Increasing CO2 threatens human nutrition. Samuel S. Myers, Antonella Zanobetti, Itai Kloog, Peter Huybers, Andrew D. B. Leakey, Arnold Bloom, Eli Carlisle, Lee H. Dietterich, Glenn Fitzgerald, Toshihiro Hasegawa, N. Michele Holbrook, Randall L. Nelson, Michael J. Ottman, Victor Raboy, Hidemitsu Sakai, Karla A. Sartor, Joel Schwartz, Saman Seneweera, Michael Tausz, Yasuhiro Usui, Nature, May 7, 2014, DOI: 10.1038/nature13179
The Big Question:
Discussions of climate change and food supply most often focus on questions of quantity. Will changes in rainfall and temperature interfere with the increased production that will be needed to support a growing population? A paper just published in the journal Nature grapples with a more insidious impact of increased carbon dioxide (CO2 ) concentration: how might it affect the quality of the global food supply?
The authors — a team of scientists from a dozen institutions, led by Samuel Myers, a research scientist at the Harvard School of Public Health — focused on concentrations of zinc, iron and protein in a variety of grains. “This study is the first to resolve the question of whether rising CO2 concentrations—which have been increasing steadily since the Industrial Revolution—threaten human nutrition,” said Myers, the study’s lead author.
Zinc and iron are a big deal because deficiencies in these nutrients reduce the immune system’s ability to defend against disease. World Health Organization (WHO) scientists already estimate that two billion people are not getting enough zinc and iron in their diets — shortening lives by 63 million life-years annually.
Study Nuts and Bolts:
Previous studies — mostly much smaller in size — have yielded mixed results on the question of whether carbon dioxide affects the nutritional value of plants.
The Nature study is the largest yet, drawing on the results of 143 comparisons from seven experimental sites in Japan, Australia and the United States. Researchers included only results from free air CO2 enrichment (FACE) experiments, which are more reliable because they increase the CO2 levels for an entire system — as opposed to chamber experiments, which use potted plants in artificial conditions.
At all the sites, researchers compared the edible portions of identical cultivars under ambient (~380 ppm at the time most of the studies were performed) and experimentally-enriched CO2 conditions (546 to 586 ppm).
Differences were clearest and most dramatic in wheat, where the concentration of zinc declined by almost 10% and the concentration of iron and protein by about 5% under high CO2 conditions. Zinc and iron in rice, field peas and soy beans responded similarly, though less dramatically, with decreases between 3 and 8%.
The researchers saw no decrease in the protein content of peas and beans, which are legumes and can acquire extra nitrogen through their relationship with symbiotic nitrogen-fixing bacteria.
Corn and sorghum showed no significant difference in nutrient content. That makes some sense because they are C4 plants, meaning they use a different method of photosynthesis than C3 plants such as wheat and rice. In C4 plants, CO2 is concentrated inside the plant before being taken up.
What It All Means:
In just the next 40-60 years, global average CO2 concentrations are predicted to reach 550 ppm – the level tested in this study. While a 5 or 10 percent loss of a few nutrients may not sound like much, it could have a large health impact, especially in places where the variety of foods is limited.
According to data from the United Nations Food and Agriculture Organization (FAO), 1.9 billion people live in countries where at least 70% of their zinc or iron comes from C3 plants. Reduction in the nutritional content of their food puts people at greater risk for diseases such as malaria, pneumonia and diarrheal diseases.
Different cultivars of the same crop did not always respond similarly — suggesting that it may be possible to partially counteract the effect by breeding for increased nutrient content (as is now done in golden rice for vitamin A). But plant breeding for specific traits often entails trade-offs in taste and yield; and improved seeds are often more expensive than traditional varieties. So there is no guarantee that farmers would adopt improved cultivars, even if they existed.
Many of the risks typically associated with climate change — such as drought, flooding, and species losses — are tied to predicted changes in temperature, moisture or ecosystem dynamics. Karla Sartor, who worked on the study before coming to the Conservancy to do grassland restoration, was struck by the direct line between increased CO2 and health effects.
“One of the things that I find powerful about this study” she said, “is that this is not modeled at all. We know that CO2 has increased and will continue to increase.”