Last month, our Florida report demonstrated how rising temperatures on the Earth’s surface could be negatively affecting the quality of certain crops. Now, a Southwestern University study confirms this notion. According to the study, rising CO2 levels in the atmosphere could decrease the nutritional value of many major food crops in the years to come.
Professor Max Taub says that the idea for the study grew out of a question posed in his Global Change biology class at Southwestern. Two of his students helped him analyze more than 200 experiments before making any conclusions. They were inspired by the fact that CO2 levels are up from 280 parts per million before the industrial revolution (late 18th century) to approximately 380 parts per million today. Since this concentration is expected to hit 540 to 958 parts per million by 2100, Taub’s students wondered what effect this would have on our food supply.
“Various studies had reported that CO2 has a large effect on crop protein concentration, or that it had little or no effect. The value of a meta-analysis such as ours is that rather than focusing on the results of one or a few experiments, ours comprehensively addresses the totality of the research literature. In this case, the literature as a whole clearly shows decreases in protein concentrations for several important crops,” says Taub.
The Southwestern study found that crops grown in atmospheres containing elevated levels of carbon dioxide had significantly lower protein concentrations. Potatoes showed a 14% decrease in protein, barley showed a 15.3% decrease, rice was down 9.9%, wheat down 9.8%, and soybeans showed reductions of 1.4%.
One reason for this decrease, says Taub, is that plant water use is decreased under CO2. Plants tend to keep their stomata (pores in the leaf or stem) more closed, which can affect uptake of minerals from soil. Another reason is that plants show higher levels of carbon in their tissues when exposed to higher levels of carbon dioxide in the atmosphere. As carbon levels increase in a plant, it simultaneously reduces the concentration of other elements, like nitrogen. As it turns out, nitrogen is a key component of proteins.
“Increased CO2 typically increases photosynthetic rates, since photosynthesis consists of an assimilation of atmospheric CO2 by the plant. This makes the elements assimilated through photosynthesis, like carbon, more available to plants, but does not in itself increase the availability of the elements that plants obtain from soils, like nitrogen,” says Taub.
Crops grown at higher temperatures have a shortened life cycle, and that affects quality. Changes in taste can be frustrating to retailers and consumers, but changes in nutritional content can be devastating – especially to poorer communities.
“For consumers, I think the effects will be seen most on populations that derive a high percentage of dietary protein from grains such as wheat and rice. Globally, this will affect certain poorer countries, such as Bangladesh and other countries in Asia. In the U.S. it might impact those on vegetarian diets. For the food-processing industry it could impact situations where protein concentrations are critical, such as bread wheat. It might also affect feed quality for livestock,” says Taub.
Most crop species utilize one of two different biochemical/physiological processes of photosynthesis, called C3 and C4, respectively. The physiology of C4 plants makes photosynthesis relatively unresponsive to atmospheric CO2 concentrations in these species. C3 species, on the contrast, show a pronounced photosynthetic response to increasing atmospheric CO2. Prominent C3 crops include wheat, rice, barley, oats, and rye.
Nearly 40% of human dietary protein worldwide, says Taub, comes from C3 grains and root crops, which makes this a serious matter. Taub says that genetic modification or conventional breeding to increase protein concentrations is one approach that ought to have success. Additional fertilization with nitrogen might partially help as well, although the studies that Taub looked at indicated that it would not entirely alleviate the effect.
If no changes are made, Taub expects a gradual decreasing in protein over a fairly long time span. Sadly, he says, this isn’t a surprising hypothesis. A 2004 analysis in the Journal of the American College of Nutrition found that USDA data suggested a widespread decrease in protein concentrations across 43 crops from 1950 to 1999. Taub’s research compared today’s concentrations of CO2 to those likely to be seen in the year 2100.