Crop nutrients may drop as carbon dioxide rises

In addition to mucking up the planet’s climate, carbon pollution spewed into the atmosphere by burning fossil fuels may also make food less nutritious.

Experimentally elevated levels of carbon dioxide in the air lowered many plants’ levels of iron, zinc and protein — required nutrients for human health. Researchers found these changes in the edible bits of staple grains and legumes such as wheat, rice and soybeans.

Appearing May 7 in Nature, the finding has alarming implications for global health, the authors say. Around 2 billion people already suffer from iron and zinc deficiencies, the authors report, and about the same number get 70 percent of their zinc and iron from these crops.

“That’s a really big deal,” says public health researcher Samuel Myers of Harvard. “If everyone was getting their dietary iron and zinc from fish, this wouldn’t matter much.”

Myers and his colleagues aren’t the first to find that food will change with rising levels of CO₂, which hovers around 400 parts per million in the atmosphere. But past studies have generated contradictory results, perhaps because they have been small-scale experiments or conducted in laboratories instead of outdoors.

To get a clear answer, Myers enlisted international collaborators already doing experiments on CO₂’s impact on crops. Though each experiment looked at different variables such as yield and water usage, they all used a common method and similar ranges of CO₂ levels. Researchers grew crops outdoors in circles, usually about 3 meters in diameter, and surrounded the circles with vents blowing CO₂. Because the greenhouse gas is denser than air, it stayed at crop level, raising CO₂ within the circle.

The team collected archived samples from 143 experiments conducted from 1998 through 2010 that tested six staple crops on three continents. The experimentally elevated CO₂ levels ranged from 546 to 586 parts per million, levels that scientists expect to see in the atmosphere by 2050.

Myers and colleagues found that wheat, rice, peas and soybeans generally contained around 5 to 10 percent less iron and zinc when grown at the higher levels. The researchers also found that wheat and rice had around 5 to 10 percent less protein. The other two crops analyzed, maize and sorghum, showed little or no response to higher CO₂.

Low levels of dietary iron and zinc can lead to anemia, a weakened immune system, low IQ and reduced energy levels. How low protein in the crops might affect human health is unclear, Myers says.

Also unknown, Myers says, is why the plants have less iron, zinc and protein. He and his colleagues are working to reveal the mechanism.

The impact of CO₂ on food quality is a neglected problem, says agricultural researcher Hans J. Weigel of the Johann Heinrich von Thünen-Institut in Brunswick, Germany. But Weigel cautions that it may still be too early to predict whether CO₂ will alter food in ways that affect human health. “Current experimentation causes an abrupt increase of atmospheric CO₂ concentration,” he explains. “This does not mimic the consequences of a gradual increase in CO₂,” he adds, suggesting that plants may adapt to new conditions over time.