Global Material Cycles

Abstract

This chapter discusses prominent examples of global material cycles. This is of major significance in order to understand potential perturbation of the natural material cycles caused by man’s production or use of energy. As selected examples, carbon, water, nitrogen and oxygen cycles will be treated, and in addition aspects of some other material cycles (sulfur, phosphorus, chlorine) as well as interactions of cycles. Significant simplifications must be used in order to focus on the major points.

The Earth as a closed system (according to Sect. 1.2) is represented in the present discussion by a selection of several open (sub-)systems which exchange material species (and energy) by various processes (chemical reactions, transport processes, etc.). In the subsequent figures, subsystems are always displayed as boxes. Components in these subsystems may include chemical species and/or different physical phases (vapor, liquid, solid). Flows of species (potentially connected to energy flows) are depicted as arrows and may be linked to (bio)chemical reactions, phase changes or transport processes across subsystem boundaries. According to the rates of flow and to the reservoir inventories of individual components, a formal residence time τ can be defined as a characteristic quantity to represent a characteristic time scale for component i in a reservoir j (Eq. 4.1).

$$ \tau = \frac{m_{i}}{F_{m,i}} $$

((4.1))

All examples discussed in the following will be simplified as a (quasi-)steady-state situation, with time-averages of all annual flows involved. Rates of individual flows will differ significantly, based on different characteristics of the individual processes (e.g. chemical reaction, mass transfer, phase change etc.). Connections between material flows and energy flows have to be taken into account for processes where significant energy changes/heat effects are involved (chemical reactions, phase transitions).