Fertilizing hidden hunger

A healthy meal is a complex cocktail of macroand micro-nutrients. Yet, when it comes to discussing diets, we typically consider calories to be the central drivers of hunger and obesity, disregarding other factors. The threat that climate change poses to agricultural productivity and food security around the world, especially in the tropics and sub-tropics, is also usually analysed only in terms of yields and calories1. The primary driver of anthropogenic climate change — the emission of CO2 into the atmosphere — has long been known to stimulate photosynthesis and plant growth, an effect that has the potential to compensate much of the negative impact of climate change. This so-called CO2 fertilization increases nitrogen use efficiency, reduces water use2, and is especially relevant for stimulating photosynthesis in the large group of C3-plants, which include important crops like wheat, rice and soy. A focus on calories, however, may be greatly misleading when judging whether the effects of CO2 fertilization are beneficial for food security. In a Letter published in Nature, Myers et al.3 present compelling evidence, based on a large meta-analysis of published studies, that CO2 fertilization will have negative effects on the nutritional value of many key food crops by reducing the concentrations of essential minerals and protein. This could have serious implications for hunger and health in many parts of the world where the quality of food is just as important as its quantity. Myers et al.3 compiled data from free-air carbon dioxide enrichment (FACE) trials in which different crops and varieties were grown under ambient and elevated atmospheric CO2 concentrations. Focusing on the edible part of the plants, they found that zinc and iron contents decrease significantly under CO2 fertilization in all C3-crops studied, whereas C4-crops, like maize and sorghum, are less responsive. Protein content was also found to decrease in all C3-crops that cannot fix additional nitrogen from the atmosphere. Concentrations of other micro-nutrients are affected as well, but the picture is more diverse and hints at complex interactions yet to be understood. Owing to the complexity of plant growth mechanisms and their dependence on environmental conditions and farm management practices, the extent to which CO2 fertilization can help farmers to increase food production remains highly uncertain4. The altered chemical composition of food crops under elevated CO2 can also affect food quantities, through hormone-controlled growth effects5. In addition, the increased feeding rates of herbivorous insects6 may lead to greater crop damage. However, threats to the nutritional value of crops are perhaps the most worrisome and yet are typically neglected in assessments of future food security. This leads to the possibility that assessments that focus on food quantity could be comparing apples to oranges. In other words, even if CO2 fertilization has the potential to compensate much of the negative climate change effects on agricultural yield, nutritional value may nevertheless be compromised. To illustrate this, we draw on global gridded crop model simulations from the large and open-access database of the InterSectoral Impact Model Intercomparison Project (www.ISI-MIP.org; ref. 7). Using the simplest possible assumption of a linear decline in iron and zinc for the C3-crops wheat, rice and soy, and also in the protein content of wheat and rice, increased atmospheric CO2 leads to a substantially lower supply of all three nutrients compared with a world implementing strong climate change mitigation, even though food Current Climate change CO2 fertilization and climate change