What was done
A semidwarf rice (Oryza sativa) cultivar was grown to maturity in open-top chambers, located in the Philippines, at atmospheric CO2 concentrations of ambient, ambient +200, and ambient +300 ppm during the wet and dry seasons of two consecutive years.  In addition, plants were grown at ambient (26 °C) or elevated (30 °C) air temperature to study the interactive effects of elevated CO2, soil moisture, and temperature on growth and yield.

What was learned
Over the two years of study, biomass was about 50% greater during the dry seasons than it was during the wet seasons.  On average, elevated CO2 increased total biomass by about 31 and 40% in the moderate and high CO2 treatments, respectively.  However, air temperature had no effect on plant biomass production, and there were no interactive effects between CO2 concentration and air temperature.

With respect to grain yield, dry season yields were also substantially greater than wet season yields.  On average, elevated CO2 increased grain yield by 15 and 27%, respectively, for the moderate and high CO2 treatments at ambient air temperature.  At the high air temperature treatment, however, this stimulation was reduced and there were significant yield differences among CO2 treatments.

What it means
As the atmospheric CO2 concentration increases, rice growth and yield can be expected to increase significantly during both wet and dry seasons.  However, if air temperatures rise by 4°C, it is possible that increasing CO2 concentrations will not lead to substantial grain yield enhancements unless heat-tolerant rice cultivars are grown in the fields.  Therefore, these data suggest that even with a global warming worse-case-scenario 4°C rise in air temperature, rice yields would still be maintained with or without further increases in the air's CO2 content.  Thus, rice production would likely remain unaffected if air temperatures warmed in the future.