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Hot and Hungry: How Climate Change Affects the Nutrient Content of Our Food

Modern crops have lower nutrient density than they used to — and agricultural practices may not be the only cause.

| September/October 2018

  • Corn is one of the food crops most affected by rising CO2 levels.
    Photo by Adobe Stock/TTstudio
  • "Stomata" are specialized structures that act as a plant's nostrils.
    Photo by Getty Images/barbol88
  • All fruit and nut trees store more carbohydrates than other nutrients in response to high CO2 levels.
    Photo by Getty/N8tureGrl
  • Rice is a staple crop for millions of people around the world.
    Photo by Adobe Stock/Cardaf
  • Legumes, such as beans and peas, are less affected by high CO2 levels.
    Photo by Getty Images/brytta
  • Selective soft focus of Sorghum field in sun light
    Photo by Phathomporn Sihasena
  • Wheat contains as much as an additional teaspoon of sugar per 100 grams in response to a 200 ppm increase in atmospheric CO2.
    Adobe Stock/Sebastian

For more than 25 years, researchers have been aware of the declining nutrient density in our crops. Many believe that the industrialization of agriculture is to blame, due to chemical fertilization techniques and declining soil quality. While these factors certainly contribute to the decline of our foods’ nutrient value, Irakli Loladze, a mathematician with a keen interest in biology, discovered a more ominous source of nutrient depletion: rising carbon dioxide (CO2) levels in the atmosphere.

Photosynthesize Me: C3, C4, and CAM

Some background on how plants use carbon dioxide will be helpful to understand why rising CO2 levels matter so much. All plants “fix” carbon dioxide from the atmosphere for fuel, using one of three photosynthesis methods, dubbed “C3,” “C4,” and “CAM.”

C3 photosynthesis transfers the carbon atom from carbon dioxide to a three-carbon molecule, which the plant then uses to produce sugars and starches. Of the three methods of photosynthesis, C3 is considered the least efficient, because in hot, dry conditions, the enzyme that “grabs” carbon dioxide from the air sometimes binds oxygen molecules instead. The plant then has to use extra energy — and release one of its hard-earned carbon atoms — to correct the mistake. About 85 percent of plant species are C3 plants. Rice and potatoes, as well as barley and wheat (the most studied of the cereal crops), are C3 plants, as are all woody trees, including nut and fruit trees.

Plants that use C4 photosynthesis have adapted to hotter, drier climates and have a more complicated process for carbon fixation that reduces the risk of fixing oxygen molecules. They use an enzyme with no affinity for oxygen to fix atmospheric carbon initially, and move the resulting four-carbon molecule into another cell, where the same enzyme used in C3 plants transforms it into sugars and starches. In hot, dry conditions, C4 photosynthesis is much more efficient than C3, but only 3 percent of plant species use this method of photosynthesis. Corn is the most commonly cultivated C4 crop; amaranth, millet, and sorghum are other commonly cultivated C4 crops.

CAM photosynthesis is mainly used by plants adapted to extremely hot, dry environments, such as pineapples, cacti, and other succulents. They alternate between fixing CO2 at night, and converting it to sugars and starches during the day. Very few CAM plants are used as crops, so we won’t go into further detail about this process.

Designing CO2 Studies

Researchers interested in the impact of rising CO2 levels can pump CO2 into a controlled greenhouse lab and study the results, but lab environments don’t accurately represent the complexity of open spaces. Weather patterns, interspecies competition, and other factors may all be of interest in studying the effects of CO2 levels on plants. Free air carbon dioxide enrichment (FACE) is a method developed to address this issue. Researchers can raise the concentration of CO2 in a specified open area and measure plants’ responses. It’s the current gold standard for measuring CO2’s effects on plants, and FACE studies can also include plants that can’t be grown in small spaces, such as trees. However, FACE experiments are extremely costly and have their own limitations. Most FACE experiments are conducted in temperate climates, and because of the high cost of CO2, don’t keep CO2 levels elevated at night. We don’t yet have much information on how agriculture in tropical and arid climates may be affected by elevated CO2 levels, nor on the possible effects of constantly high CO2 levels.

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