New research, published in the scientific journal Nature this week, has revealed that higher levels of carbon dioxide in the atmosphere will have serious effects on the nutrients in a range of crop plants – bad news for farmers, and consumers.

Based on predicted atmospheric CO2 levels for 2050, crops that provide a large share of the global population with most of their dietary zinc and iron will have lost a significant amount of these nutrients within the next thirty years.

The contribution of humanity's reliance on fossil fuels to rising atmospheric CO2 levels is beyond dispute, though scientists are still discovering new impacts that a rapidly changing climate will have on the planet and its inhabitants.

Researchers at eight institutions, from Australia, Israel, Japan and the United States, contributed to the newly published analysis. The researchers looked at multiple varieties of wheat, rice, field peas, soybeans, maize and sorghum – 41 cultivars in all – grown in fields with atmospheric carbon dioxide levels like those expected in the middle of this century.

Atmospheric CO2 concentrations are expected to rise to 550 parts per million (ppm) by 2050, even if emissions mitigation measures are taken now. April this year was the first month for millions of years in which atmospheric CO2 concentrations were above 400ppm every day.

Teams conducting the Nature research simulated high CO2 levels in open-air fields using a system called Free Air Concentration Enrichment (FACE), which pumps out, monitors and adjusts ground-level atmospheric CO2 to simulate future conditions. They compared crops grown in these conditions to an otherwise identical set of control crops. The researchers found that the nutritional quality of a number of the world's most important crop plants dropped in response to higher CO2 concentrations.

Zinc and iron content fell significantly in wheat, rice, field peas and soybeans. Wheat and rice also saw notable declines in protein content when grown in high CO2 conditions. Zinc, iron, and protein concentrations in wheat grains grown at the US sites were reduced by 9.3 percent, 5.1%, and 6.3% respectively. Other staple crops lost nutrients too, but by lesser degrees.

University of Illinois plant biology professor Andrew Leakey, an author on the study, said, "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 concentrations], and zinc and iron deficiency is a big global health problem already for at least 2 billion people."

"Across a diverse set of environments in a number of countries, we see this decrease in quality," Leakey continued. The Illinois professor said sorghum and maize remained relatively unaffected by protein loss seen in other crops, as these plants use a type of photosynthesis which already concentrates carbon dioxide in their leaves.

Loss of nutrients 'most significant' health threat associated with climate change

According to Samuel Myers and his colleagues, researchers from Harvard University who worked on the Nature study, "this is the first [paper] to resolve the question of whether rising CO₂ concentrations – which have been increasing steadily since the Industrial Revolution – threaten human nutrition." Myer and his colleagues said, "The reduction in these nutrients represents the most significant health threat ever shown to be associated with climate change."

Almost half of the current global population rely on the crops tested for vast majority of their zinc and iron intake. Myers warned that simply eating more to counteract the effects of higher CO2 concentrations is not a viable option, given concerns about the availability of adequate food for a higher global population by the middle of the century.

The Harvard researchers said breeding programmes would need to factor in the changes climate change is set to bring, noting that certain varieties fared better than others at high levels of CO2, especially amongst rice plants, but pointed out that farmers select seeds based on a wide range of criteria, including marketability, yield and tradition, and that especially in developing regions improved seeds may prove unaffordable for many.

Myers cautioned, "Humanity is conducting a global experiment by rapidly altering the environmental conditions on the only habitable planet we know. As this experiment unfolds, there will undoubtedly be many surprises. Finding out that rising CO₂ threatens human nutrition is one such surprise."

Commenting on future areas of focus, Illinios Professor Leakey said, "It's important that we start to do these experiments in tropical climates with tropical soils, because that's just a terrible gap in our knowledge, given that that's where food security is already the biggest issue."

The researchers are unsure of the precise mechanism that caused the plants' nutritional value to decrease.