Human Activities Are Responsible for an Increase in the Amount

Human activities are responsible for an increase in the amount of atmospheric carbon dioxide. Many feel that the buildup of this gas is at least partly responsible for the greenhouse effect and global warming. Increased carbon dioxide may actually be a benefit to some plants since they need carbon dioxide for carbon-fixation. How will the greenhouse effect influence the biosphere? Even though experiments have shown increased carbon dioxide to be a boon for test plants, some scientists are still concerned about this phenomenon. Why?