There is now strong scientific evidence that human activities have led to the accumulation of a variety of trace gases in the Earth’s atmosphere, particularly in the past few decades. These global scale changes in the chemical composition of the atmosphere are expected to influence the climate, and have already led to significant depletion of the ozone layer of the stratosphere, which shields the Earth’s surface from harmful ultraviolet radiation from the sun. The ozone layer is particularly vulnerable to emissions of chlorofluorocarbons (CFCs), which are industrial chemicals used mainly as refrigerants, solvents, propellants for spray cans, etc. After their release to the environment, these very stable compounds slowly diffuse into the stratosphere, where they decompose, releasing chlorine atoms and leading to ozone destruction. In the past decade a very significant depletion of the ozone layer has been observed over Antarctica and in recent years also at northern latitudes. Laboratory and field experiments have very clearly pointed to the CFCs as the dominant cause for this depletion. Climate change may result as a consequence of the enhancement of the greenhouse effect. The energy which the Earth receives from the sun is balanced by an equivalent amount of energy emitted by the planet in the form of infrared radiation. There are some gases in the atmosphere which trap a portion of this outgoing radiation; the most important one is water vapor, followed by carbon dioxide, methane and nitrous oxide and the CFCs. The net result is that the average temperature of the Earth’s surface is higher than it would otherwise be; this is the process known as the “greenhouse effect”. The science of climate change is at present rather uncertain, but there is little doubt that the levels of some greenhouse gases -- notably carbon dioxide and methane – have increased significantly in recent decades as a consequence of human activities, and that these gases will affect the Earth’s climate; the question is when, and to what extent. It is also clear that the climate is changing: the average surface temperature has increased a fraction of a degree in this century; mountain glaciers are receding, and sea level is rising. The question is whether or not human activities are largely responsible for these changes; the consensus among many climate experts is that “the balance of evidence suggests a discernible human influence on global climate.”