Increased CO2 and Food Quality

This is Part 1 of a three-part series on Food and Farming.

1. Increased CO2 and Food Quality
2. Farm Animals and Methane
3. "Food Mile" Complexities


The author of today's post, Lisa Moore, Ph.D., is a scientist in the Climate and Air program.

When people talk about the harmful effects of fossil fuels, they usually focus on global warming. But as I explained in my post about nitrogen pollution, and Bill discussed in his post about ocean acidification, fossil fuel use has other unintended consequences.

Here is yet another example: increasing levels of carbon dioxide (CO2) can affect the food chain. There’s a lot to be concerned about, but today I’ll focus on livestock.

This issue got some coverage last week, when Grist described some of the effects of CO2 on food quality, based on a news story in Nature (paid subscription required).

As Grist and Nature described, plants grown in high-CO2 conditions contain less nitrogen and other nutrients. From an animal’s perspective, this can be a problem because these plants have less protein and are less nutritious.

For example, researchers at Kansas State University have been analyzing the effects of rising CO2 on ruminants (animals like cattle, sheep and goats). In one study, they exposed sections of a tallgrass prairie to CO2 levels that could easily be reached later this century. Then they measured the nutrient content of the plants and found that the plants were less digestible, with lower nitrogen and protein content. The change in forage quality was bad enough that, according to the scientists’ calculations, beef cattle grazing on prairie in a high-CO2 future would not gain as much weight as today’s cattle, even if they ate as much grass. More than that, the authors pointed out that cattle tend to eat less in warm conditions. So the higher temperatures coming with global warming would mean that cattle gain even less weight.

The Nature article mentioned the possibility that fertilizer might offset some of the effects of CO2 on crop quality. Setting aside for a moment the unintended ecological and health effects of fertilizer use, not to mention the expense and logistics of fertilizing large pastures, would that even work for ranchers? A study in Colorado’s shortgrass steppe addressed this question, and the conclusion was that no, "nitrogen fertilization did not ameliorate the negative effects of elevated CO2".

One final thought: the consequences of decreased plant nutrition won't be confined to domesticated animals. Scientists anticipate that many wild animals, from insects to antelopes, will feel the effects. In case we needed yet another reason to cut carbon pollution from fossil fuels, keeping food quality high is a good one.

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One Comment

  1. Posted August 28, 2007 at 6:32 pm | Permalink

    Some background on CO2 and Food Quality…

    CO2 is a fertilizer for plants because plants use sunlight, water, and CO2 to synthesize the glucose they need to grow (a process called "photosynthesis"). Since increased atmospheric CO2 accelerates plant growth, commercial greenhouses artificially raise CO2 to what it would be under gloomy global warming scenarios. Thus many see accelerated plant growth as a silver lining to the greenhouse effect.

    But as described in the Nature article "The Other Greenhouse Effect", this silver lining may be insidiously tarnished. There isn't a lot of research into the effects of CO2 on plant nutrition, but the studies that exist suggest a variety of negative effects. Researchers have observed significantly lowered protein levels (especially wheat gluten, which reduces baking quality), lowered trace mineral content, lowered Vitamin C in potatoes, and lowered calcium in soya beans (problematic since soya beans are used to make dairy substitutes).

    There is even evidence that plant yields in the real world of global warming will not be higher, since other factors such as higher temperatures and drought will negate the effect of increased CO2.

    Humans aren't the only ones to eat plants, as Lisa points out in her post. Grazing livestock also eat plants, and we eat the livestock. So the full range of what we eat is affected.

    Other parts of the food chain are also impacted. One of the studies cited in the Nature article found that certain types of plant pests thrive on the altered soya bean leaves, laying eggs that destroy the following year's crop.

    Moral: It's not nice to fool Mother Nature!

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