Climate change with increasing levels of the greenhouse gas carbon dioxide could lead to crops having significantly reduced concentration of the elements basic to human health, a study suggests.

By 2050, the study predicts, crops that deliver dietary iron and zinc to much of the world's population will be showing a reduction of the elements if atmospheric levels of CO2 reach anticipated levels.

Around the world as many as two billion people are already suffering from forms of malnutrition brought on by iron and zinc deficiencies, so any further reduction in the amounts of the elements available in crops would present the most serious health threat yet associated with the effects of climate change.

The researchers at the Harvard School of Public Health analyzed data on 41 types of legumes and grains from a number of different locations in the United States, Japan and Australia, all grown using a technique known as free air carbon dioxide enrichment, or FACE, to gauge the effect on the crops of elevated CO2 levels.

Global atmospheric levels of CO2 currently stand at 400 parts per million; the levels were enriched in the study fields to around 550 ppm.

Wheat, rice, soybeans, maize, sorghum and peas were tested for nutritional content in iron and zinc over six growing seasons.

"[W]e find that the edible portions of many of the key crops for human nutrition have decreased nutritional value when compared with the same plants grown under identical conditions but at the present ambient" carbon dioxide, the researchers said.

The most significant decline in zinc and iron levels was found in wheat, a drop of 9.3 percent.

In many regions of the world as much as 60 percent of dietary iron and zinc people need is provided by the crops studied, the United Nations Food and Agriculture Organization says.

"Humanity is conducting a global experiment by rapidly altering the environmental conditions on the only habitable planet we know," says study lead author Samuel Myers of the Harvard school's Department of Environmental Health. "As this experiment unfolds, there will undoubtedly be many surprises. Finding out that rising CO2 threatens human nutrition is one such surprise."

The world may have to resort to developing new breeds of wheat, peas, soybeans and rice that show the ability to resist higher CO2 levels, Myers says.

That the current varieties are being affected is beyond doubt, he warns.

"Crops are losing nutrients as CO2 is going up."

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