What Are Systems?
Systems theory is a powerful conceptual tool that is useful in understanding complex phenomena, such as weather and climate. The atmosphere is certainly one of the most complex phenomena known to humanity, so it probably comes as no surprise that it has been subjected to systems-based analysis for many decades. Likewise, conceptualizing climate as a system gives us a way to both understand how it functions and predict how it might change in the future.
A system can be defined as a group of interrelated components that make up a dynamic and complex "whole" and interact as a structured, functioning unit. This definition sounds complex, but the concept is quite basic. We often refer to electrical systems, central heating systems, and even the central nervous system. In each case, the system is made up of component parts (or even sub-systems) that work together to move matter or energy around and have a power source (electric power plant, natural gas, electrochemical reactions) that drives its operation.
Every system must have a clear demarcation between what is included in the system and what is excluded. Quite a bit of effort and research is dedicated to determining system boundaries. This is not to say that systems exist in complete isolation-- far from it. Truly isolated systems (which have zero transfer of matter or energy across their boundaries), simply do not exist in the natural world. Likewise, closed systems (which transfer energy, but not matter, across their boundaries) are rare.
The earth-atmosphere system can be thought of as a closed system. Energy in the form of solar radiation (sunlight) enters the system and eventually exits in the form of terrestrial and atmospheric thermal radiation (heat), while only negligible amounts of matter are exchanged between the earth and space.
Most systems are open systems that freely transfer both matter and energy across their boundaries. The climate system is an excellent example of an open system. This begs an interesting question: What is the climate system? What features and phenomena make up this so-called system? Let's take a few minutes to explore this question.
The many faces of the climate system
As you can see in the images below, there are many ways of defining "the climate system." Clicking on each image will display a larger version, while clicking on the source link below them will take you to the page where each was found. Performing an image search in Google for the term "climate system" will return hundreds, if not thousands, of illustrations.
It is quite intuitive to include the atmosphere as a key component of the climate system, but most experts agree that it also includes the oceans as well as the cryosphere, biosphere, and geosphere. It is important to understand that the system also includes the interactions between the components.
System-based approaches offer insight on how the atmosphere functions, and how the climate might respond to either natural or anthropogenic (human-caused) forcings. You must be careful, though-- never mistake the conceptual system model for the real thing! No conceptual model is ever complete and the complexity of the atmosphere ensures that including all of its components and processes into a fully-integrated model will remain an elusive objective for a long time. In the meantime, we can make informed guesses in answer many "what if" questions using this systems approach incorporated into powerful computer models.