Concern for the health of the stratospheric ozone layer led to an international agreement in 1987, the landmark Montreal Protocol, that restricted CFC production. This international agreement and its amendments have led to a curtailment of CFC production around the world. Ground-based measurements have shown that CFC concentrations have stopped growing, and have slightly decreased in the troposphere. Stratospheric CFC concentrations lag tropospheric CFC concentrations by a few years, and it is expected that tropospheric concentrations should begin to decrease within the next few years. As these chlorine and bromine concentrations start to decrease, Antarctic and Arctic ozone amounts should begin to recover (see Chapter 12 on modeling and the future of the ozone layer).

While the CFC impact on the ozone layer is being cured, other effects such as greenhouse-warming and aircraft pollution need to be studied further. Recent research has suggested that greenhouse-warming effects may lead to significant cooling of the polar regions. This is because greenhouse gases, principally carbon dioxide, actually cause cooling in the stratosphere even as they warm the troposphere. If so, this cooling may exacerbate polar ozone losses in spite of decreasing chlorine and bromine levels. Why this is so is explored in Chapter 11.

We still need to be vigilant about substances and processes that may impact the ozone layer. The United States is currently investigating the construction of a new supersonic transport (see Chapter 10). This High Speed Civil Transport (HSCT) would travel at twice the speed of sound in the stratosphere. Such an aircraft would significantly reduce the travel time between North America and Asia, Europe, and Australia. However, the exhaust from these aircraft will alter the stratospheric ozone layer. As part of the US technology investigations, we are also researching the stratospheric impact. At this early development stage, careful consideration of stratospheric chemistry can feed into the design of fuel and engine components to minimize the impact on the stratosphere. Scientist and engineer partnering should lead to an aircraft that is a technological wonder and environmentally safe.

The story of ozone and CFCs has evolved greatly over the last 30 years as our understanding of the stratosphere has expanded. Our ability to monitor the stratosphere, investigate its phenomena, and assess its future has dramatically improved because of the investments by government, industry, and the academic community Forecasting the future is always a tricky process (ask any weather forecaster), but we are now able to largely determine the stratospheric effects of new chemicals and technologies, and thereby heal and preserve the ozone layer for future generations and for the ecological health of the planet.