Source: News about Rice and People, IRRI

Direct observation of the link between rising temperatures and falling rice yields reveals that crop simulation modelers were in the dark about damage at night - the hours worst affected by global warming

Field studies conducted at the International Rice Research Institute (IRRI) have confirmed predictions from theoretical studies that global warming will make rice crops less productive. Combining a quarter century of climate data collected at IRRI with yield trends in adjacent fields over the past dozen years, researchers further discovered that simulation models underestimated the problem by half because they overlooked the pernicious effect of high minimum nighttime temperatures.

The study, reported in PNAS, the proceedings of the National Academy of Sciences of the United States of America, found that rice yields at IRRI declined by 15% for every 1ºC increase in mean daily temperature.
Temperatures are projected to rise globally by 1.5-4.5ºC in the coming century - or 3 to 9 times more than in the past century. Global warming thus threatens to erase the hard-won productivity gains that have kept the rice harvest in step with population growth.

Since the dawn of the Green Revolution - which began in Asia with IRRI's release in 1966 of IR8, the first modern, high-yielding semidwarf rice variety - the global rice harvest has more than doubled, racing slightly ahead of population growth. This achievement not only averted the mass famine that 4 decades ago appeared poised to engulf much of Asia, it has also made rice more affordable and so helped unshackle many Asian communities from the treadmill of subsistence agriculture.

Yet rice remains the staple food of more than half of humanity. In Asia, where 90% of all rice is grown and consumed, more than 2 billion people obtain 60-70% of their calories from rice. More than half of the world's 1.3 billion poor are Asian rice farmers and consumers, and Asia's poorest - urban slum dwellers and rural landless - still spend up to 40% of their income on rice.

The poor suffer first and foremost when a succession of meager harvests causes rice shortages and price hikes like those now making headlines in China. The shortfall in the world's largest rice producer and consumer has many apparent causes, and government-orchestrated restoration of planted area will likely revive the Chinese rice sector before it upsets the calm and predictability with which the international rice trade has been blessed for the past couple of decades.

However, China's rice woes demonstrate that ongoing food security cannot be taken for granted - even in one of the world's most dynamic economies.
Harvest shortfalls induced by global warming would likely be much more widespread and persistent. Left unchecked, they could undermine global food security and political stability.

The study recorded that the mean minimum nighttime temperature during dry season at IRRI has risen since 1979 by 1.13ºC, or 3 times the 0.35ºC rise in mean maximum daytime temperature. This difference is an expected consequence of increased greenhouse gas concentrations in the atmosphere, and IRRI's climate records are consistent with warming trends found elsewhere in the Philippines and globally.

The news is that high nighttime minimum temperatures clearly and strongly suppressed rice yields in the seasons in which they occurred, while high daytime temperatures had no measurable effect. Yields fell by 10% for every 1ºC increase in mean nighttime minimum temperature. Because the increase in night temperature was 3-fold greater than the increase in daytime temperature, rice yields declined by 15% for every 1ºC increase in daily mean temperature - double the 7% decline that emerged from theoretical models.

"Most studies of temperature and global warming effects on crop growth and grain yield are based on daily mean air temperature, which assumes no difference in the influence of day versus night temperature," wrote the nine-member research team from IRRI, China and the United States led by IRRI crop physiologist Shaobing Peng. "This report provides direct evidence of decreased rice yields from increased night temperature associated with global warming."

The scientists attributed more than 70% of the year-to-year variation in rice grain yield to nighttime temperature but could do little more than speculate on the mechanism involved.

"The results of this study highlight the need for greater fundamental understanding of the effects of night temperature on physiological processes governing crop growth and yield development," concluded the authors of Rice yields declining with higher night temperatures from global warming. Dr.
Peng added, "The challenges now is to get the financial resources needed to develop new rice varieties that can still yield well despite any increase in temperatures in rice growing areas," Dr. Peng said.

The International Rice Research Institute (IRRI) is the world's leading rice research and training center. Based in the Philippines and with offices in 10 other Asian countries, it is an autonomous, nonprofit institution focused on improving the well-being of present and future generations of rice farmers and consumers, particularly those with low incomes, while preserving natural resources. IRRI is one of 15 centers funded through the Consultative Group on International Agricultural Research (CGIAR), an association of public and private donor agencies. Please visit the Web sites of the CGIAR or Future Harvest Foundation, a nonprofit organization that builds awareness and supports food and environmental research.


