Hidden impacts: as carbon dioxide goes up, crop nutrients go down

Sep 27, 2018


Philip Thornton (CCAFS), Andy Jarvis (CCAFS), John McDermott (4NH), Bruce Campbell (CCAFS)

While agricultural scientists were busy understanding the impacts of climate change on crop yields, rising atmospheric carbon dioxide was chipping away at the nutritional content of our food almost undetected.

The carbon dioxide story is complicated. For some food crops like maize, rising carbon dioxide levels don’t affect their growth much, because of the way they do photosynthesis (called C4). But many of the other crops we eat, like wheat, rice and potatoes, do photosynthesis differently (C3). For these crops, rising carbon dioxide levels actually have a beneficial effect on their growth. We’ve known about this for a long time, and it’s one of the reasons why climate impacts are projected to not be so bad in the first few decades of the 21st century.
But one downside of this ‘fertilisation effect’ is that these crops produce more carbohydrates at the expense of other nutrients that we need for healthy diets. This effect was essentially unknown until after the turn of the 21st century. A 2004 study found declines in nutrients for 43 food groups between 1950 and 1999. The authors attributed this to widespread use of higher-yielding crop varieties that inadvertently traded-off nutrient content.
Several recent studies, however, have identified the true villain as rising carbon dioxide concentrations. The data we have now indicate losses of minerals by up to 8 percent. A new study by Matthew Smith and Samuel Myers looks at the projected impact of elevated carbon dioxide concentrations in 2050 on dietary intake of iron, zinc and protein in 151 countries. They  predict that an additional 175 million people will be zinc deficient, and an additional 122 million people will be protein deficient.
It’s not only crops: forage quantity and quality are also affected by elevated carbon dioxide concentrations, sometimes substantially. About 60% of grasses globally are C3 plants and thus susceptible to carbon dioxide effects on nutritional quality. These impacts will result in greater nutritional stress in grazing animals, as well as reduced meat and milk production.

How can we address the issue? For some crops, breeding programs could select cultivars with reduced carbon dioxide sensitivity, as well as other traits like heat and drought tolerance. There’s also a lot of ongoing work developing new varieties with enhanced micronutrient content. Efforts are underway to mainstream enhanced micronutrients into multi-trait breeding programs for staple food crops, which could help offset declines in nutrient density in the places that need it the most.
Can we do this breeding fast enough? It’s probably a close-run thing. New tools like speed breeding and CRISPR-9 may be able to accelerate breeding progress substantially, but there’s no time to lose. Other nutrition options for national governments are to enhance consumption of nutrient-dense foods such as vegetables, fruits and animal-source foods and to provide micronutrient supplements and fortified foods for vulnerable populations.
But ultimately, we need to redouble efforts to curb all greenhouse gas emissions. In a hothouse earth at 550 ppm CO2 and +4°C, who knows what other “hidden effects” of climate change we may stumble across? Much better to not find out.