-paddy ecosystem coupling model that considers the interaction between the temperature change due to the stomatal response of rice paddy and the atmospheric layer above. They conducted a simulation targeting the area where paddy fields are distributed, including the urban area near the Kanto region.
As a result, it was shown that the maximum daytime temperature of paddy fields on a typical sunny day during summer is about 2°C lower than that of the urban area in the target area. However, under the condition when the concentration of atmospheric CO2 is doubled, it was found that the transpiration is reduced and the effect of mitigating the temperature rise is decreased. Also, the temperature of paddy fields increased by 0.2 to 0.7°C (0.44°C on average). In urban areas, the average temperature rises by 0.07°C due to the effect of increasing temperature in the paddy field, it is estimated that the temperature would increase by up to 0.3°C in the adjacent areas near the paddy fields in the urban areas.
It was revealed from this research result that, if the CO2 concentration in the atmosphere rises in the future, "heat mitigation effect of paddy fields" will decrease and the maximum daytime temperature of paddy fields and surrounding areas will increase. In addition, there is concern that the risk of damage such as deterioration of rice quality and sterility due to high temperatures in summer may increase in paddy fields along with the rise in temperature due to global warming.
This research result was published online edition 5th March 2021 in International Magazine Boundary-Layer Meteorology.
Heat-Mitigation Effects of Irrigated Rice-Paddy Fields under Changing Atmospheric Carbon Dioxide based on a Coupled Atmosphere and Crop Energy-Balance Model. Boundary-Layer Meteorology, https://doi.org/10.1007/s10546-021-00604-6