Before going any further, we need to discuss what is meant by a system; it is a somewhat vague term that means different things to different people. In this book, we will be talking about dynamic systems -- systems that involve change -- because change at all timescales is a major theme in the study of the Earth. Some aspects of Earth, like its weather, change dramatically on short timescales, while others, like the positions and sizes of the continents, show dramatic change only when viewed over hundreds of millions of years. But, getting back to the definition, dynamic systems are coordinated or related sets of processes and reservoirs (places where things can reside or forms in which matter or energy exists) through which material or energy flows, characterized by continual change. This is a complex definition, but I think it is fairly precise, so it is worth thinking about carefully. A few examples will help.

A bathtub is a simple example of a dynamic system. Water flows into the tub through a faucet and leaves the tub through a drain. The faucet and the drain represent processes that are related because they both involve water moving into and out of the same reservoir, which is the tub itself. The balance between the inflow through the faucet and the outflow through the drain determines how much water accumulates in the tub. If the inflow and outflow rates are the same, then there will be no change in the amount of water in the tub, so the system would appear to be unchanging and not a true dynamic system. But of course, individual water molecules are moving through the system; material is flowing through this system.

Another example of a dynamic system is a pot of water set on a burner. In this case, energy, rather than matter, flows through the system. Energy is added to the pot via the burner and it is absorbed by the water in the pot, raising the temperature of the water. Energy escapes the system via infrared radiation (heat waves) and through vapor loss. When water changes from a liquid to a vapor, a process called evaporation, it requires a good deal of energy and this energy comes from the body of liquid water. This is why evaporation from a film of water covering your skin cools you. So in this system, there is one process adding energy to the system and two processes removing energy to the system and a reservoir of water through which the energy passes. We'll look into these systems in a more detailed manner in the next chapter.

While we're on the topic of basic definitions, we should also consider the global climate system and its important components -- the atmosphere, hydrosphere, biosphere, and geosphere -- so that you have a clear idea of what it meant by these terms.

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