Grains are the bedrock of civilization. They led humans from hunting and gathering to city-building. According to the Food and Agriculture Organization, the fruits of three grasses provide the world with 60 percent of its total food: corn, wheat and rice. Aside from energy-rich carbohydrates, grains feed us protein, zinc, iron and essential B vitamins.
But rice as we know it is at risk.
As humans expel billions of metric tons of carbon into the atmosphere and raze vast swaths of forests, the concentration of carbon dioxide in our air hurries ever higher. That has the potential to severely diminish the nutritional value of rice, according to a new study published on Wednesday in Science Advances. For people who depend heavily on rice as a staple in their diets, such a nutritional loss would be devastating, says Kristie Ebi, a professor at the University of Washington and an author on the study. "When you look at a country like Bangladesh, three out of every four calories comes from rice. Obviously, that means any decline in nutritional value is very significant."
To study how rice responds to different concentrations of carbon dioxide, the researchers grew several varieties of rice in experimental fields in Japan and China. For each variety, they set aside one rice paddy as a control, and one neighboring enclosed rice paddy with tubing running through the field. They blew carbon dioxide out of the tubing, raising the ambient carbon dioxide inside the enclosure to some 580 parts per million, the expected carbon dioxide concentration in the next half century if there are no further attempt to curb emissions or deforestation. "The fields have the same sunshine, same water, same characteristics," Ebi says. "So the experiment sees what happens to the same rice under different carbon dioxide concentrations."
Ebi says that the rice grown under the elevated carbon scenario lost substantial amounts of protein, zinc, iron and B vitamins per grain. Iron, zinc and protein losses ranged from 5 percent to 20 percent. Vitamins B1 and B5 dropped up to 30 percent, depending on the variety. "Folate [vitamin B9] declines across the nine rice varieties ranged from 10 percent to 45 percent. So, it's a lot," she says.
Of course, many foods can be a rich source of these essential nutrients, but Ebi says food other than rice is not always available to people. Among the poorest in the world, the grain can make up an overwhelming portion of their diet, Ebi says. "In the paper, we looked at the most rice-dependent countries in Asia. Using a weighting scheme focusing on those with the fewest resources, we estimate this decline in nutrient quality will affect about 600 million people."
A mass deficiency in vitamin B9, also known as folate or folic acid, would have particularly severe public health consequences. The nutrient is critical to fetus development, and a lack of vitamin B9 can result in defects of the brain, spine or spinal cord at birth. "It can be a catastrophic birth anomaly," Ebi says.
The study's finding is disheartening, if not surprising, to researchers in the field. Scientists already knew that higher carbon dioxide concentrations can decrease protein, zinc and iron levels in important crops, and this study shows a similar effect for B vitamins, says Camilo Mora, a climate change scientist at the University of Hawaii Manoa who was not involved with the new work. "It's just another piece of evidence to show how bad climate change is," he says.
For non-experts, the phenomenon may seem a little odd, considering carbon dioxide is food for plants. Plants that share the same photosynthesis pathway as rice and wheat do indeed grow larger and produce greater yields in higher carbon dioxide concentrations by creating more carbohydrates, says Lisa Ainsworth, a biologist at the University of Illinois at Urbana-Champaign and the U.S. Department of. Agriculture who did not work on the study. But they don't increase the amount of other nutrients in their grains relative to that yield gain. "They're basically getting a dilution effect of the nutrients in the grains," she says.
Some varieties of rice may not experience as severe of a nutrient loss as carbon dioxide levels go up. For instance, the rice variety Liang You 84 (a hybrid style grown in China) lost around 45 percent of its vitamin B9 content whereas the Koshihikari rice (a short-grained sushi rice from Japan) lost roughly 30 percent of its B9 content in the elevated carbon scenarios. That knowledge gives researchers an opportunity, given enough funding, to breed climate change-resistant strains of rice.
But getting people to switch to new grains is not always easy, Ainsworth says. "I think culturally it is difficult. People eat different rice for different meals and events."
Other strategies are available, too. Investing in ways to increase access to different kinds of food beyond rice would help, for instance, she points out.
Or, Mora says, humanity could always work together to mitigate climate change and carbon emissions so the problem doesn't arise in the first place.
Angus Chen is a journalist in New York. He's on Twitter @angrchen.