Rising levels of carbon dioxide (CO2) will badly hit the nutritional value of key cereals, including rice and wheat, scientists warned on Wednesday.

Crop breeders should focus on the worrying vulnerability of these staples to surging carbon emissions, they added.

Reporting in the journal Nature, researchers said they had tested 41 strains of six crops grown in open fields at seven sites in Australia, Japan and the United States, where plants were exposed to higher levels of CO2 released through horizontal gas pipes.

Normal air has CO2 concentrations of around 400 parts per million (ppm), which is currently rising at around two or three ppm annually.

In their "carbon-enriched" environment, the experimental plants grew in conditions of 546-586 ppm of CO2 -- a figure that under pessimistic scenarios may be reached by as soon as mid-century.

It translates into warming of more than three degrees Celsius (5.4 degrees Fahrenheit) over pre-industrial levels; UN countries have vowed to limit temperature rise to 2 C.

Zinc, iron and protein concentrations in wheat grown at the sites were reduced by 9.3 percent, 5.1 percent and 6.3 percent compared with wheat grown in normal conditions, the scientists found.

In rice, levels of zinc, iron and protein tumbled by 3.3 percent, 5.2 and 7.8 percent, although the figures varied widely across the different strains that were tested.

Other declines were seen in zinc and iron for field peas and soybeans, but there was little change in their protein levels.

In contrast, the impact of "enriched" CO2 on corn and sorghum was relatively minor.

"This study is the first to resolve the question of whether rising CO2 concentrations, which have been rising steadily since the Industrial Revolution, threaten human nutrition," said Samuel Myers, a researcher in environmental health at the Harvard School of Public Health.

"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."

The study called on crop breeders to look at tweaking essential cereals to make them less sensitive to CO2 rise.

Without help, poorer countries could be exposed to worsening nutrition, it said. Around two billion people suffer from zinc or iron deficiencies, which can affect the immune system and cause anaemia respectively.