The carbon dioxide humans are pumping into the atmosphere at a rate of 2.4 million pounds per second is sapping the nutrients in a major food source for 2 billion people, a big study published Wednesday found. It’s the latest sign that we’re not just changing the climate by burning fossil fuels; we’re also changing the food we eat.

Essential nutrients found in rice, including protein, iron, zinc, and B vitamins, are all poised to drop, according to the study in Science Advances. And that could be dangerous for people who count on the grain as their main source of nourishment.

Levels of CO2 in the atmosphere keep climbing — the monthly average for global CO2 concentration topped 410 parts per million in April, a new record high. And scientists are finding that while CO2, a greenhouse gas, helps plants grow, it can also radically alter the nutrient content in food crops.

Earlier studies have found that staples like barley and potatoes have seen their carbohydrate levels go up and protein content go down as CO2 levels have ticked upward. And experiments show that as CO2 emissions continue to rise, critical nutrients will decline further, even as we harvest more beans, grains, and seeds.

Already about 1 billion people in the world are deemed food-insecure and 795 million are undernourished. If staple foods like rice fail to provide critical vitamins and minerals, illnesses like anemia and beriberi will rise, stalling progress in the climb out of poverty.

In the new study, researchers showed how rising CO2 levels would impact nutrition in the 10 countries that eat the most rice as a proportion of their daily diets, suggesting that a dire future may be in store. Let’s walk through the details.

Rice is delicious but becoming less nutritious

From the stubby round grains of Japonica to the long fluffy grains of Indica, rice is the culinary bedrock for much of the world, providing 25 percent of the total calories consumed globally. But it’s especially critical in developing countries like Myanmar, Madagascar, and Cambodia, where more than 600 million people rely on rice for more than half of their daily energy and protein.

“It is the primary food available for the poorest people in the world, particularly for those in Asia,” said Lewis Ziska, a research plant physiologist at the US Department of Agriculture and a co-author of the new study.

Consequently, the people who eat the most rice stand to suffer the worst health problems as the crop becomes less nutritious.

Ziska and his colleagues studied 18 rice strains grown around the world using a technique called free-air CO2 enrichment at sites in Japan and China. This involved building an octagon of tubing around a section of a rice paddy and injecting CO2 while still using the standard commercial applications of fertilizer and pesticides to simulate real-world growth.

They then exposed the rice to CO2 concentrations between 568 parts per million and 590 parts per million to see what might happen to rice nutrition in the future. They chose those levels because climate models suggest that CO2 concentrations will likely top 570 parts per million by 2100 and children born today will likely eat rice grown in 550 parts per million CO2 levels within their lifetimes.

After harvesting the rice, researchers found an average of 10.3 percent reduction in protein across all the tested varieties, with one tested cultivar showing a 20 percent drop in protein. Iron content fell by an average of 8 percent and dropped by as much as 20 percent, while the average decline in zinc was 5.1 percent, though some strains experienced an almost 15 percent fall.

B vitamin concentrations also fell as CO2 levels rose. B5 levels dipped by 12.7 percent on average while B9 plummeted 30 percent on average.

This is a worrying development because rice is going to remain a staple crop for the foreseeable future, and countries like Guinea and Senegal are becoming increasingly reliant on it. Declining nutrition from rice is likely to increase the burden of disease, hitting the poorest the hardest.

Scientists don’t know why CO2 reduces plant nutrients, but they have some theories

Despite the scale of the threat, it’s been hard for scientists to figure out exactly why CO2 hurts crop nutrients.

“We have a firm possibility of a definite maybe of several things,” Ziska said. “It’s not a one-size-fits-all explanation.”

It could be that if plants can draw on more carbon dioxide, they need less of other nutrients. It could also be that as crops produce more grains, the nutrients are diluted. Carbon dioxide also causes stomates, pores in plant leaves, to close. This in turn changes the flow of water through the plant, which may affect how water-soluble nutrients accumulate.

Though the mechanism remains a mystery, there are some potential solutions, ranging from varying diets to engineering crops to soil enrichment.

But picking a different dinner staple won’t be easy for the world’s poorest; countries that have shifted away from rice, like Japan, did so because they became wealthier. In 1959, Japan received 62 percent of its calories from rice. It fell to 40 percent in 1976, and now rice provides less than 20 percent of calories to Japan.

“As you increase your overall GDP, you also tend to increase your diversity of diet, and that in turn is going to affect whether you’re impacted by these changes in rice,” Ziska said.

Countries may also have to breed or genetically engineer varieties of rice that remain nutritious at higher carbon dioxide levels. The most famous example is golden rice, which was genetically modified to produce more vitamin A. This a tremendously time-consuming and expensive option, and right now we don’t have a rice cultivar that compensates for all the nutrients lost due to increasing CO2.

Another approach may be to fortify soils with the requisite nutrients, but that also adds to costs and demands energy, forcing farmers over time to invest more to get the same or less from their food.

And more CO2 may not always benefit crops. For plants cultivated in poor soils or in water-stressed regions, extra CO2 won’t do much to help them grow.

All the while, the other major consequence of rising carbon dioxide levels — climate change — remains a threat to crops. Increased global temperatures could cut rice yields by 20 to 40 percent by 2100, and higher temperatures alone can reduce protein content in crops. That’s before factoring in crop failures from more severe storms and droughts.

The global population is still growing too, so more people are going to have fewer crops to eat, and the food itself will provide less nourishment. So while some climate change skeptics argue CO2 is plant food and that more of it means a greener earth, the science points to a scenario where food insecurity and malnutrition increase in the coming decades even as crop yields go up, and the world’s poorest will once again face the harshest repercussions.