Climate change’s effects often aren’t obvious.

In a particularly lateral example of how our planet’s changing environment is coming to affect our lives, scientists are now warning that our increasingly CO2-suffused atmosphere is causing the plants we eat to be less nutritious. Though the changes are subtle, they could already be endangering millions of people with poor diets, and hundreds of millions if the trend holds in the coming decades.

“The diets we eat today have less nutritional density than what our grandparents ate, even if we eat exactly the same thing,” Kristie Ebi, a professor at the University of Washington’s Center for Health and the Global Environment, told The Washington Post.

In a study published in the journal Global Change Biology in November, a team of researchers in the Netherlands surveyed the nutrient levels of 43 crops, ranging from rice to soybeans to wheat, in an assortment that represents the vast majority of plants that humans eat. The team found that nutrients like protein, iron, and zinc have fallen by 3.2 percent across all plants since the late 1980s. The culprit, they found, was an uptick in atmospheric carbon dioxide.

It’s not a drastic plummet, but it’s the beginning of an alarming trajectory. As billions of peoples’ diets already teeter on the verge of malnutrition, study lead author Sterre ter Haar warned that those few percentage points could push millions of people into a health crisis. A quarter of the world’s population is already believed to have anemia, a condition in which the body is incapable of producing enough blood cells to carry oxygen.

Plants thrive off CO2, converting it into sugars that provide them energy, so it may sound counterintuitive that more CO2 could be making them less nutritious. But their nutritional value, to us at least, comes from minerals absorbed through the soil. The greenhouse gas may help the crops grow bigger and faster, but their nutrient uptake remains the same, diluting their concentration, according to WaPo.

That’s not all. The over-abundance of CO2 means plants don’t need to open their microscopic pores, called stomata, to let in air. When the stomata open, some of the water a plant contains evaporates. Since the stomata are open less frequently, they’re also drawing less water though their roots — water that crucially contains the minerals we need.

“The plant is becoming more efficient, but it’s occurring at a price, from a human perspective,” Lewis Ziska, a plant biologist at Columbia University who studied the phenomenon for more than two decades, told WaPo.

The price is steep. An additional 175 million people could suffer problems from a zinc deficiency due to plunging nutrient levels in food from CO2 pollution, a 2018 study found. And some 1.4 billion women and children could lose four percent of their dietary iron, worsening their anemia, a condition that can cause pregnancy complications, developmental problems, and even death.

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