Energy Flows, Material Cycles and Global Development

This chapter discusses the relevance of environmental effects caused by humans and the concept of sustainable development. Many of these effects take place on a global scale. Thus they are relevant for the Earth system (Steffen et al. 2005) and presently constitute a major challenge for human societies. The second section deals with the definition of systems, their balances, and their properties with special emphasis on dynamic and feedback situations. In the third section, it is described how systems respond to internal processes and to changes of their surroundings. Classical thermodynamics distinguishes between different kinds of energies that can be interconverted without altering the sum of all energies. However, different kinds of energy have not the same quality and low-level energy cannot be completely transformed to higher levels. Second-law analyses using the concepts of entropy and exergy are required to describe the efficiency of these transformations. Finally, non-equilibrium thermodynamics devoted to real systems far away from equilibrium, where fluctuations and instability are of great importance, are briefly discussed.

The properties of the Earth and especially of the biogeosphere as an environment that has allowed for the development of life and human civilization are discussed in this chapter. The first section describes the position of the Earth in the solar system and the present conditions on Earth as a whole as well in its constituents, e.g. crust, ocean, and atmosphere. The second section deals with the driving forces for changes in the biogeosphere and the resulting developments of the atmospheric composition and the global climate. Finally, the development of life on Earth and of the human civilization are briefly reviewed.

In this Chapter the energy transformations on Earth are described. The surface temperature, which is a key variable for the climate, is on the one hand determined by the global energy balance between radiative energy coming from the Sun and radiative energy emitted back to space by the Earth (Fig. 3.1a). On the other hand, the atmosphere has a strong impact on the global energy balance as it efficiently absorbs infrared radiation coming from the surface of the Earth. As a consequence, the surface temperature is much higher than it would be without the atmosphere.

From an engineering point of view, the Earth is an energy converter like the technical converters described in the subsequent Chapters of this book (Fig. 3.1b). Shortwave radiation from the Sun is transformed into longwave radiation that is re-emitted by Earth, but also into latent heat contained in the atmosphere and the ocean, as well as into mechanical energy in wind, rivers, and ocean currents. The complexity of the Earth system is tremendous and a full understanding of the processes taking place in it has not yet been achieved. However, several general properties, limiting cases, and typical phenomena occurring within the Earth system can be derived from relatively simple energetic analyses.

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).

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).

During the Earth’s history, there have been significant variations in the global carbon cycle, caused by natural factors (e.g. changes in surface temperature due to changes in solar irradiation). Given these natural changes, variations due to human activities and their causes are difficult to identify.

As human activities in recent (industrial) times cause large flows of materials and energy with significant impact on the environment, public discussion was stimulated in the 1970s and 1980s by publications about potential limits (e.g. by Meadows et al. 1972 or Barney 1981). The following discussion introduces four factors representing limits in different respects. Intriguing is the question, which of these factors might become most determining and when. Besides the four factors introduced, there are other aspects like availability of capital or political or military conflicts limiting human activities which in this discussion here are not included.

This chapter discusses different aspects of future global development, as a consequence of the flow analyses presented earlier. Criteria for future development are presented, as well as technology and general policy strategies. The effect of individual life styles or living conditions on per-capita energy demand and emissions is demonstrated with some selected examples. Scenarios for future global CO₂ emission and atmospheric carbon inventory are presented which are helpful in setting supranational agreements for future climate change policies.

The Earth is a complex system and a complete understanding of the interaction of all processes taking place has not yet been achieved. However, we believe that a number of fundamental constraints, limitations and guidelines for future development can be derived from the process engineering approach presented in this book. As a result, the following conclusions are based on the view that humans should avoid significant perturbations of natural cycles in the biogeosphere. They are comprised of some facts regarding natural and anthropogenic flows in the biogeosphere as well as important aspects of sustainable global development.