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MYTH #74: Carbon dioxide is plant food

DENIERS SAY:

CO2 helps agriculture flourish. It is the gas of life.

SCIENCE SAYS:

Plants need water, but is a flood a good way to water your crops? Too much carbon dioxide is bad news.

To grow, plants need three main things: sunlight, water and carbon dioxide. But that doesn’t mean we can jump to the conclusion that more carbon dioxide will be good for plants. More isn’t always better. A flood is not a good way to supply water to a forest. While experiments have shown that some plants respond well to higher carbon dioxide levels, others have shown that abnormally high levels of carbon dioxide can cause damage. Keep in mind that as carbon dioxide increases, temperatures also increase, rainfall patterns change, and some kinds of extreme weather events become more common and severe. This certainly spells bad news for plants. Don’t believe me? Ask the peanut farmers in drought-ridden Texas. Find out about the wine grapes in Australia and California, or the African cacao trees that are used to make chocolate. Climate change has not been kind to these plants so far.

Additional info from Climate Central


Plants need CO2 to live. So is more of it a good thing?

We do not yet know enough to make adequate projections of the global trends for plant life in a world with higher levels of carbon dioxide, or CO2. It is clear, however, that there can be both positive and negative responses.

One of the first things taught in biology class is that animals breathe in oxygen and exhale CO2, while plants take in CO2 during the day and release oxygen. In a process called “photosynthesis,” plants use the energy in sunlight to convert CO2 and water to sugar and oxygen. The plants use the sugar for food — food that we use, too, when we eat plants or animals that have eaten plants — and they release the oxygen into the atmosphere. If it were not for plants, we would have no oxygen in our air!

So, if we’re putting more CO2 into the atmosphere by burning fossil fuels, you might expect plants to grow better. But the story is not quite that simple. When biologists have grown crops like wheat, soybeans, and rice inside greenhouses with extra CO2 present, the plants have indeed grown more rapidly and more abundantly. For the past several years, scientists all over the world have also been doing a series of experiments called “Free-Air Concentration Enrichment,” or FACE. Instead of using greenhouses, they grow crops in open fields to give them the most natural environment possible and pump in extra CO2 from a network of pipes.

The results of these experiments have shown that the crops do not thrive as well in this environment. Plants do need CO2, but they also need water, nitrogen and other nutrients. Increase one of these without increasing the others and there’s a limit to how much the plants will benefit. Some do not grow much more at all. Others, like wheat, grow bigger but end up with less nitrogen. As a result, insects end up eating more to get the nitrogen they need. The nutritional value of food plants would be similarly reduced for other animals — including humans. Also, we could end up with vegetables that have too much carbon — perhaps producing spinach that would be very tough to chew!

Adapted from © Climate Central
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