Finally, the most long-lasting and potentially least reversible global problem is the greenhouse effect. As noted above, this effect is induced by carbon dioxide, chlorofluorocarbons, methane, and more than a dozen other gases in concentration in the atmosphere. The role played by carbon dioxide is the most significant. The amount of CO{sub 2} in the atmosphere has risen steadily since the mid-1800s largely as a result of the combustion of coal, oil, and natural gas on an ever-widening scale. In 1850 the global CO{sub 2} level of the atmosphere was roughly 280 parts per million, whereas by the late 1980s it had increased to approximately 350 parts per million.

Should present trends in the emission of greenhouse gases, particularly of CO{sub 2} , continue beyond another 100 years, climatic changes larger than any ever experienced during recent geologic periods can be expected. This could substantially alter natural and agricultural ecosystems, human and animal health, and the distribution of climatic resources. In addition, any significant greenhouse warming could cause a rapid melting of some polar ice, resulting in a rise in sea level and the consequent flooding of coastal areas.

In spite of these long-term possibilities, the greenhouse problem has received the least policy-oriented attention of any of the three major issues at hand. There are various reasons for this: (1) The problem is fraught with technical uncertainties. (2) It has perceived "winners" and "losers"--economic and otherwise. (3) No one nation acting alone can do much to counteract the CO{sub 2} buildup in the atmosphere. (4) Dealing with the problem substantively could be expensive and even alter life-styles. (5) There is no way of proving the validity of the greenhouse theory to everyone's satisfaction except by "performing the experiment" on the real climatic system, which would necessarily involve all living things on Earth. (6) The principal greenhouse gas, CO{sub 2} , is an inherent by-product of the utilization of a commodity that is most fundamental to the economic viability of the world--fossil-fuel energy. (This fact more than any other explains why the greenhouse problem is so difficult to solve.)

It seems appropriate to break down the issue of greenhouse warming into a series of stages and then consider how policy questions might be addressed against the background of these more technical stages. The present discussion will deal with the problem specifically as it relates to increasing atmospheric CO{sub 2} for the sake of simplicity, though other related questions certainly can be dealt with in the same manner.