Rising carbon dioxide (CO2) levels are zapping greens and legumes of their nutrients, according to a new study in the journal Nature.

Atmospheric CO2 concentrations are currently approaching 400 parts per million (ppm), and are expected to jump to 550 ppm by 2050.

To see what would happen to some of the world's crops in a future, more CO2-dominant environment, a team of researchers used a system called Free Air Concentration Enrichment (FACE). It pumps out, monitors and adjusts ground-level atmospheric CO2 to simulate future conditions. Researchers grew 40 varieties of six different grains and legumes, including wheat, rice, field peas, soybeans, corn and sorghum, at seven locations throught Japan, Australia and the United States.

Predictably, the nutritional value of these crops, including zinc and iron levels, dropped when more CO2 filled the air.

Zinc and iron are important nutrients, and yet more than 2 billion people in the world are deficient in both. Consuming foods containing less of these nutrients can only exacerbate the problem.

"When we take all of the FACE experiments we've got around the world, we see that an awful lot of our key crops have lower concentrations of zinc and iron in them (at high CO2)," Andrew Leakey, a University of Illinois botanist and genomic biology professor, said in a statement.

Zinc and iron went down significantly in wheat, rice, field peas and soybeans. Wheat and rice also saw notable declines in protein content at higher CO2.

"Across a diverse set of environments in a number of countries, we see this decrease in quality," Leakey added.

But sorghum and maize were not as affected, nutritionally speaking, at higher CO2 levels because these crops use a type of photosynthesis, called C4, which already concentrates carbon dioxide in their leaves, Leakey noted.