Humans’ greenhouse gas emissions have caused nothing but trouble for the environment — this much is obvious. From the public health problems caused by heavy air pollution to the myriad problems associated with climate change, there have never been more reasons to cut down on our carbon emissions. And a new study, published Wednesday in The Lancet, has just given us one more: Higher levels of carbon dioxide in the atmosphere could be causing nutrient deficiencies around the world.

The research finds that increasing amounts of carbon dioxide in the atmosphere will lower the content of zinc — an important nutrient — in certain major food crops, leading to an increase in nutrient deficiencies around the world. The study predicts that by the year 2050, when the carbon dioxide concentration in the Earth’s atmosphere is expected to reach 550 parts per million, at least 138 million more people will be placed at risk of a zinc deficiency.

This is alarming because zinc is an important nutrient for human health, helping to boost proper immune function, particularly in young children and pregnant women. A zinc deficiency can increase the likelihood of death by all kinds of common diseases, such as diarrhea and pneumonia. Already, more than a billion people worldwide are thought to be at risk of such a deficiency, according to the study.

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Led by Samuel Myers, a senior research scientist at Harvard University’s T.H. Chan School of Public Health, the researchers drew on a variety of data sources to come up with their conclusions. First, they referenced data from a series of experiments that grew crops under different carbon dioxide concentrations to see how they would react. The experiments showed that some food crops, including common staple items like wheat, rice, barley, soy and field peas, had a lower zinc content when grown under higher concentrations of carbon dioxide.

The researchers also examined other published data on the amount of zinc humans require in order to be healthy, as well as the types and amounts of food eaten in 188 countries around the world to see where different societies were getting their nutrients. When they compared this information with the decreased levels of zinc scientists expect crops to experience at higher carbon dioxide concentrations, they found that nearly 140 million more people around the world will probably not be getting enough zinc by 2050.

According to Paul Ehrlich, president of Stanford University’s Center for Conservation Biology (who was not involved with the research), the study is “a very important paper because it emphasizes what is so often ignored in the horrendous food problem the world faces, and that is that several billion people may be micronutrient malnourished. And this shows how that’s going to get worse.” And not only will malnutrition get worse — it will get worse in areas of the world that are already disproportionately affected by environmental concerns, such as climate change.

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The study indicates that most of the new zinc deficiencies will occur in developing nations, with populations in Africa and parts of Asia likely to be worse affected — in fact, the research predicts 48 million of the newly at-risk people will be found in India, alone.

It’s an aspect of the study that highlights a major inequality between the parts of the world most responsible for greenhouse gas emissions and the areas most affected by them. “It’s very clear that there’s this weird inversion where the wealthiest people are putting the most carbon dioxide in the atmosphere, and the poorest people are experiencing the most vulnerability to this effect,” says Myers, the lead author.

This inequality is compounded by the ways different populations get their nutrients. Eating meat from animals can be used to compensate for nutrients that are lost in plants. But people that can afford to eat meat often are usually “wealthier segments of populations and wealthier populations in general,” Myers says.

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To top it all off, Myers and his colleagues have reason to believe their results actually underestimate the number of people who will suffer in the future. For one thing, the researchers didn’t factor population growth into their calculations. “We all know that there are going to be something like another two to three billion people sharing the planet with us by 2050,” Myers says. “And so if you just scale the effect, it would be much higher numbers [of affected people].”

Another issue that didn’t factor into the study was whether human diets will change in the next 50 years, and by how much. Any changes in diet could affect the way humans get their zinc and other nutrients. From an optimistic standpoint, economic growth and development could allow for more dietary variety and higher calorie consumption around the world, which would be a nutritional bonus. But Myers believes it’s much more likely that a growing population, coupled with environmental issues like climate change and global water shortages, will continue to strain the world’s agricultural production instead, causing more hunger.

Ehrlich agrees that feeding and nourishing the world’s populations will only grow more challenging in the future for a variety of reasons. “A: We have too many people,” he says. “B: The rich are consuming too much. And C: We’re not distributing the food we produce the way it should be distributed.” These issues only exacerbate the problems that carbon emissions, climate change and other environmental issues are causing for global agriculture production.

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And it’s not just zinc intake that’s likely to be affected, either. Research has shown that high carbon dioxide levels can affect iron and protein content in some plants as well, Myers says. And another study Myers was involved with, also published today in The Lancet, suggests that declines in pollinators — such as bees and butterflies — could cause major drops in the world’s supplies of fruits, vegetables, nuts and seeds, leading to more nutrient deficiencies in people around the globe.

Altogether, the research paints a dire picture — although there are still some steps humans can take to try and improve the situation. “The thing that needs to be done immediately, of course, is basically get rid of the use of burning fossil fuels,” Ehrlich says, a step that some nations are already working toward.

Other interventions could include zinc supplementation programs — basically providing zinc supplements to people in need — as well as research on breeding crops that have higher zinc and other nutrient contents to begin with, Myers says.

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He adds that one of the paper’s biggest takeaways is the need for an awareness about the surprising ways human activities can affect our own survival.

“Who would have sat down 10 years ago and anticipated that burning fossil fuels and putting carbon dioxide into the atmosphere would alter the nutrient profiles of the foods that we eat, which would therefore lead hundreds of millions of people to be pushed into zinc deficiency?” he says. “One theme is that we’re going to experience a lot of these kinds of surprises as we continue to really change the playing field on which we’re trying to generate the world’s food.”

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