Climate Change Affecting Quality of Rice in East Asia

A staple food for billions of people worldwide, rice is predominantly grown in Asia and exported globally. Rice crops are sensitive to weather conditions, and it is vital to understand how yields are affected by climate change to ensure sustainable food supplies in the future.

Aside from yield, climate change may also affect rice quality. Rice quality is determined by a combination of appearance, palatability, nutritional profile, and milling properties. Recent research published in Geophysical Research Letters highlighted a decline in rice quality in East Asia coincident with changing temperatures.

Dr. Xianfeng Lu of China’s Shaanxi Normal University, along with his colleagues, used data collected over 35 years to explore patterns in rice quality from Japan and China. This is based on head rice rate (HRR), which is a measure of the fraction of milled rice kernels that retain 75% of their length after milling, during which the husk and bran are removed.

The research team explored a number of climate variables via modeling to ascertain which had the greatest effect on HRR. The variables were nighttime temperature, daytime temperature, diurnal temperature, daily averaged temperature, hot days (above 30/35 degrees C), precipitation, precipitation frequency, soil moisture, solar radiation, cloud cover, relative humidity, daytime vapor pressure deficit, transpiration, and carbon dioxide concentration.

The scientists ultimately determined that overnight temperatures are the main driver of reduced rice quality. According to their data, as the nighttime temperatures become warmer, a critical threshold for decline begins at 12 degrees C and 18 degrees C for Japan and China respectively. When flowering and grain growth occur in such conditions, rates of photosynthesis and starch accumulation in the grain decrease, leading to lower rice quality as more grains are susceptible to breakage.

The daily solar radiation was the second most important factor contributing to rice quality changes as higher solar radiation led to lower HRR. Next was daily precipitation (less precipitation corresponding to lower HRR), and lastly daytime vapor pressure deficit (beyond a threshold of 0.5 to 1kPa, HRR declines.)

For China, the average rice quality across all sites and years had an HRR of -62% but was reduced by 1.45% every decade. As for Japan, the average HRR was slightly higher at -66% with a prominent rate of decline of 7.6% every decade.

Researchers noticed that in both countries, there is a strong gradient of diminished rice quality from northern to southern provinces and prefectures. This is likely linked to the southern regions being closer to the equator and therefore experiencing higher nighttime temperatures.

Dr. Lu and his team noted that the models consistently showed the average climate data 40 days in advance of the harvest date was a good predictor of rice quality for that yield. The research is viewed to be an important one as projections under both moderate and high emissions scenarios suggest rice quality will continue to decline in decades to come as climate change progresses.

In the study, HRR decreased by 0.5% and 1.5% for Japan and China respectively in projections between 2020 and 2100 for lower emissions scenarios. However, HRR is projected to decrease more prominently after 2050, potentially exceeding 5% in China by 2100 as modeled emissions increase.

The study also showed that the negative impact of climate change on rice quality in China is predicted to be harsher than for Japan with spacial sensitivity (HRR decline of -1.2% per degree of temperature rise) being higher than temporal sensitivity (-0.7% per degree of temperature rise.) China’s southern provinces may be more limited in adapting to increasing nighttime temperatures, which is a significant challenge to overcome as these are the main areas of rice cultivation.

With the data and research, the likelihood of rice cultivars adapting in tandem with the rate of climate change may pose a hazard to sustainable food supplies, human nutrition, and economic stability in the near future.