Understanding how carbon dioxide impacts food quality is vital to tackle malnutrition effectively, says agricultural researcher Lewis Ziska.
Researchers are focusing much attention on how to adapt agriculture to ensure steady food supplies in the face of climate change. But it is equally important to preserve the quality of these supplies as well as the quantity.
Researchers, policymakers and the public are increasingly aware that climate uncertainty — characterised by shifting rainfall patterns, increased desertification and warming temperatures — threatens to decrease people's ability to grow food sustainably in many parts of the developing world.
But few know that rising levels of carbon dioxide (CO2) — the principal greenhouse gas — is also expected to affect the nutritional value of many basic food crops.
At first glance this may seem counter-intuitive, since CO2 stimulates plant growth in several basic crop species such as wheat and rice, cereals that supply the bulk of calories for most of the world's poor.
But the nutritional value of these potentially bumper yields is unlikely to improve because extra CO2 is often converted into carbohydrates such as starch, meaning that the relative levels of other components may fall. For example, the 20 per cent or so rise in atmospheric CO2 since 1960 may have already caused a significant decline (5–10 per cent) in protein concentration in wheat flour.
And a recent study, by researchers in Southwestern University, Texas, of major food crops including barley, wheat, soya bean and potato, reveals a significant decline (10–15 per cent) in protein content if atmospheric CO2 reaches 540–960 parts per million — a range anticipated by the middle to end of this century.
In addition to 'diluting' protein levels, rising CO2 levels may reduce water flow through a crop plant, affecting the uptake of micronutrients from the soil, lowering concentrations of key nutrients such as sulphur, magnesium, iron, zinc and manganese.
The UN Food and Agriculture Organization estimates that more than 1 billion people worldwide are malnourished. Malnutrition generally results from a lack of either protein (which is needed for muscle development and maintenance) or micronutrients such as iodine, vitamin A or iron, which boost immunity and healthy development.
Malnutrition contributes to at least half of the 10.8 million child deaths each year, exacerbates the effect of childhood diseases such as measles, malaria, pneumonia and diarrhoea, and can have long-term effects on cognitive development and economic productivity.
For many populations in the developing world, meat is scarce, and plants provide the primary source of both protein and micronutrients.
If, as we expect, rising CO2 levels decrease plant levels of these critical components, impoverished areas of the world already threatened by shortages in food supply may face an additional burden of 'hidden hunger'.
A choice of strategies
Existing strategies could ease this decline in food quality. Perhaps the most obvious is to increase nitrogen fertilisation to compensate for lower protein levels. But nitrogen fertiliser may be too expensive, or not available, particularly in developing countries.
Consuming more food could also provide the necessary protein or nutrients. But this option is impractical, particularly if food distribution, supply and cost are already threatened by climate change.
A simpler solution may be to distribute more food supplements, building on practices already common among many nongovernmental organisations.
The most promising long-term option may be biofortification — developing nutritionally improved crops with increased protein or micronutrient levels. This can be done through traditional plant breeding — for example selecting crops with higher protein concentrations to compensate for rising CO2 levels. Alternatively, genetic engineering can insert specific genes that enhance nutrient or vitamin concentration, for example vitamin A-enriched 'golden rice'.
We also need to determine urgently the exact role that CO2 plays in the production of secondary compounds of nutritional value in plants. While its negative impact on protein and micronutrient levels are well-documented, there is still a dearth of information on potential benefits. For example, some studies suggest that more CO2 increases levels of anti-oxidants in strawberries.
But any effort to address malnutrition in the developing world will take both time and recognition. Despite the potential advantages of golden rice, for example, use of the crop remains trapped in regulatory red tape more than a decade after its creation.
Acknowledgement of the 'hidden hunger' aspect of nutritional quality in the context of the rising CO2 level and climate change remains unappreciated by scientists, private groups and policymakers.
Understanding how CO2 impacts food quality remains a critical, if over-looked, aspect of the global debate on climate change and food security in the 21st century.
Lewis Ziska is a plant physiologist at the Agricultural Research Service, United States Department of Agriculture.