by Sarah Derouin Friday, December 21, 2018
Plants absorb carbon dioxide to fuel their growth. As humans increase the amount of the greenhouse gas in the atmosphere, more will be available to vegetation around the world. But according to a new study, too much carbon dioxide might eventually lead to plants that are deficient in key nutrients for humans, which could be especially detrimental in the developing world.
Current atmospheric carbon dioxide concentrations are more than 400 parts per million (ppm) and are steadily rising. Within 30 to 50 years, scientists predict atmospheric carbon dioxide will surpass 550 ppm, but it is unknown how such levels will affect the global food supply.
Experiments in the 1990s focused on greenhouse-grown crops to gauge what might happen in high-carbon atmospheres. Samuel Myers, a research scientist and director of the Planetary Health Alliance at Harvard and co-author of the new study, says these studies showed that some nutrient concentrations dropped significantly in plants grown in elevated carbon dioxide conditions. The reason for the nutrient reductions isn’t known, Myers says, but researchers hypothesized that the increased plant growth spurred by extra carbon dioxide may act to dilute nutrients. The results of these early studies were contested in the scientific community, however, because of small sample sizes — both in the number of crops and years of sampling — and the artificial growing conditions.
To address these issues, in 2014, Myers and his colleagues tapped a global consortium of researchers to gather data on open fields of cereal grains and legumes — two classes of crops humans eat in abundance. In the 2014 paper in Nature, they looked at 10 years of data on nutrient levels in 41 cultivars and six staple food crops from seven locations on three continents. They found that in most field crops, elevated carbon dioxide concentrations resulted in decreased levels of iron and zinc — both micronutrients that affect human immune systems, IQ and strength — and protein, a macronutrient that plays an important role in human mortality and malnutrition.
The multicontinent study raised another question, Myers says: How many people will be impacted from a nutrition standpoint? “That question is a tough nut to crack,” he says. “You have to estimate what the entire global population is eating, then estimate per capita intake of all the foods they’re consuming, and the density of nutrients in each one of those [foods].”
In the new study published in Nature Climate Change, the researchers estimated how rising carbon dioxide will impact the risk of iron, zinc and protein deficiencies for people in those countries using the Global Expanded Nutrient Supply (GENuS) database — which quantifies 23 nutrients in 225 food categories (for example, cereals, fruits and meat) for people in 152 countries (representing 95 percent of the world population) based on historical trends in diets.
They again found that levels of zinc, iron and protein in some grains and legumes will drop between 3 and 17 percent with higher carbon dioxide concentrations in the atmosphere. In particular, areas in Africa, China, India, the Middle East and Southeast Asia — where people consume less meat and rely on legumes and grains like wheat and rice for much of their micronutrient supply — will be at the highest risk of nutritional deficiencies.
Based on population projections, the team reported that if carbon dioxide levels reach 550 ppm by 2050, an additional 175 million people worldwide could experience zinc deficiency and an additional 122 million people could be protein-deficient, beyond the roughly 2 billion people globally already thought to suffer from some sort of nutrient deficiency. Additionally, a combined 1.4 billion children under 5 and women of childbearing age would be at a high risk of iron deficiency. “Chances are we are talking about more than one deficiency in these populations, which can’t be good from a human health standpoint,” Myers says.
Measuring the impacts of rising atmospheric carbon on nutrition is important, says Frances Moore, an environmental economist at the University of California, Davis, who was not involved in the study. “The magnitude of the effect [of rising carbon dioxide on plants] wasn’t well quantified” in past work, she says. And the “full chain of causal connections” from how changes in carbon dioxide concentrations affect crops to the ultimate effect on dietary nutrition, “hasn’t really been made, certainly not for multiple nutrients.”
Moore says the study is important in helping us understanding how climate change will impact human nutrition but notes that there are some problems with the projections. For example, the study projects that diets in 2050 will be the same as today, which “is a little strange, because we are reasonably sure these countries are going to be growing economically over the next 30 years.” And dietary improvements could change the number of those at risk, she notes: As people get richer, she says, one of the first things they do is “improve their diet — they diversify their diet and add meat.” Still, Moore says, the huge number of people currently at risk and suffering from nutritional deficiencies is “a serious problem already that should be receiving more attention.”
And the increased risk and sheer magnitude of potential malnutrition and nutrient deficiencies with rising carbon dioxide shown in the new work is surprising to many scientists, including Myers and Moore. “It’s a very unanticipated consequence,” Myers says.
“Maybe 10 or 20 years ago, the impression was that [rising] carbon dioxide was going to be beneficial [to plants],” Moore says. “The picture emerging now, however, is that carbon dioxide is somewhat beneficial, but that those gains are pretty quickly offset by the negative effects of high temperatures — which are widespread and large.” Nutrient losses will especially hit people with lower incomes, she says, as they tend to get much of their nutrient supply from vegetables.
Reducing carbon dioxide emissions is “the first-order preventative” step to avoid compounding the risk to those already struggling with malnutrition, Myers says. Regardless of what happens with emissions, he says he hopes the new research will be useful in identifying vulnerable populations. There’s no single solution, he says, but there are ways to mitigate vulnerability once you know where it is.
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