## The climate system

Usually it is the value the various variables take at some particular time that is of concern; this is called the system state. The number of variables to be considered is determined by the length of time chosen to study the system. Over short time-scales some variables may hardly alter and so they can effectively be considered as constant. For longer time-scales, however, more variables will have to be studied as more will change. This is particularly true of the climate system.

As a system changes from one state to another state it is said to be in transition. Some systems jump from one to state to another and can only exist in one state at any one time. This is called a discrete system. A good example of a discrete system is channel hopping on a TV remote control. Other systems move gradually from one system state to another and these are called continuous systems. The climate system is an example of a continuous system. The links which bind the climate system together are the transfers of energy and mass. What makes up a system is rather arbitrary. A system is usually a collection of objects that human beings want to study. Systems can be classified in terms of how they relate to their outside environment (Lockwood, 1979).

1 Isolated systems: These systems do not transfer energy or mass to, or from, their surrounding environment.

2 Closed system: Exchanges energy with its surrounding environment but there is no exchange of matter.

3 Open systems: These exchange both mass and energy with their surrounding environment.

Examples of isolated systems in the real world are rare. They are much more likely to be located in a laboratory. The usual example is of a closed test tube containing a gas. The mass and energy remain locked within the test tube. If there are any temperature gradients within the gas these will eventually even out. A very good example of a closed system is the planet Earth in space. There is some loss of mass through the atmosphere escaping into space but it is so small as to be negligible. There is, however, energy exchange, the Earth receives energy from the Sun and also loses energy to space.

Open systems are very common in the natural environment. River catchments, hurricanes and the climate system are all examples of such systems (Lockwood, 1979). The open system can be in dynamic equilibrium. This means that over long enough time-scales the features of the system appear constant but at any moment may vary from that. Open systems can be divided into three categories: decaying, haphazardly fluctuating and cascading. While some open systems will always belong to one category, some will change categories. Decaying systems use up the energy, and/or mass that are feeding them. As the name suggests, haphazardly fluctuating systems change in an unpredictable and random way. A cascading system is one which is composed of a series of subsystems. Mass and energy cascade from one subsystem to the next, so the output from one forms the input for the next.

The climate system is such a cascading system, the subsystems being the individual components of the climate system.

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