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.
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. Read more...