The Changing Role of Non-CO2 Greenhouse Gases

During the last century, the concentrations of several greenhouse gases have increased considerably - most notably carbon dioxide, methane and nitrous oxide. In addition, new, entirely man-made gases have been put into the atmosphere that also cause the greenhouse effect; these include the chlorofluorocarbons. Calculations have shown that during the last century the non-CO₂ greenhouse gases could together be almost as effective as the increase of carbon dioxide in causing global warming. These and similar gases were therefore included in the Kyoto Protocol to develop a comprehensive plan for controlling potentially unfavorable climatic change. Studies show however that the other gases, with few exceptions, are likely to play a smaller than expected role in future global warming. The most significant non-CO₂ manmade greenhouse gases are methane and nitrous oxide. Methane rose from 700 ppbv some 200 years ago to about 1750 ppbv in recent times, while nitrous oxide rose from about 285 ppbv to 318 ppbv over the same time. These trends made methane the most important gas for global warming after carbon dioxide. But now, the rate of increase of methane has slowed down considerably. Budget analyses suggest that we may not see major changes of concentrations in the future comparable to the trends of the last century. Thus the role of methane in future global warming may be less than expected earlier. Nitrous oxide on the other hand, has increased slowly but steadily during recent decades. It is likely to become more important in the future compared with methane. While other greenhouse gases such as the perfluorocarbons, sulfur hexafluoride and hydrochlorofluorocarbons are included in the Kyoto Protocol, these are present in such minute concentrations that it is unlikely that they will have an important role in future global warming.