July 2, 2004

Little White Exaggeration

Filed under: Agriculture

New study claims rice yields will decline as temperatures warm. But bushels of evidence show otherwise.

A recent study promising that rice yields will decline due to global warming fails to take into account the hundreds of articles showing myriad benefits of higher temperatures and carbon dioxide concentrations.

Rice is one of the world’s most important food crops. Any reduction in yield would create problems for literally billions of humans who rely heavily on rice for survival.

But could climate change really reduce rice crop yields? According to an article in the most recent issue of the prestigious Proceedings of the National Academy of Sciences, the answer is yes.

A team of researchers from the Philippines, China, and the United States analyzed weather data from the International Rice Research Institute farm in the Philippines from 1979 to 2003 and compared trends in variations in temperatures with rice yields from irrigated field experiments. As with many other parts of the world, the Peng et al. team found that the maximum temperature increased by a third of a degree Celsius, while the minimum temperature increased by more than a degree in the 25-year study period.

Peng et al. reported that “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.” They firmly concluded that their work provides “direct evidence of decreased rice yields from increased nighttime temperature associated with global warming.”

We get the message: The United States is causing global warming and will starve billions because higher temperatures lower rice yields. But that message is ill-conceived. Indeed, the authors largely ignore the hundreds of other papers that have been published showing that rice responds favorably to elevated CO2 concentrations.

It is true that atmospheric CO2 concentrations will rise in the future. It is likely true that nighttime temperatures will increase as well. It is not true that rice will suffer these consequences. In fact, hundreds of published studies find that, as CO2 levels rise, rice, along with essentially every other plant in our biosphere, will

• increase its rate of photosynthesis
• grow bigger faster
• produce more yield
• increase water use efficiency
• decrease the effects of stresses from pollutants and/or herbivores.

There is a drumbeat of upbeat news about the future of rice: Any relatively small reduction in yield from elevated nighttime temperatures will be dwarfed by the biological benefits of elevated CO2. Evidence of those facts is not hard to find. A quick visit to the library netted these results:

More yield, consistent nutrient value. A team of scientists from New Zealand, South Korea, and Japan, wondered how elevated CO2 would influence nutrient concentrations of staple food crops such as rice. They grew rice outside with natural and elevated CO2 levels, and, like everyone else, they found that “elevated CO2 increased biomass and grain production” in the rice. With respect to nutrient concentrations, they “found no changes in the concentrations of any of the other elements analysed.” Elevated CO2 produced more grain yield with absolutely no decline in nutrients!

Up to 71% more rice. A scientist with the USDA grew a variety of rice cultivars commonly raised in the southern United States outdoors in controlled chambers with natural and doubled CO2 concentrations and varying day-night temperatures. Although unrealistically high temperatures cooked the plants, Baker reported that “At the 28°C temperature treatment, CO2 enrichment increased grain yield by 46% to 71% among the three cultivars.”

He concluded that future rice growers will be able to take advantage of “the possibility of selecting or breeding rice cultivars with enhanced capability to take advantage of future global increases in [CO2].” Even if nighttime temperatures remotely approach threatening levels, future agriculturalists will be able to identify cultivars that are more tolerant of heat – rice farmers are not stupid!

The nitrogen factor. A team of Japanese scientists grew rice at ambient and elevated CO2 with variations in nitrogen (N) solution in the soil. The Yamakawa et al. team found that “The dry matter of rice was increased by elevated CO2” and that “the amount of N uptake seemed to limit rice growth.” Assuming rice farmers of the future do not run out of nitrogen (which would be like running out of air), their yields will increase substantially.

Microbial mass increases. Another team of Japanese scientists examined how the beneficial soil microbial biomass beneath the rice plants was impacted by elevated CO2. Li et al. reported “Elevated [CO2] significantly increased microbial biomass carbon in the surface 5 cm soil when N (90 kg ha-1) was in sufficient supply.”

Spend 10 more minutes in a good science library and discover at least a dozen more published experiments on the future of rice in the 2004 literature alone; there are hundreds more on rice and elevated CO2 in the older stacks.

This recent warning from the Peng et al. group regarding rice and global warming may not be a little white lie, but they certainly commit a sin of omission. Overwhelming evidence demonstrates that rice yields will increase significantly in the years to come, with or without the predicted rise in nighttime temperature.


Baker, J.T. 2004. Yield responses of southern US rice cultivars to CO2 and temperature. Agricultural and Forest Meteorology, 122, 129-137.

Li, K. Yagi, H. Sakai, K. Kobayashi. 2004. Influence of elevated CO2 and nitrogen nutrition on rice plant growth, soil microbial biomass, dissolved organic carbon and dissolved CH4. Plant and Soil, 258, 81-90

Lieffering, M., H.-Y. Kim, K. Kobayashi, and M. Okada. 2004. The impact of elevated CO2 on the elemental concentrations of field-grown rice grains. Field Crops Research, 88, 279-286.

Peng, S., J. Huang, J.E. Sheehy, R.C. Laza, R.M. Visperas, X. Zhong, G.S. Centeno, G.S. Khush, and K. G. Cassman. 2004. Rice yields decline with higher night temperature from global warming. Proceedings of the National Academy of Sciences, 101, 9971-9975.

Yamakawa, Y., M. Saigusa, M. Okada, and K. Kobayashi. 2004. Nutrient uptake by rice and soil solution composition under atmospheric CO2 enrichment. Plant and Soil, 259, 367-372.

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