In a study to resolve whether rising carbon dioxide concentrations are a threat to human nutrition, researchers have discovered that if levels reach those expected by 2050, crops yielding a large portion of the global population's dietary zinc and iron will have reduced concentrations of those nutrients.
The researchers, from the Harvard School of Public Health (HSPH), publish their findings in the journal Nature.
They note that around 2 billion people around the world have zinc and iron deficiencies, which results in a loss of 63 million life years each year from malnutrition.
If these nutrient concentrations are reduced, the team adds that this will represent the "most significant health threat" associated with climate change.
Zinc is a vital nutrient for human health. It is found in cells throughout the body and helps the immune system fight bacteria and viruses. Additionally, the body needs zinc to make proteins and DNA.
Meanwhile, iron is a crucial component of hemoglobin, a protein that transfers oxygen from the lungs to the tissues. Iron is also necessary for growth, development, cellular functioning and synthesis of certain hormones and connective tissue.
Decreased iron and zinc levels found in wheat and rice
To conduct their study, the investigators used data involving 41 genotypes of grains and legumes from the groups of plants that use C3 and C4 carbon fixation from seven different free air carbon dioxide enrichment (FACE) locations in Japan, Australia and the US.
High CO2 levels resulted in decreased zinc, iron and protein concentrations in wheat and rice.
The team says experiments using FACE technology have become the "gold standard," as this technique allowed plants to be grown in open fields exposed to elevated levels of carbon dioxide (CO2), whereas prior studies used crops grown in greenhouses under artificial growing conditions.
After testing iron and zinc concentrations of the edible parts of wheat, rice, maize, sorghum, soybeans and field peas, the researchers found that the level of CO2 across all seven sites was between 546-586 parts per million.
Overall, their findings showed there was a significant decrease of zinc, iron and protein concentrations in wheat and rice.
In detail, these concentrations in wheat grains grown at the FACE sites were reduced by 9.3% for zinc, 5.1% for iron and 6.3% for protein, compared with wheat grown at moderate CO2 levels.
The team notes that zinc and iron were also reduced in legumes, whereas protein was not.
'Humanity is rapidly altering environmental conditions'
But how do these findings affect humans? Lead author Samuel Myers, research scientist in the Department of Environmental Health at HSPH, estimates that around 2-3 billion people on our planet receive over 70% of their dietary zinc or iron from such crops.
They add that this is the case in the developing world particularly, as zinc and iron deficiencies are already a major health concern there.
Crops like maize and sorghum were less affected by the increased levels, which the researchers say is consistent with their underlying plant physiology. Such plants concentrate CO2 inside the cell for photosynthesis, so they theorize this makes the plants less sensitive to changes outside the cells.
The team was surprised that zinc and iron varied so significantly across different genotypes of rice, but they add that there could be a possibility of breeding reduced sensitivity to increasing CO2 levels into crops in the future.
"Humanity is conducting a global experiment by rapidly altering the environmental conditions on the only habitable planet we know. As this experiment unfolds, there will undoubtedly be many surprises. Finding out that rising CO2 threatens human nutrition is one such surprise."
He adds that alongside trying to reduce CO2 emissions, breeding plant genotypes with reduced sensitivity and nutritionally supplementing human populations most affected could reduce the health impacts of these environmental changes.