Rising carbon dioxide levels in the atmosphere will strip nutrients from major staple crops, potentially putting millions of people at risk of malnutrition, according to a new paper from Harvard University.

The impact of greenhouse gas emissions is sapping protein and other nutrients from staple crops including rice and wheat, according to new research from Harvard’s TH Chan School of Public Health, though researchers admitted they don’t know why this is happening. The leading theory is that plants could increase their starch content in response to higher concentrations of CO2, at the expense of other nutrients.

In the paper, published on Wednesday, researchers said that this trend represents a significant food security worry, as it could potentially expose an extra 150 million people to protein and nutrient deficiencies by mid-century.

The study used data from open field experiments, in which crops were exposed to higher levels of CO2, replicating the future atmosphere, and researchers used dietary information from the UN to understand the impact of global warming on crop nutrition. They found that under higher CO2 concentrations, the protein contents of rice, wheat, barley, and potatoes decreased by 7.6%, 7.8%, 14.1%, and 6.4%, respectively.

Looking at these potential impacts, the Harvard researchers recommended urgent action to reduce atmospheric CO2 levels and moves to diversify diets, particularly in the global south, where people tread a much thinner line when it comes to nutrient intake.

"This study highlights the need for countries that are most at risk to actively monitor their populations' nutritional sufficiency, and, more fundamentally, the need for countries to curb human-caused CO2 emissions," said Samuel Myers, senior research scientist in the Department of Environmental Health, who led the research. Other research by Myers has indicated that greenhouse gas emissions and human-driven climate change also stand to affect the iron and zinc content of staple foods.

The Harvard researcher added, "Strategies to maintain adequate diets need to focus on the most vulnerable countries and populations, and thought must be given to reducing vulnerability to nutrient deficiencies through supporting more diverse and nutritious diets, enriching the nutritional content of staple crops, and breeding crops less sensitive to these CO2 effects. And, of course, we need to dramatically reduce global CO2 emissions as quickly as possible.”

It’s not only people who are at risk from the effects of global warming on crop nutrition. Also reporting in Environmental Research Letters, where the Harvard study was published, a private research team from Colorado reported that there has been a significant decline in the protein available to grazing animals from US grassland over the past 22 years.