QUESTIONS & ANSWERS

What is importance of the findings?
The paper reports strong evidence of a reduction in rice yields caused by rising temperatures consistent with trends in global warming. These findings are important for predicting the effects of global warming on food security because rice is the staple food for two billion people, or about one-half of the human population.

Who and where?
The study was led by Dr. Shaobing Peng and was conducted in rice production fields at the International Rice Research Institute (IRRI) in the Philippines, where it is possible to grow two crops of rice per year on the same piece of land. These intensive irrigated rice systems are practiced on about 24 million hectares of land in Asia and account for about 40% of the global rice supply. At the research site, detailed climate data have been collected for the past 25 years, and rice has been grown with optimal management such that yields are largely
dependent on year-to-year variations in climate-primarily solar radiation and temperature.

What are the climate trends?
Since 1979, the climate data documents a significant increase in air temperature that was 3 times greater in the night than in the daytime. For example, there was an increase of 1.1C in the mean minimum nighttime temperature and a 0.35C increase in mean maximum daytime temperature during dry season, and these warming trends are consistent with increases found elsewhere in the Philippines and globally.

What are the effects on yield?
More than 70% of the year-to-year variation in rice grain yield was attributable to nighttime temperature, while there was no detectable effect on yields from the much smaller increase in daytime temperature. Rice yields were found to decrease by 10% for each 1C increase in nighttime minimum temperature. But, because the increase in night temperatures was threefold greater than the increase in daytime temperature, rice yields declined by 15% for every 1C increase in mean daily temperature. This estimate is two times greater than previous estimates of global warming effects on rice yields obtained from simulation modeling studies.

What are the causes of yield reduction?
At this point, the researchers speculate that the greater sensitivity of rice yields to nighttime temperatures is due to increased maintenance respiration, which forces the plant to divert more energy to maintaining metabolic functions as opposed to producing greater biomass and grain yield. Detailed physiological studies will be needed to verify this hypothesis.

What are the scientific implications of this work?
Previous modeling studies on the effects of global warming were based on increases in mean daily temperature and did not account for the greater effects of global warming on nighttime temperatures. Therefore, future studies must focus on developing crop simulation models that are more sensitive to the differential effects of global warming on night versus daytime temperatures, and on better understanding the reasons for greater sensitivity to nighttime temperatures.

Proceedings of the National Academy of Sciences
10.1073/pnas.0403720101

Rice yields decline with higher night temperature from global warming
Shaobing Peng, Jianliang Huang , John E. Sheehy, Rebecca C. Laza, Romeo M. Visperas, Xuhua Zhong, Grace S. Centeno, Gurdev S. Khush, and Kenneth G. Cassman
Crop, Soil, and Water Sciences Division, International Rice Research Institute, DAPO Box 7777, Metro Manila, Philippines; Crop Physiology and Production Center, College of Plant Science and Technology, Huazhong Agricultural University, Wuhan, Hubei 430070, China; Rice Research Institute, Guangdong Academy of Agricultural Sciences, Guangzhou, Guangdong 510640, China; University of California, Davis, CA 95616; and Department of Agronomy and Horticulture, University of Nebraska, Lincoln, NE 68583
Contributed by Gurdev S. Khush, May 27, 2004

The impact of projected global warming on crop yields has been evaluated by indirect methods using simulation models. Direct studies on the effects of observed climate change on crop growth and yield could provide more accurate information for assessing the impact of climate change on crop production. We analyzed weather data at the International Rice Research Institute Farm from 1979 to 2003 to examine temperature trends and the relationship between rice yield and temperature by using data from irrigated field experiments conducted at the International Rice Research Institute Farm from 1992 to 2003. Here we report that annual mean maximum and minimum temperatures have increased by 0.35°C and 1.13°C, respectively, for the period 1979-2003 and a close linkage between rice grain yield and mean minimum temperature during the dry cropping season (January to April). Grain yield declined by 10% for each 1°C increase in growing-season minimum temperature in the dry season, whereas the effect of maximum temperature on crop yield was insignificant. This report provides a direct evidence of decreased rice yields from increased nighttime temperature associated with global warming.