A symbolic threshold was crossed in 2016 when carbon dioxide concentrations in the atmosphere from a monitoring station at the South Pole recorded 400 parts per million (ppm) for the first time in possibly four million years. Much study has focused on the effects these rising carbon dioxide levels could have on weather patterns and global temperatures, but could elevated atmospheric CO2 levels negatively affect the nutritional value of the food we grow?

While the crossing of the 400 ppm barrier doesn't immediately mean much, examining the dramatically increasing rate of atmospheric CO2 over the past 50 years reveals a cause for concern. In 1958 the first dedicated CO2 measurement station began tracking the atmosphere on the slopes of the Mauna Loa volcano in Hawaii. CO2 in the atmosphere initially was growing at a rate of 0.7 ppm per year, but that has steadily risen to a rate of 2.1 ppm per year over the past decade.

The atmosphere and our food

In 2014, two landmark research papers were published, both investigating the effects of atmospheric CO2 levels on the nutritional properties of the food we eat. Both studies were relatively conclusive – higher CO2 levels in the atmosphere result in lower concentrations of protein and minerals in some key crops.

The first study, from Irakli Loladze, was the largest and most comprehensive examination to date, looking at how elevated atmospheric CO2 levels could affect the nutrient levels in food. Loladze compiled nearly 8,000 observations as part of his study, comparing the exposure to CO2 against the overall concentration of 25 minerals in 130 different plant varieties.

He identified elevated CO2 levels reducing the mineral content of plants by 8 percent on average. He also found that increasing a plant's exposure to CO2 increased the ratio of carbohydrates to minerals. Essentially, as the carb content rose, the mineral content dropped, making the plant less nutritionally valuable to humans as food.

"Every leaf and every grass blade on earth makes more and more sugars as CO2 levels keep rising," Loladze recently said in an interview with Politico. "We are witnessing the greatest injection of carbohydrates into the biosphere in human history – [an] injection that dilutes other nutrients in our food supply."

The other study came from Harvard's Samuel Myers and a large team of researchers. This study was very clearly targeted at trying to understand what effect elevated CO2 levels would have on specific major human food crops – rice, wheat, maize and soybeans were among the crops examined. Nutrient concentrations were compared between crops grown at ambient (or current) CO2 levels, and crops grown at elevated levels (noted as in the range of 546 to 586 ppm across seven different study sites).

Decreases in zinc, iron and protein were seen in all the studied crops. Wheat grown at elevated CO2 levels for example, displayed a 6.3 percent decrease in protein content, 9.3 percent lower zinc and 5.1 percent lower iron. Similar results were also identified in rice crops, although that common food crop showed an even higher protein drop, down 7.8 percent.

So what does all this mean?

Well, first we have to think about a planet where atmospheric CO2 levels exceed 550 ppm. Climate scientists aren't entirely in agreement over what will happen to the planet if we hit the 550 ppm point, but there will be "some" level of global temperature rise, and that will result in "some" sea level rise. How much no one is really sure, and estimates range from the mildly damaging to the catastrophically disastrous. We can be reasonably confident that at some point, probably over the next 50 years, we will cross the 500 ppm threshold (unless we see some significant global action to tackle emissions).

But assuming our environment stays more stable than we expected after atmospheric CO2 levels have risen, what effect would these seemingly minor protein and nutrient drops have for people's health?

Focusing solely on the lowering of zinc levels in some food crops, Myers calculated that if CO2 levels hit 550 ppm by 2050 the subsequent nutritional effects would place 138 million people at a new risk of zinc deficiency. The numbers are even higher when we look at the effects of a drop in protein concentrations.

"Wheat and rice, among the most sensitive crops to eCO2 (elevated atmospheric CO2 concentrations), are primary protein sources for 71% of the world's population," writes Myers and his team in a recent research paper. "By 2050, 148.4 million people worldwide may become at risk of protein deficiency from rising CO2. In India, expected to be the world's most populous country, and a country that is highly dependent on rice, 53.4 million people may be newly at risk of protein deficiency."

These small shifts in a crop's nutritional content can rapidly tip a person in a developing or third world country into an unhealthy, nutritionally deficient state. Those already in poverty, or already with poor diets, will be the ones to suffer most from this nutritional hit.

Reality is always much more complicated though, and there are so many other factors that will affect where the world ends up in 50 years time. Some argue that rising CO2 levels will actually increases crop yields, and there is some evidence to back this up.

So maybe we will just grow more food to compensate? Or maybe increasing atmospheric CO2 levels will result in extreme weather conditions such as heatwaves and floods, eliminating any net yield growth from increased CO2.

Genetically modified crops could certainly be developed to increase the nutritional value of a food, but will they be available to all and safely tested before being implemented widely?

The most fundamental takeaway from this research is that rising atmospheric CO2 levels will affect the planet in a variety of unpredictable ways, right down to affecting the nutritional value of our food.

"It is very hard to predict all the challenges to human health resulting from climate change," said Myers in an interview with The Guardian. "My guess is there will be many more surprises as we remake the environmental conditions on the planet. As a civilization we are now living with 400 ppm for the first time: it's a new world."