World rice production must increase by ≈1% annually to meet the growing demand for food that will result from population growth and economic development (1). Most of this increase must come from greater yields on existing cropland to avoid environmental degradation, destruction of natural ecosystems, and loss of biodiversity (2, 3). Achieving greater yields depends on increasing total crop biomass, because there is little scope to further increase the proportion of that biomass allocated to grain (4). Total crop biomass is determined mainly by crop photosynthesis and respiration losses, both of which are sensitive to temperature (5). Future crop yields will be influenced by complex interactions between the effects of increases in atmospheric concentrations of CO2 (6) and trace gases such as ozone (7) as well as the effects of temperature increases brought about by climate change (8).
Global mean surface air temperature increased by ≈0.5°C in the 20th century and is projected to further increase by 1.5 to 4.5°C in this century (9). In the past century, daily minimum nighttime temperature increased at a faster rate than daily maximum temperature in association with a steady increase in atmospheric greenhouse gas concentrations (10, 11). Although the effects of projected climate change on crop yields have been evaluated by using crop-simulation models (8), there are few studies on the effects of observed climate change on crop growth and yield (12, 13). In the present study, we analyzed weather data at the International Rice Research Institute (IRRI) Farm (Los Baños, Laguna, Philippines) from 1979 to 2003 to evaluate trends in mean maximum and minimum temperatures and solar radiation in both dry and wet cropping seasons. Relationships between grain yield and temperature or radiation were evaluated by using yield data from field experiments conducted under irrigated conditions with optimal management at the IRRI Farm from 1992 to 2003. Our objective was to determine whether there were significant time trends in changes of temperature or radiation and whether these changes had an impact on grain yield.