Rising levels of carbon dioxide in the atmosphere may cut the nutritional quality of some of the world's most important food crops, researchers reported on Wednesday after conducting experiments simulating conditions expected by mid-century.
The amounts of two important nutrients, zinc and iron, were found to be lower in wheat, rice, soybeans and field peas grown in open-air fields where the scientists created CO2 concentrations at the level they forecast for Earth by roughly 2050, about 550 parts per million.
They grew 40 varieties of six different grains and legumes, also including corn and sorghum, at seven locations on three continents, in Japan, Australia and the United States.
"This is important because almost two billion people globally receive most of these two nutrients (zinc and iron) by eating crops," said University of Illinois plant biology professor Andrew Leakey, one of the researchers.
The researchers said these findings point to one of the most important health threats shown to be linked to climate change.
Dr. Samuel Myers of the Harvard School of Public Health, who led the study published in the journal Nature, said there already is a significant public health problem in parts of the world due to inadequate intake of zinc and iron.
Myers noted that inadequate zinc intake affects the immune system and makes people more vulnerable to premature death from maladies like malaria, pneumonia and diarrhea. Myers said iron deficiency is linked to increases in maternal mortality, anemia, reductions in IQ and reduced work productivity.
Scientists have sought to gauge the impact of climate change on humankind in the coming decades including the effects of CO2 levels that have been rising due to the burning of fossil fuels since the advent of the Industrial Revolution.
Earth's atmospheric CO2 concentration currently is about 400 parts per million and continues to rise.
The study found that in wheat grown under elevated CO2 conditions there were about 9 percent lower levels of zinc and 5 percent lower levels of iron compared to wheat grown under normal conditions. The rice grown with elevated CO2 levels had 3 percent less zinc content and 5 percent less iron.
In wheat and rice, there also was lower protein content at the elevated carbon dioxide levels, the researchers said.
Nutrients in sorghum and corn remained stable at the higher carbon dioxide levels because these crops use a kind of photosynthesis that concentrates CO2 in their leaves, the researchers said.
The scientists simulated higher CO2 levels in open-air fields using a system known as Free Air Concentration Enrichment (FACE), which pumps out, monitors and adjusts ground-level CO2 in the air to simulate future conditions.
"You have an open agricultural field that looks like any other except that in the middle of the field you have rings of carbon dioxide-emitting jets. At the center of the ring you have a sensor that tells you the CO2 concentration and the wind direction. When the CO2 falls below your prescribed level the upwind jets emit some more CO2 to keep it at a constant level," Myers said.
These crops were grown in identical conditions to crops grown outside the ring in terms of weather, pests, pathogens and soil nutrients, with CO2 concentration as the only variable. The scientists then compared nutrient content using those crops grown in matching plots without the added CO2.
Leakey said rice, wheat and soybeans made more sugars through photosynthesis at the elevated CO2 levels and produced about 15 percent more seeds but had decreases in zinc and iron content.
He said more work is needed to nail down the details of why higher CO2 levels drive down the nutrients. He also said it will be important to study how crops in tropical countries will respond to elevated CO2 and other aspects of global environmental change.
