Samuel Myers, a researcher at Harvard's School of Public Health, says these experiments take place in open fields, "except that in the field are placed rings of carbon dioxide jets." These jets release just enough carbon dioxide to simulate the atmosphere that crops will almost certainly experience 40 to 60 years from now.

In general, the experiments show that crops grow faster when there's more carbon dioxide, and yields are often 10 percent higher, compared to plants in normal atmosphere.

But Myers and his colleagues took a closer look, examining not just the quantity of the harvest, but also its quality.

"What we found were five to ten percent reductions in nutrients like iron, zinc, and protein," he says.

Myers isn't sure what's causing this. One theory is that when a plant produces more grain or beans, the trace nutrients get diluted.

No matter what the cause, Myers says the effects could be really significant — and harmful.

Worldwide, about two billion people already are getting too little iron and zinc in their diets, and it's damaging their health. Zinc deficiency causes increased child mortality due to infectious diseases, because it prevents the immune system from working properly. Lack of iron increases the death rates of mothers and lowers the IQ of children.

If some of the world's most important crops provide even lower levels of these nutrients in a future, high-CO2 world, Myer says, it's likely to make the problem even worse.

The problems of iron and zinc deficiency have gotten increasing attention in recent years.

The ideal solution would be for people to eat a wider range of foods, since some of the world's major food crops — rice and corn in particular — don't supply much iron or zinc at all, even without any rise in carbon dioxide. Many people rely on those crops, however, because they can't afford anything else.

An international effort called HarvestPlus is trying to create new crop varieties, through plant breeding, that contain higher levels of these nutrients. The initiative has succeeded in creating lines of rice and wheat that are high in zinc.

But Michael Grusak, a researcher with the US Department of Agriculture's Children's Nutrition Research Center in Houston, says it's proven quite difficult to boost the levels of nutrients, especially iron, in certain crops. "If elevated CO2 or other climate change processes are working against us, we're going to have to work even harder to raise these levels," he says.

The report appears in this week's issue of the journal Nature.

Copyright 2014 NPR.

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Less Nutritious Grains May Be In Our Future