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!