Rising CO2, Crop Quality and Human Nutrition: INTENSIFYING Hidden Hunger & Obesity
Rising CO2 affects human nutrition: systemic depletion of essential minerals in crop and wild plants.
In 2002, while doing my postdoc at Princeton University, I postulated that rising atmospheric CO2 should affect the quality of human nutrition by reducing the concentrations of essential minerals in crops worldwide. I specifically singled out iron (Fe), iodine (I) and zinc (Zn), which had already been deficient in the diets of the half of the world. Deficiencies of these essential elements contribute to 'hidden hunger' - a widespread nutritional disorder stemming from diets that are calorie-rich but nutrient-poor.
My prediction was rooted in the theory of Ecological Stoichiometry. I was fortunate to be exposed to it when its founders -- James Elser and Robert Sterner -- were developing it in 1990s. This theory focuses on a few chemical elements shared by all life. This makes large-scale predictions possible by using simple and rigorous mass balance arguments.
The key physiological mechanism driving the mineral depletion in plants is the increased carbohydrate synthesis caused by elevated CO2. More CO2 means more carbohydrate production for most plants (so-called C3 plants, such as rice and wheat). Carbohydrates are made of carbon (C), oxygen (O), and hydrogen (H). However, it's naive to expect the dilution of nutrients caused by these three elements to be the only change in plants grown in high CO2. In my 2002 paper, a stoichiometric thought experiment is run to deduce that rising CO2 should affect plant minerals unevenly, with some minerals changing more than the others. This is because elevated CO2 alters other key aspects of plant physiology. For example, elevated CO2 makes plant leaves lose less water, the effect referred to as 'reduced transpiration.' This, in turn, leads to the reduced mass flow of mobile minerals, such as calcium, toward the roots, while leaving immobile minerals less affected. Hence, for this and other reasons (e.g. interactions with mycorrhizae) , elevated CO2 should change concentrations of some minerals more than others. The overall tendency, however, should be toward lower mineral concentrations in various plant species and tissues. This was the conclusion of my 2002 "stoichiometric thought experiment." More importantly, the paper linked the predicted decline in the quality of plants to human nutrition.
Back in 2002, the published experimental data on the issue were fragmented and limited. Understanding the importance of the issue, I attempted to compile whatever little data were available at the time. This first meta-analysis on the issue supported my hypothesis: indeed, the levels of nearly all minerals declined in plants exposed to elevated CO2, with some declining more than others. What I found particularly worrisome, was the decline in the concentrations of iron and zinc (I could not find a single data point on iodine, unfortunately.) Because these two minerals, Fe and Zn, are deficient in the diets of hundreds of millions of people, it became clear to me that rising CO2 should affect the quality of human nutrition. This is why I linked the issue of rising CO2 and declining mineral concentrations to human nutrition and titled my 2002 paper as "Rising CO2 and human nutrition."
However, some of the declines were statistically nonsignificant, which was not surprising considering the limited amount of data relative to all the noise inherent to plant systems. Furthermore, a large part of the data came from closed-chamber and greenhouses experiments, while the rest from open-top chambers and Free-Air Carbon dioxide Enrichment (FACE) centers (and I run a FACE 'thought experiment' in my 2002 paper).
Unfortunately, some plant experts perceived me, a mathematician by training, as an outsider, and considered my 2002 argument too simplistic to be true. The only way to change their opinion was to generate new results. This required more data, much more data. A very cost efficient way to generate new data is to analyze archived & fresh plant samples collected at the existing facilities, including FACE centers. Despite the enthusiastic support from many CO2 FACE facility directors, who were keen to collaborate with me, the NSF and the DOE rejected all my requests for funding. Some research groups in the US were particularly dismissive of my 2002 hypothesis such as the CO2 group at the University of Illinois at Urbana-Champaign.
Meanwhile, some researchers used flawed meta-analytical methods to challenge altogether the notion that elevated CO2 could consistently lower the mineral content in crops. That these researchers have consulting ties to fuel industry, including BP, does not add to their credibility. Unfortunately, their irreproducible findings have had an impact. The new IPCC (2014) WR2 AR5 report, affected by their findings, muddles the issue of elevated CO2 and mineral content in plants. While citing the empirical findings of several groups and my 2002 work, the IPCC AR5 comes to a vague and confusing conclusion:
"Since 2002, studies generally find decreases in Zn, S, P, Mg and Fe in wheat and barley grain, increase in Cu, Mo and Pb (from a limited number of studies) and mixed results for Ca and K"
This gives readers an impression that elevated CO2 tends to decrease only a few minerals in wheat and barley, but increases other minerals. The last IPCC 2014 assessment treats the global CO2 effect on crop quality and human nutrition as a minuscule issue. Out of its 2,600 pages, the report barely devotes a page (one page!) to the issue CO2 and mineral decline in crops.
Update: To their credit, the authors of the study that claimed no consistent CO2 effect on plant minerals, retracted the study. Unfortunately, the retraction came after the study impacted the IPCC 5th assessment.
Over the last 12 years, dedicated researcher groups, most notably from Germany, Sweden, Australia, USA, China, Japan, and India, published new data. Soon there will be enough data to provide a definitive answer: does elevated CO2 diminish the mineral content in crops - the foundation of human nutrition?
The lack of research funds and multiple NSF rejections have not stopped me from pursuing the issue. Over the last 12 years, I've been manually compiling all the relevant data and have amassed thousands of observations on the effect of elevated CO2 on the elemental content of plans. Analyzing all these data requires sound statistical methods to assure that the robustness and reproducibility of my results. Doing all this with $0 funding is no walk in the park, but persistence and diligence in this direction will inevitably reveal the truth.
Update: On May 7, 2014 eLife published the results of my meta-analysis covering 7,761 paired observations - the largest compilation to date on the issue. Aside from showing the systemic and pervasive mineral depletion in crop and wild plants, I use the stoichiometric theory to show that the shift in crop quality can affect the prevalence of obesity.
My analysis shows that the effect of elevated CO2 on the quality of plants is apparent in both non-FACE studies (for example, greenhouses) and FACE studies. The latter are expensive to set up and run and create hurdles to new data generation. We need to move away from FACE studies to much cheaper and faster data generation experiments.
I took me three submissions just to eLife alone to publish my paper. In a move that promotes transparencyy, eLife made all the past history of my paper available online. This including referee comments related to the rejected versions of my paper (an astute reader can figure out what research group is responsible for delaying the release of my work):
Links to my 2002 paper and some media coverage of my work:
Mongabay (2016) "Rising CO2 is reducing nutritional value of food, impacting ecosystems"
Politico (2017) "The Great Nutrient Collapse"
