"But how plants respond to that sudden increase in food will impact human health as well, from nutritional deficits, to ethnopharmacology, to seasonal pollen allergies - in ways we don't yet understand", said Lewis Ziska, a plant physiologist with the US Department of Agriculture research service, one of the authors.
The study puts the case more coolly: "For those populations that are highly rice-dependent, any CO2-induced change in the integrated nutritional value of rice grains could disproportionately affect human health". Rice grown under the higher carbon dioxide concentrations expected in the second half of this century (568 to 590 parts per million) is less nutritious, with lower amounts of protein, vitamins, and minerals. Both proteins and iron decreased by 10 percent, while zinc levels decreased by 5 percent.
"Rice is not just a major source of calories, but also proteins and vitamins for many people in developing countries and for poorer communities within developed countries", said Professor Kobayashi. Beyond physically changing weather conditions and the land on which farmers grow crops, new evidence shows excess carbon dioxide is deteriorating the nutritional quality of some food plants.
The rice was grown at research sites in China and Japan using an open-field method where researchers build 17-meter-wide (56-foot-wide) plastic pipe octagons elevated about 30 centimeters (1 foot) above the tops of plants within standard rice fields.
Their data revealed an inverse relationship between Carbon dioxide levels and nutritional qualities - the higher the Carbon dioxide level, the lower the levels of vitamins and minerals. Aside from energy-rich carbohydrates, grains feed us protein, zinc, iron and essential B vitamins.
Earlier this year, NOAA scientists announced the month of April averaged global Carbon dioxide concentrations above 410 parts per million, a new record.
Some possibilities include that the Carbon dioxide takes the place of other nutrients, diminishing the need for them, or that as crops produce more grains the nutrients are in turn diluted, or the fact that Carbon dioxide causes the plant's stomates, pores in plant leaves, to close, changing the water flow and potentially affecting how water-soluble nutrients react. "Reductions in the nutritional quality of rice could affect maternal and child health for millions of people".
The research was funded by National Basic Research Program of China, Natural Science Foundation of Jiangsu Province in China, and Youth Innovation Promotion Association of Chinese Academy of Sciences. As carbon dioxide levels got higher, certain nutrients became less powerful. That knowledge gives researchers an opportunity, given enough funding, to breed climate change-resistant strains of rice.
The consequences for wheat are tied to rising temperatures, but with rice, the immediate issue appears to be the growing concentrations of carbon dioxide in the atmosphere.