More carbon dioxide in the air makes crops less nutritious, exacerbating threat of hunger in warmer world
By Gerard Wynn
Higher levels in the atmosphere of the greenhouse gas carbon dioxide will reduce the nutritional value of major food crops, said a study published on Wednesday.
The study casts doubt on previous assumptions of a crop benefit from continued carbon emissions.
Such assumptions had rested on the fact that plants need CO2 to grow, meaning that crop yields could rise if people continue to emit carbon from burning fossil fuels. It was hoped that such higher yields might help offset the impact on farm production from climate change including more frequent, intense droughts, heat waves and floods.
However, the extra crop growth has lower nutritional value, eliminating some of the benefit, found Wednesday’s article, supporting other recent studies.
The authors found a lower zinc and iron content in a wide range of major food crops including wheat, rice, peas and soy beans, at elevated atmospheric CO2.
“We found that elevated CO2 was associated with significant decreases in the concentrations of zinc and iron in all C3 grasses and legumes,” found the study’s 20 authors, from multiple academic institutions, publishing in the journal Nature.
“Dietary deficiencies of zinc and iron are a substantial global public health problem. An estimated two billion people suffer these deficiencies, causing a loss of 63 million life-years annually.”
“We also found that elevated CO2 was associated with lower protein content in C3 grasses, with a 6.3% decrease in wheat grains and a 7.8% decrease in rice grains. Elevated CO2 was associated with a small decrease in protein in field peas, and there was no significant effect in soybeans.”
The study was significant because it examined newly acquired data from seven experiments in Japan, Australia and the United States, equivalent to more than ten times the data so far reported in the published literature, its authors said.
Some plants (called C4, such as maize) are less affected by higher levels of CO2, regarding both their yields and nutritional quality, because they are already adapted to growing at lower CO2 levels. C3 plants are less well adapted, and so see a bigger growth spurt – called carbon fertilisation – when CO2 levels rise, but their nutritional quality also suffers, Wednesday’s report said.
The study examined the impacts of atmospheric levels of CO2 expected in the second half of this century. CO2 concentrations are presently around 400 parts per million and rising by 2 ppm a year; the study examined levels of around 550 ppm.
The findings could have important implications for estimates of the health impact of climate change, given that many studies to date have assumed a crop yield benefit from elevated CO2.
Such supposed benefits had wiped tens of millions off the numbers of people projected to suffer hunger by the middle of the century.
The actual impact on health may be more complicated, depending on what people most lack, whether protein and minerals, which may fall under elevated CO2, or calories, which may rise.
“The public health implications of global climate change are difficult to predict, and we expect many surprises,” the Nature study said.
“The finding that raising atmospheric CO2 lowers the nutritional value of C3 food crops is one such surprise that we can now better predict and prepare for. In addition to efforts to limit increases in CO2, it may be important to develop breeding programmes designed to decrease the vulnerability of key crops to these changes.”
“Nutritional analysis of which human populations are most vulnerable to decreased dietary availability of zinc, iron and protein from C3 crops could help to target response efforts.”