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Atmospheric CO2 levels are expected to be significantly elevated by around 2050.

And researchers have warned that crops that provide the global population with most of their dietary zinc and iron will be affected.

That's according to a new study led by the Harvard School of Public Health (HSPH), who say plants will have significantly reduced concentrations of those nutrients.

A new study from the Harvard School of Public Health (HSPH) says plants will be significantly less nutritious by 2050 if levels of carbon dioxide continue to elevate

An estimated two billion people suffer from zinc and iron deficiencies.

This results in a loss of 63 million life years annually from malnutrition.

Thus the reduction in these nutrients represents the most significant health threat ever shown to be associated with climate change, says the HSPH.

'This study is the first to resolve the question of whether rising CO2 concentrations - which have been increasing steadily since the Industrial Revolution - threaten human nutrition,' said Samuel Myers, research scientist in the Department of Environmental Health at the HSPH and the study's lead author.

Some previous studies of crops grown in greenhouses and chambers at elevated CO2 had found nutrient reductions, but those studies were criticised for using artificial growing conditions. 

Experiments using free air carbon dioxide enrichment (FACE) technology became the gold standard as FACE allowed plants to be grown in open fields at elevated levels of CO2, but those prior studies had small sample sizes and have been inconclusive.

This study now, however, shows that the loss of iron and zinc from grains and legumes at elevated CO2 is significant.

In the study they found that grains and legumes lost significant amounts of iron and zinc at elevated levesl of CO2. The researchers say two to three billion people around the world receive 70% of their dietary zinc and/or iron from crops like these

Myers and his colleagues estimate that two to three billion people around the world receive 70% or more of their dietary zinc and/or iron from C3 crops, particularly in the developing world, where zinc and iron deficiency is already a major health concern.

C4 plants are those that have long growing seasons and spend a lot of time in the sun.

C3 plants, meanwhile, are found in a broader range of environments.

In the study C4 crops appeared to be less affected by higher CO2, which is consistent with underlying plant physiology.

This is because C4 plants concentrate CO2 inside the cell for photosynthesis so they might be expected to be less sensitive to changes outside the cell in CO2 concentration.

But the researchers were surprised to find that zinc and iron varied substantially across cultivars (plants chosen for their desirable characteristics) of rice.

That finding suggests that there could be an opportunity to breed reduced sensitivity to the effect of elevated CO2 into crop cultivars in the future.

In addition to efforts to reduce CO2 emissions, genetically engineering crops to increase their nutritional value (known as biofortification) with regards to iron and zing could reduce the human health impacts of these changes, said Myers.

He also suggests we could breed cultivars with reduced sensitivity to CO2 and provide nutritional supplementation for the populations most impacted if rising CO2 levels cannot be quelled.

'Humanity is conducting a global experiment by rapidly altering the environmental conditions on the only habitable planet we know,' he said.

'As this experiment unfolds, there will undoubtedly be many surprises.

'Finding out that rising CO2 threatens human nutrition is one such surprise.'