11.7. The atmospheric carbon dioxide concentration has been increasing at a greater rate during recent years than during the middle of this century, raising concerns that the balance between generation and removal rates may now be upset so that more than half of the carbon dioxide produced may be retained. In addition, some other gases, such as Freon, which result from human activity, tend to “block the atmospheric window,” similar to carbon dioxide.

11.8. Energy from the sun is primarily in the shortwave and visible portions of the spectrum, only a small fraction of which is absorbed or scattered back to space by the atmosphere. However, heat reradiated from the earth’s surface is mostly in the infrared portion of the spectrum, in which carbon dioxide, methane, nitrous oxide, ozone, Freon, and water have some absorption bands. Thus, the various complicated rate processes that establish a thermal equilibrium may tend to shift to a position favoring a warmer earth surface temperature. On the other hand, an increase in particulate matter in the atmosphere favors a cooling trend as a result of greater reflection of the incident radiation back into space. Green plants absorb carbon dioxide by photosynthesis but discharge it by respiration. Although modeling of these rate processes on a global scale is very difficult, it does appear that warm climatic periods favor carbon dioxide emission, thus introducing a positive feedback effect.

11.9. Since warming predictions regarding the rate of possible long­term global warming depend on the sophisticated modeling of many world­wide climatic and other processes, the results remain somewhat uncertain. However, a conservative approach would be to accept the possibility and to limit carbon dioxide emissions as much as possible and discourage further reductions of green plant growth areas.