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Carbon Dioxide (CO2) C arbon dioxide is a very familiar molecule that is the product of human respiration and of the combustion of fossil fuels. This gas also puts the “fizz” in soda pop. It turns out that the simple, familiar CO2 molecule has profound implications for our continued existence on this planet. The concentration of carbon dioxide in the earth’s atmosphere has risen steadily since the beginning of the Industrial Revolution and is expected to double from today’s levels in the next 50 years. This change will have important effects on our environment. Consider the case of global warming. Many scientists feel that the increased CO2 levels will trap more of the sun’s energy near the earth, significantly increasing the earth’s average temperature. Some evidence suggests that this is already occurring, although scientists disagree about the contribution of CO2 to the changes. One thing that everyone does agree on is that the increased CO2 levels will make plants grow faster. On the surface the increase in plant growth due to increased CO2 might seem like a good thing. Surprisingly, this situation could spell disaster for plant eaters, from caterpillars to antelopes, and for the animals that eat these herbivores. Faster plant growth often leads to lower nutritional value. As the plants increase their rate of photosynthesis and use the carbon in CO2 to build more fiber and starch, the amount of nitrogen—which indicates the amount of proteins present—declines. Studies show that new leaves on plants grown in a CO2-rich atmosphere are starchy, but protein-poor. This is bad news for caterpillars, which need to bulk up before they pupate. Studies have indicated that caterpillars eat 40% more of the starchy, protein-poor leaves but grow 10% slower and produce smaller than normal adult butterflies. Studies on larger herbivores, such as cows and sheep, have been more difficult to carry out. Nevertheless, indications are that plants grown in a CO2-enriched environment provide less protein and produce slower growth in these species as well. Research is continuing to try to assess the effects of the increasing CO2 levels on the food chain. A Monarch butterfly caterpillar eating a leaf.