# 15.1: Introduction - Systems, Cycles, Reservoirs, and Fluxes

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Thinking about the Earth as a system has become, in recent years, much in vogue. The basic idea of a system is that it is something that consists of a number of distinctive and diverse parts that function together by a variety of interactions and exchanges of energy and matter. Your motor vehicle is a good example of a fairly small and simple system: it is a machine with a number of moving or nonmoving parts, which consumes motor fuel and emits various exhaust substances as it travels from place to place. The Earth is, of course, a far larger and also far more complicated system, whose component parts operate on time scales from seconds to many millions of years. This is the basis for what is nowadays called Earth system science. Earth system science includes many aspects of Environmental Science including hydrologic cycles and biogeochemical cycles which we will discuss in this chapter.

The concept of cycles plays a fundamental role in systems thinking— although it’s not easy to frame a suitable definition of such cycles. A cycle might best be described as a characteristic succession of events and processes, involving certain kinds of Earth materials, by which the materials reside in certain kinds of places and move among such places in certain ways. Cycles operate through an indefinitely long span of time. We can think about cycles at global scales or at the scale of a single organism. It is important to remember that while matter and energy are processed in cycles, they aren't necessarily moving in a simple circle and don't really have a beginning or an end.

Energy flows directionally through ecosystems, entering as entering as sunlight (or inorganic molecules for chemoautotrophs) and leaving as heat during the transfers between trophic levels. Rather than flowing through an ecosystem, the matter that makes up living organisms is conserved and recycled. The law of conservation of mass states that matter is neither created nor destroyed. For example, after a chemical reaction, the mass of the products (ending molecules) will be the same as the mass of the reactants (starting molecules). The same is true in an ecosystem. Matter moves through different media, and atoms may react to form new molecules, but the amount of matter remains constant.

The six most common elements associated with organic molecules—carbon, nitrogen, hydrogen, oxygen, phosphorus, and sulfur—take a variety of chemical forms and may exist for long periods in the atmosphere, on land, in water, or beneath Earth’s surface (Figure $$\PageIndex{1}$$). The cycling of these elements is interconnected. Geologic processes, such as weathering, erosion, water drainage, and the subduction of the continental plates, all play a role in the cycling of elements on Earth. Because geology and chemistry have major roles in the study of this process, the recycling of inorganic matter between living organisms and their nonliving environment is called a biogeochemical cycle. Today, anthropogenic (human) activities are altering all major ecosystems and the biogeochemical cycles they drive.

The most important biogeochemical cycle, in the context of the Earth’s surface environment, is the carbon cycle. The carbon cycle is especially important because carbon is the fundamental basis for life. It is also of great importance for the Earth’s climate, because carbon dioxide (CO2) is the second most important of the greenhouse gases, which play a key role in climate. (Water vapor is the most important, but we humans have virtually no control over the content of water vapor in the atmosphere, whereas we have great influence on the content of carbon dioxide in the atmosphere, owing to the burning of fossil fuels.) Water, which contains hydrogen and oxygen, is essential to all living processes. The hydrosphere is the area of Earth where water movement and storage occurs: as liquid water on the surface (rivers, lakes, oceans) and beneath the surface (groundwater) or ice, (polar ice caps and glaciers), and as water vapor in the atmosphere. Nitrogen is a major component of our nucleic acids and proteins and is critical to human agriculture. Phosphorus, a major component of nucleic acids, is one of the main ingredients (along with nitrogen) in artificial fertilizers used in agriculture, which has environmental impacts on our surface water. Sulfur, critical to the three-dimensional folding of proteins (as in disulfide binding), is released into the atmosphere by the burning of fossil fuels.

What pops into your mind when you hear the word reservoir? Probably a body of water, small or large, that is impounded behind a dam. In Earth systems science, the term reservoir is used for a distinctive kind of place where a certain kind of material is stored, or resides, for some period of time. This part of a cycle that holds an element or water for a short period of time is sometimes also called an exchange pool, or pool. For example, the atmosphere is an exchange pool for water. It usually holds water (in the form of water vapor) for just a few days. Some other examples of reservoirs or pools for water you will encounter in the course include glaciers; the soil layer; the aggregate of bodies of fresh water on the continents (rivers and lakes). Material moves into and out of reservoirs. The rate at which a given material moves between reservoirs is called a flux. If the flux of material into and out of a given reservoir is the same for some period of time, that reservoir is said to be in a steady state. Commonly, however, the flux in and the flux out are not equal.