The earth is a complex system, continually evolving and changing. Today, ever-increasing evidence points to the roles that humans play in modifying Earth systems. As striking as human changes are, natural geologic, hydrologic, atmospheric and biotic factors still control the environmental context in which anthropogenic activities take place. Together, these processes are continuously altering the globe, with the continents, oceans, and atmosphere that exist today only temporary features when considered on the geologic time scale. The processes that shape the Earth can be divided into two broad categories: those that are internal to the Earth, and those that are external. The motions of plate tectonics, which act to build mountains, create new sea floor and fuel volcanic eruptions, are of the internal type, driven by the heat produced from radioactive decay within the earth. These processes are natural to the system, though their impacts on mankind can be severe. Solar energy drives the external heat engine of the earth and thus regulates the oceanic and atmospheric processes which sculpt and mold the earth's surface. Interactions between the amount and distribution of solar energy reaching the Earth's atmosphere, the composition of the atmosphere, and the nature of the surface of the Earth all may produce changes in the climate system (Barron, 1994). Variability in the amount and distribution of solar energy reaching the earth is a function of both natural and anthropogenic processes.

Global change occurs over a wide range of temporal and spatial scales (Figure 1). For example, changes in the configuration of the continents occur slowly over millions of years as a result of plate tectonics. As the distribution of land and ocean changes, so do oceanic and atmospheric circulation systems, hence altering earth's climate. At the other end of the time spectrum are events such as volcanic eruptions, which can alter the regional landscape and global atmospheric chemistry in a matter of hours.

This chapter focuses on climate as an example of an integrated system continually subject to change via both natural and anthropogenic influences, on time scales from days to millennia to eons. The climate system consists of several subsystems - the atmosphere, hydrosphere, lithosphere, and biosphere - which interact with one another through both positive and negative feedback processes (Figure 2). A change in any one of the subsystems will in turn affect the others and result in modifications and changes to global climate.

Based on studies of the geologic record, we know that climate change has occurred throughout Earth's history. For example, ice cores retrieved from glaciers existing at high latitudes and altitudes have archived information about past climates. Analysis of the ice itself, and of gas and particulates trapped in the ice, provide data that reflect changes through time in atmospheric chemistry and circulation. Marine and lake sediment cores similarly record changes in local, regional and even global climate. By comparing and contrasting evidence for climatic variation through time from a diverse suite of geologic records covering wide geographic regions, scientists are attempting to understand the history of climate on earth. Such reconstructions include information on the range and rate of climate variability and help to tease apart the tangle of information concerning natural versus anthropogenic influences on global climate.

Considering the naturally-occurring wide swings and rapid changes in climate which characterized the pre-industrial world, you may ask why we are concerned with present anthropogenic influences on the climate system. Of what significance is increasing the average Earth temperature by a mere 1-2°C, or the addition of 3 gigatons of carbon dioxide (1 gigaton = 1015 gram) to the atmosphere each year? The significance lies in the unique ability of planet Earth to sustain life. The composition of the atmosphere and the presence of water make the Earth habitable for the biosphere, including humans. Changes that we make to one part of the climate system have feedbacks that affect all other parts, which in turn affect us. As a result of anthropogenic changes to the climate system, we are now faced with such concerns as global warming (the greenhouse effect) and the depletion of stratospheric ozone, which have the ability to affect the basic environment necessary for our existence.