Rising carbon dioxide levels may reduce the nutritional value of important foods

A study in the journal Nature finds that rising concentrations of carbon dioxide threaten global human nutrition by significantly reducing the levels of nutrients important to human health.

Researchers cultivated 41 different varieties of staple crops on three continents to examine how they might be affected by the expected increase of CO2 in coming decades. The crops included rice, wheat, soybeans, maize, field peas and sorghum — plant groups that are central to human nutrition around the world.

The study’s lead author, Sam Myers, says they found significant reductions in zinc, iron and protein in grain crops like rice and wheat, and similar reductions in zinc and iron, but smaller reductions in protein, in legumes like soybeans and field peas.

The reductions are statistically highly significant and represent a serious threat to public health, Myers says. Roughly two billion people around the world already suffer from zinc and iron deficiencies.

Without adequate zinc, Myers says, our immune systems don’t function properly, leading primarily to an increase in child mortality from diseases like pneumonia, diarrhea, malaria, measles and other infectious diseases. Iron deficiency is more complex: the risk of maternal mortality rises sharply and there are also effects on IQ and work productivity, Myers explains.

“When you combine all of those different kinds of effects for both iron and zinc deficiencies together, it’s estimated that we [already] lose about 63 million life years annually from those nutrient deficiencies,” Myers says.

The researchers grew the crops under conditions that simulated CO2 levels of 550 parts per million — the concentration most scientists agree will exist by the year 2050. They used a technique called Free Air Carbon Dioxide Enrichment, in which the crops are grown in open-field conditions surrounded by a ring of carbon-dioxide-emitting jets. A sensor in the center of the ring calculates wind speed and carbon dioxide concentration and signals the jets to adjust their emissions as needed.

Different cultivars of crops might do better in a high-carbon dioxide environment, Myers says. For example, the researchers looked at 18 rice cultivars and found that some of them were more affected by rising CO2 than others. So, it may be possible to breed reduced sensitivity to carbon dioxide into rice as well as other crops.

Myers is not so sanguine about the so-called CO2 ‘fertilization effect’ some argue will make plants grow bigger and faster. There is data to suggest there is a small fertilization effect that can increase yields, Myers says, but this occurs only in the context of adequate irrigation. The unreliability of water supplies in many regions of the world renders this effect negligible at best.

The world is facing a deeply concerning confluence of difficult conditions, Myers says: CO2 levels rising to 550 parts per million over the next 40 years; the world population increasing from seven to nine billion; and a warmer planet that is likely to have less arable land than it does now.

“We are essentially transforming all the natural systems on the planet in a way that is both pervasive and accelerated,” Myers says. “That transformation will lead to a host of surprises, and it’s very difficult to anticipate how these environmental changes will ripple through ecological systems and ultimately impact our own health and well-being.”

“All we can really say with certainty is that we’ll be surprised many times in the future. To me, this research is an example of one of those surprises.”