World Cultural Council : Response delivered by Prof. Ralph J. Cicerone

The changing chemical composition of the atmosphere is a subject that attracts scientists from around the world and which reveals much about our environment. Atmospheric chemistry involves purely scientific puzzles and it also raises deep questions about the sustainability of various human activities.

Only forty years ago, our knowledge of the chemical composition of the air was limited to easily measurable gases, nitrogen, oxygen, carbon dioxide, water vapor and a few inert gases. Ozone's presence was known, but not the forces that control it. Then new instruments made it possible to measure and observe otherwise invisible quantities and how they were changing. Carbon dioxide amounts were seen to increase, chlorofluorocarbons (CFC's) were observed and measured, methane and nitrous oxide increases were observed worldwide.


The concept that human activity can alter the global environment was demonstrated by discovering the great power of chemical catalysts to bring about chemical transformations in the atmosphere, notably the destruction of stratospheric ozone by chlorine from organic chemicals like CFC's. Similarly, atmospheric chemicals (greenhouse gases) can exert influence over Earth's climate that is much stronger than one might guess from their atmospheric amounts (parts per million and less).

Today, the global atmosphere is changing under human influence. A large ozone hole develops each year over Antarctica due to ozone destruction by chlorine and bromine atoms (from synthetic chemicals) that are released by hetereogeneous reactions on the surfaces of icy particles. Conditions for ozone destruction are favorable at first sunlight in August and the hole persists until early summer sunlight creates new ozone and stratospheric winds allow mixing of air to and from lower latitudes.

International agreements that limit emissions of synthetic halogenated chemicals are preventing the ozone hole from continuing to grow and as atmospheric amounts of these chemicals decrease, the hole will cease to form, perhaps in some tens of years. Biological sensitivities to ultraviolet light which passes through air with subnormal ozone amounts, are being studied as are natural, biological sources of some brominated and chlorinated chemicals. Knowledge of the human-perturbed ozone layer preceded our understanding of the natural system that controls it.

Scientists from many countries have worked separately and together to understand the importance of the Earth's ozone layer, to map out the processes that control it and to assess the extent of human impact. Because the ozone layer is global, the involvement of scientists from around the world is only appropriate.

Now we are also facing difficult scientific questions about climatic change -- for example, how are human activities changing global climate, how climate will change in future decades in response to more human activity such as increased fossil-fuel burning -- while we are also sorting through various possible modes of natural climate change such as the El Nino regional patterns. Once again, atmospheric chemical composition is central to these questions, and atmospheric amounts of carbon dioxide, methane, nitrous oxide, chlorofluorocarbons and ozone must be well measured and understood, while they also trace human activities. Particulate material and cloud-forming processes are also essential to understand.

Scientists from many disciplines and nations are needed in this effort because climate is inherently complex and effective collaboration between scientific disciplines and nations is required. Society needs our results and our best judgements.

Print version Page updated on 2009-01-27