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    Based on that assumption, the study team established seven agricultural sites across Australia, Japan and the United States. In turn, 41 versions of grains and legumes were planted in open-air conditions, with CO2 levels set between 546 and 586 parts per million.

    Nutritional testing revealed that some crops -- such as sorghum and maize -- fared better than others, probably due to pre-existing CO2 exposure, the study authors suggested. Some forms of rice also seemed to hang on to their nutritional content despite elevated CO2 levels.

    But, many varieties of rice, wheat, peas and soybeans lost significant amounts of iron and zinc. Zinc levels in wheat, for example, dropped by more than 9 percent, with iron dropping by 5 percent. Wheat also saw protein levels fall off by more than 6 percent, the investigators found.

    In the end, Myers' team concluded that the nutritional threat posed by carbon dioxide is all too real.

    "And I think it's very important not to conflate the CO2 issue with climate change," he said. "Because while climate change is, for some, a matter of vigorous debate in terms of how it will unfold, there is no debate about the simple fact that CO2 in the atmosphere is rising. It's rising. And the nutritional impact we have identified here is entirely dependent on that rise, and nothing else."

    So is a nutritional calamity inevitable?

    "There are two possible avenues to address the issue," Myers said. "One is to try to contain CO2 levels. But the problem is that the CO2 levels that most people believe we'll see by 2050 are expected regardless of any efforts to mitigate against climate change that might theoretically be taken now, because most of the efforts being discussed are about trying to reduce even higher levels of CO2 in the future."

    Myers acknowledged the importance of trying to reduce CO2 levels, but "unless we develop some completely unanticipated technology to remove CO2 from the atmosphere in huge amounts, the nutritional impact we saw will come into play," he said.

    "The other avenue is to try and reduce vulnerability by developing crop breeds that are less sensitive to this effect," he suggested. "And there is some grounds for believing that's possible, by, for example, bio-fortifying grains with additional amounts of iron and zinc. Or, on the other hand, launching aggressive global mineral supplementation programs. All of this might help."

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