Day-to-day changes in weather are associated with a remarkably rich variety of complex phenomena that include individual thunderclouds, tornadoes, warm and cold fronts, hurricanes, cyclones and anticyclone, and meanders of the Jet Streams. At longer time scales we sometimes experience exceptionally severe winters or hot summers, persistent floods or prolonged droughts. These are not strictly atmospheric phenomena because fluctuations in oceanic conditions are also involved. An outstanding example is the major warming of the water of the tropical Pacific referred to as El Niño. It occurs every few years, lasts for a year approximately, involves drastic changes in the circulation of the Pacific Ocean, and contributes to unusual weather throughout the world.

     On time scales of decades and much longer, trends of global climate changes are evident in the records. These trends may reflect natural variability of the ocean-atmosphere system. However, there is increasing concern that human activities may be contributing significantly to a warming trend. For example, burning of fossil fuels has led to a large and continuing increase in atmospheric carbon dioxide. What factors determine how much of the carbon dioxide we emit into the atmosphere remains, and how much the oceans and the plants absorb? What global climate changes are likely in response to the rise in atmospheric carbon dioxide, a powerful greenhouse gas?

     At still longer time scales, geological evidence reveals that the Earth has experienced climates very different from that of today during its long history. Temperatures were far lower than today during the recurrent Ice Ages of the past three million years. Before that temperatures were higher than they have been in the recent past. These phenomena are not merely of academic interest but are invaluable tests for climate models, those used to predict future global warming for example. Confidence in the forecasts of those models will be bolstered considerably if they can explain and simulate past climates.

     Meteorologists and oceanographers in Princeton's Atmospheric and Oceanic Sciences Program (AOS) are attempting to understand the nature and causes of an impressive range of observed and human-induced phenomena. To do so they use the tools of mathematics, physics and chemistry, and collaborate closely with colleagues, in various departments including the department of geosciences, who are engaged in laboratory experiments and field programs. Because the environmental problems being studied are of obvious human concern, and because the solutions influence policies to deal with those concerns, there is also close collaboration with colleagues in the Woodrow Wilson School of Public and International Affairs.