By Ellie Fung
When it comes to the relationship between climate change and food, many would cite the heightened risk of crop failure and starvation as a result of intensified natural disaster, extreme weather and pest infestation events. Decreased crop yield is understandably a major concern and cause for carbon reduction efforts, but it often overshadows another equally important aspect of global food security: the nutritional quality of food. Elevated atmospheric carbon dioxide levels (eCO2) shifts crop nutrient composition, potentially causing an increased prevalence of nutrient deficiencies and burden of associated diseases. Given the dire implications for public health, there needs to be a greater focus on the impact of climate change on food quality alongside food quantity.
As of August 2020, global atmospheric CO2 levels are around 413ppm (Global Monitoring Laboratory, 2020), but are expected to reach 550ppm in the next 40-60 years (Myers et al., 2014). Setting aside other climate change impacts, it is generally accepted that eCO2 increases overall crop yield by promoting net photosynthesis in plants (Dong et al., 2018). While quantity improves, the quality of food crops is compromised as tissue concentrations of protein and minerals decline under eCO2. The specific mechanism for the decline is not well understood, but a few likely causes have been put forward. The carbohydrate dilution hypothesis suggests that increased carbon fixation results in greater carbohydrate production and plant biomass, consequently diminishing the relative concentrations of other nutrients (Myers et al., 2014). Another proposes that eCO2 reduces stomatal conductance and transpiration, subsequently decreasing nutrient mass flow and root uptake. A third hypothesis postulates that the increased metabolic activity associated with greater plant growth leads to a shift in nutrient allocation away from storage towards functional proteins and cofactors involved in metabolism. It is likely that all these mechanisms occur together in plants, individually influencing nutrient content to a certain degree (McGrath & Lobell, 2013).
The public health consequences of reduced plant nutritional quality are severe, as many populations mostly depend on plants for daily total caloric intake. 76% of the world’s population currently obtain most daily dietary protein from plants and 71% rely on wheat and rice as their primary protein source (Harvard T.H. Chan School of Public Health, 2017). Worryingly, a recent meta-analysis of published literature on the protein content of staple C3 crops revealed that the overall protein concentration of rice, wheat, barley and potato decreases under eCO2 by 7.6%, 7.8%, 14.1% and 6.4% respectively, with individual studies consistently resulting in protein losses across C3 crops. 1.4 billion people worldwide are already at risk of protein deficiency (Nate, 2017), and eCO2 would put 148.4 million more at risk by 2050 given current demographic and carbon emission trends, mostly in poorer and rapidly growing regions of Sub-Saharan Africa, South America and South Asia. A third of the newly at-risk would be from India, where CO2-sensitive rice and wheat forms a major part of daily protein intake in a largely undiversified diet (Medek, Joel & Myers, 2017). Similarly, soybean grown in eCO2 demonstrated lower protein and free amino acid content. As soybean is also a major food source for livestock, the reduced nutrient value of soybean may also influence the quality of animal-based food (Li et al., 2018).
Globally, 2 billion are currently iron or zinc deficient (Tulchinsky, 2010), many of whom rely upon C3 cereals and legumes to supply these essential dietary minerals. eCO2 is expected to exacerbate this problem and extend it to hundreds of millions more people, disproportionately in poorer rural communities: C3 staples grown in 550ppm CO2 show a 3-11% lower zinc and iron content compared to those grown at ambient CO2. Other than a significant iron decrease in maize, protein and mineral content in C4 crops are less affected by eCO2 (Myers et al., 2014). Other essential minerals, such as calcium, potassium, magnesium and selenium, are also reduced in many fruits and vegetables under eCO2 conditions, the extent of which greatly varies between different classes and species (Dong et al., 2018; Jones et al., 2017).
It is worth noting that eCO2 does not affect all types of nutrients equally. Vegetables, especially leafy varieties, grown under eCO2 demonstrated a 50.9% greater overall antioxidant capacity and higher content of total phenols, flavonoids and ascorbic acid compared to those grown under ambient CO2, which may improve the nutritional quality of vegetables. It is suggested that antioxidant accumulation is enhanced from a higher amount of soluble sugar precursors in tissues as a result of elevated carbon fixation (Dong et al., 2018). Nevertheless, an increase in antioxidants in vegetables does not make up for the lost protein and minerals that are also indispensable to human health, especially when the most vulnerable depend upon these very plants for much of their essential dietary protein and mineral intake.
Furthermore, other environmental factors form multitudinous complex interactions with eCO2 to determine the specific response in plants, often in unpredictable ways. For instance, a low soil nutrient availability limits photosynthetic rate regardless of carbon levels: tomatoes grown under eCO2 but in low-nitrogen soil demonstrated lower soluble sugar and lycopene concentrations than those grown under eCO2 and high-nitrogen soil. In lettuce and potatoes, there were varying effects on phenolic acid, flavonoids, ascorbic acid and pigments depending on growth conditions (Dong et al., 2018). These growth factors, including soil salinity, tropospheric ozone concentrations and temperatures, may also be a consequence of eCO2, presenting greater challenges in determining the specific changes in crop nutrient composition from climate change and may hinder efforts in mitigation and adaptation strategies.
Mitigating the decrease of essential nutrients in plant tissues may involve genetic engineering, selective breeding, fertiliser application or a change in cultivation technique (Dong et al., 2018), but the optimal strategy would depend on the specific plant species and growth conditions. Thus, additional research into species-specific plant response mechanisms to changing atmospheric carbon and environmental conditions, as well as a better characterisation of the effect of eCO2 on overall food quality is necessary. This would enable the development of better strategies to safeguard not only crop productivity and yield, but also food quality and ecosystem services, to maintain good human and environmental health (Heckathorn et al., 2020). Nutritional analysis of susceptible populations may also aid in targeting adaptation efforts such as selective breeding, biofortification and nutrient supplementation (Myers et al., 2014). More importantly, these unpredictable, disproportionate and often adverse effects of eCO2 on the nutritional quality of food further adds to the urgency of the need for global decarbonisation, lest jeopardising the ability of current and future populations to fulfill one of life’s most fundamental needs.
References:
Dong, J., Gruda, N., Lam, S. K., Li, X. & Duan, Z. (2018) Effects of Elevated CO2 on Nutritional Quality of Vegetables: A Review. Frontiers in Plant Science. 9 924. Available from: https://www.frontiersin.org/article/10.3389/fpls.2018.00924.
Global Monitoring Laboratory. (2020) Trends in Atmospheric Carbon Dioxide . Available from: https://www.esrl.noaa.gov/gmd/ccgg/trends/mlo.html [Accessed 4 October 2020].
Harvard T.H. Chan School of Public Health. (2017) Millions may face protein deficiency as a result of human-caused carbon dioxide emissions [Press release]. Available from: https://www.hsph.harvard.edu/news/press-releases/climate-change-carbon-emissions-protein-deficiency/ [Accessed October 3, 2020].
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Imperial Bioscience Review 