Researchers say they still do not understand how or why carbon dioxide emissions sap protein and other nutrients from plants, but the mystery is one that could have devastating consequences across the globe.
An additional 150 million people globally may be at risk of protein deficiency by 2050 because of rising levels of carbon dioxide in the atmosphere, said the report in the Environmental Research Letters journal.
The study, led by Harvard University researchers, is the first to quantify the impacts of global warming on the protein levels of crops.
It relies on data from open field experiments in which plants were exposed to high concentrations of carbon dioxide.
Global dietary information from the United Nations was used to calculate the impact on people who live dangerously close to the edge when it comes to getting enough protein.
Without it, growth is stunted, diseases are more common and early mortality is far more likely.
Carbon dioxide is a by-product of fossil-fuel burning that helps trap heat around the earth.
Without stark action, these emissions are expected to climb in the decades to come, resulting in rising seas, hotter temperatures and more extreme weather events.
A leading hypothesis was that CO2 might increase the amount of starch in plants, thereby decreasing protein and other nutrients.
But lead author Samuel Myers, a senior research scientist in Harvard University's T H Chan School of Public Health, said that experiments did not back up the theory.
"The short answer is we really have no idea," he told AFP.
"We've looked into it pretty extensively."
Protein was not the only nutrient to take a major hit.
Other research has shown that rising CO2 will cut key minerals like iron and zinc in staple crops, leading to further nutritional deficiencies worldwide.
AFRICA, ASIA HARDEST HIT
Researchers calculated that by 2050, higher CO2 concentrations will sap the protein contents of barley by 14.6 per cent, rice by 7.6 per cent, wheat by 7.8 percent, and potatoes by 6.4 per cent.
"If CO2 levels continue to rise as projected, the populations of 18 countries may lose more than 5 per cent of their dietary protein by 2050 due to a decline in the nutritional value of rice, wheat, and other staple crops," said the report.
A full 76 per cent of the people on earth rely on plants for most of their daily protein, particularly in poor areas of the globe.
The hardest hit areas are expected to be Sub-Saharan Africa, where millions already do not get enough protein in their diets, and South Asia where rice and wheat are common staples.
India alone may lose 5.3 per cent of protein from a standard diet, putting a predicted 53 million people at new risk of protein deficiency.
Researchers said solutions may include cutting carbon emissions, supporting more diverse diets, enriching the nutritional content of staple crops, and breeding crops that are less sensitive to the harmful effects of CO2.
by AFP Agence France-Presse is an international news agency headquartered in Paris. It is the oldest news agency in the world and one of the largest.