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����[15 ] Carbon dioxide ��C02�� iS the most abundant of the greenhouse gases. It occurs as a natural consequence of respiration. However�� much larger quantities are put into the atmosphere as a waste product of energy production. Coal�� oil�� natural gas�� and biomass are all burned to provide heat and electricity for industrial processes�� home heating�� and cooking. These sources are increasing the amount of carbon dioxide in the atmosphere.����Measurement of carbon dioxide levels at the Mauna Loa Observatory in Hawaii show that the carbon dioxide Ievel has increased from about 315 ppm �� parts per miliion�� in 1958 to about 360 ppm in 1995.����[16] A major step toward slowing giobal warrrung would be to increase the efficiency of energy utilization. This would also be of value in conserving the shrinking supplies of energy resources. It makes sense to increase energy efficiency�� thus reduang carbon dioxide production�� even if global warming is not a concern. One way to stimulate a move toward greater efficiency would be the imposihon of a carbon tax. A carbon tax would increase the cost of fuels by taxing the amount of carbon put into the atmosphere by their use. This would increase the demand for fuel effiaency because the cost of fuel would rise. It would also stimulate the development of alternative fuels with a lower carbon content and generate funds for research in many aspects of fuel efficiency and altemative fuel technologies.����[17] Another approach to the problem is to increase the amount of carbon dioxide removed from the atmosphere. If enough biomass is present�� the excess carbon dioxide can be utilized by vegetation during photosynthesis�� thereby reducing the impact of carbon dioxide released by fossil-fuel burning. Australia�� the United States�� and several other countries have announced plans to plant billions of trees to help remove carbon dioxide from the atmosphere. Many critics argue that this approach will only provide a short-term benefit since�� eventually�� the trees will mature and die�� and their decay will release carbon dioxide into the atmosphere at some later time.����[ 18 ] An associated concern is the destruction of vast areas of rainforest in tropical regions of the world. These ecosystems are extremely efficient at removing carbon dioxide and storing the carbon atoms in the structure of the plant. The burning of tropical rainforests to provide farm or grazing land not only adds carbon dioxide to the atmosphere but also reduces the ability to remove carbon dioxide from the atmosphere�� since the grasslands or farms created do not remove carbon dioxide as efficiently as do the original rainforests.����Furthermore�� the grazing lands and farms in such regions of the world are often abandoned after a few years and do not return to their original forest condjtion. ����[19] Chlorofluorocarbons are entirely the result of human activity. They were widely used as refrigerant gases in refrigerators and air conditioners�� as cleaning solvents�� as propellants in aerosol containers�� and as expanders in foam products. Although they are present in the atmosphere in minute quantities�� they are extremely efficient as greenhouse gases �� about 15��000 times more efficient at retarding heat loss than is carbon dioxide��. Since all chlorofluorocarbons are made by people for specific purposes�� their levels can be easily controlled.����[ 20 ] The use of chlorofluorocarbons as propellants in aerosol spray cans or as expanders in foam products is not necessary. Other more benign materials�� such as hydrocarbons�� could be used. Since the 1970s�� when chlorofluorocarbons were linked to the depletion of the ozone layer in the upper atmosphere�� their use in aerosol cans has been banned in the United States�� Canada�� Norway�� and Sweden�� and the European Economic Community agreed to reduce use of chlorofluorocarbons in aerosol cans. However�� worldwide�� chlorofluorocarbons are still widely used as aerosol propellants. In foam and solvents�� care can be taken to recover the chlorofluorocarbons for reuse rather than allow them to escape into the atmosphere. Alternative refrigerants are available�� and care could be taken to recycle refrigerant gases rather than just venting them into the air.����In January 1991 �� DuPont announced the development of new refrigerants that would not harm the ozone layer.����These will be installed in refrigerators and air conditioners in the future. We will need to exploit all these options if we want to reduce chlorofluorocarbon production. Until substitutes become available�� the mandatory recycling of chlorofluorocarbons will reduce the rate at which new gases are added to the atmosphere.��������