Earth's climate exhibits variability on a range of time scales from seasonal to 10's of thousands of years. On time scales of a few years, climate can be described as the long-term background or "average weather" for a given region. This is easily seen by the predictable change in seasonal temperatures. On longer time scales (decades and longer) however, regional and global climate actually exhibits considerable variability. The objective of this workshop is to provide teachers with an introduction to the factors that drive climate change on a variety of time scales.

Current scientific understanding of the climate system has identified a number of processes that contribute to this variability. These include: changes in atmospheric greenhouse gas concentration, volcanic activity, planetary albedo, internal oscillations of the ocean-atmosphere- cyrosphere and shifts in the Earth-Sun orbital geometry. Differentiating the precise climate role of each of these forcing functions on various time scale is complicated by the interaction between various components of the climate system.

Despite this complexity, considerable progress has been made in the past decades toward unraveling the mechanisms of climate change. For example, it is now generally well accepted that changes in the Earth's eccentricity (the shape and of the Earth's orbit around the sun and thus the mean Earth-Sun distance), precession (the timing of the seasons in relations to the Earth's orbital position) and obliquity (tilt of the Earth's equatorial axis relative to the Earth-Sun plane) play a critical role in the shifts from glacial to interglacial climates on geologic time scales (20,000 to 100,000 year time scale).

Similar generalities can be made regarding the role of greenhouse gases on climate variability. The presence of trace amounts of carbon dioxide, methane, water vapor, and other greenhouse gases in Earth's atmosphere absorb incoming short-wave solar radiation and prevent it's escape back to space as long-wave radiation. Throughout geologic time, this thermodynamic process has helped to keep Earth habitable by warming its atmosphere and land surface and by decreasing the contrast between daily high temperatures and nightly lows. Since the beginning of the Industrial Revolution, however, fossil fuel burning, changes in land use practices and other human activities have both increased dramatically the total atmospheric concentration of natural greenhouse gases and generated new potent classes of anthropogenic greenhouse gasses, such as chloflorocarbons (CFC). The results of this unplanned, global experiment are beginning to reach the level of detectability.

Study of the Monsoon system serves as an example of how some of these climatic forcing functions interact. On seasonal time scales, Monsoon circulation systems in African, India and Asia are driven by the thermal contrast between land and sea. During times in the geologic past when orbital parameters maximized seasonal contrast, enhanced Monsoon systems existed. This effect has been successfully simulated with general circulation models (GCMs) and observed using geologic data such as lake level records.