CHAPTER 2 Energy and Earth

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1 CHAPTER 2 Energy and Earth This chapter is concerned with the nature of energy and how it interacts with Earth. At this stage we are looking at energy in an abstract form though relate it to how it affect and is affected by, Earth. Its applications and relevance will be demonstrated in later chapters. This may make it more difficult to absorb the information, so practical examples would be appropriate where possible. The nature of energy and its different forms should be considered at the start. These can then be related to the inputs and outputs of energy from Earth. The transformations of energy and internal transfers provide vital information about how energy moves around the Earth atmosphere system. In turn this forms the basis for our climate and how it may change over time. Satellite imagery is very useful in demonstrating the patterns of energy outputs, either in the visible waveband (to show reflection) or in the thermal infra-red (to show emission and therefore temperature). Concepts of energy Five forms of energy are important in environmental processes: radiant, thermal, kinetic, chemical and potential. Radiant energy is energy associated with electromagnetic waves propagating through space. It is vital in transferring solar energy to the Earth and in removing the energy surplus from the Earth. Thermal energy is the energy associated with temperature and is produced when radiant energy is absorbed by a substance. Kinetic energy is the energy of motion and operates at all scales, from the molecular to the global circulation. Chemical energy is energy stored by the combination of molecules into chemical compounds. It can be released into thermal or kinetic energy when the substance breaks down. Potential energy is due to the position of an object above the ground surface; objects further above the surface possess more potential energy. This energy can be converted to kinetic energy when it moves to lower levels or to heat if it is affected by friction. There is considerable interaction between the forms of energy. Heat is the lowest form of energy and many other forms eventually degenerate to heat. Each type of energy can be demonstrated fairly readily, apart from radiant energy. Ask for examples of each type. Sensible and latent heat Two other forms of thermal energy that are very important in the earth system.

2 Sensible heat is the exchange of warm air down a temperature gradient. By day this is from the surface to the atmosphere, by night, it is usually from air to ground. Latent heat is the energy needed or released when a substance changes its state from vapour to liquid to gas or vice versa. On Earth, it is water which is the key substance. Much thermal energy is required to evaporate water to vapour. In turn this is released on condensation. Transfers of energy The distribution of energy across the globe is not uniform. As a result there needs to be energy transfer. Energy transfers take place down the energy gradient, from higher-energy to lower-energy levels. In the case of thermal energy, transfer can take place as radiation, convection or conduction. Radiation and convection are most important for atmospheric transfers. Conduction is important into the ground surface. Energy can be transformed from one type to another. Eventually the energy is dissipated at the lowest level as heat. Patterns and principles of electromagnetic radiation We can summarize the laws of radiation as they affect Earth as follows: All substances emit radiation when their temperatures are above absolute zero. Some substances, largely gases, absorb and emit radiation at certain wavelengths only. A perfect radiator will emit radiation to an extent which is in proportion to the fourth power of its absolute temperature. The hotter is a substance, the shorter will be the wavelengths of its emitted radiation. The amount of radiation passing through a particular area is inversely proportional to the square of the distance of that area from the source. These laws are very important, as they determine the transmission of energy through the atmosphere and its distribution over the globe. In particular, they explain the difference in properties between solar and terrestrial radiation. The nature of inputs and outputs of energy In an imaginary view of the world from space, we should appreciate that virtually all energy is received from the sun. If we could measure all the energy being returned to space from Earth we would find that it balanced the amount of energy from the sun,

3 If we do not know what happens to the energy inside Earth and its atmosphere. This situation is known as a black box model. Solar radiation input Energy from the sun reaches Earth as short-wave radiation, with its maximum output at a wavelength of about 0.5 µm. This places it in the visible part of the spectrum. The gases of the atmosphere are largely transparent to visible light. Earth s rotation and the geometry of its orbit around the sun determine the diurnal and seasonal variations of radiation input. The greatest intensity of input is when the angle between the sun s rays and the surface is 90. Everyone is aware of the diurnal and seasonal variations of the position of the sun in the sky. It is more difficult to explain the terrestrial geometry which causes the changes. The use of a model Earth and torch to imitate the sun may help. Energy outputs of the globe Because the radiant emission from Earth is of longer wavelength than that of the Sun it reacts differently with the atmosphere. Much of it is absorbed and reradiated. Long-wave radiation is the means of losing energy to offset the gains of shortwave radiation from the sun. The surface emits most radiation because it is warmest. The atmosphere emits radiation; some returns to Earth s surface, some is lost to space. Global energy transfers The amounts of energy over the globe are not uniform; polar regions tend to receive less than tropical areas. Less available energy means lower temperatures, which drive the global energy circulation. The transfer of energy is not adequate to offset the radiation imbalances, so the circulation keeps going. Transfer of energy takes place as warm air masses, by warm ocean currents and by the transfer of latent heat resulting from the evaporation of water. Regional energy transfers At a more local scale we have a variety of energy transfers: Atmospheric transfers, where movements in the atmosphere transfer energy between surface and atmosphere, and from energy surplus to energy-deficient areas. Hydrological transfers, where precipitation from the atmosphere starts a cycle which involves weathering, storage and erosion before it reaches the oceans and eventual evaporation back to the atmosphere.

4 Landscape transfers, involving mountain-building processes, which generate potential energy and allow rock particles to move downslope through gravity or water movement. Ecological transfers, largely on a local scale, where the formation of soil, the growth of plants, their decay or consumption by animals, all involve energy flow. Essay and discussion questions 1 Describe and give examples of the various ways that heat can be transported in the atmosphere. 2 If the temperature of the sun was to change, what effects might this have on Earth s energy budget and climate? 3 How does the atmosphere respond differently to short-wave and long-wave radiation 4 In what ways do energy transfers at Earth s surface differ from those in the atmosphere? 5 What is the climatic significance of the tilt of Earth s axis of rotation? 6 Why is water important in energy transfer around the globe? 7 How might you confirm that latent heat is a form of energy? Web site addresses Much of this chapter is about basic physical features of energy and its importance to Earth. Useful websites are therefore those can provide a scientific introduction to these topics. This site can provide a first call for many questions or topics relevant to each chapter. However it is only edited to some degree and therefore less authoritative than more official government or educational websites. Nevertheless it is extremely useful when treated with care. and Two interlinked sites provided by NASA. Give a range of information about the atmosphere and its properties. Clouds, hurricanes and land use changes can be seen. Intended as a teaching manual, information is available about the atmosphere, principles of climate, greenhouse effect and global climate change.

5 A site that tries to give a scientific basis to the understanding of energy and electromagnetic radiation. Plenty of relevant information that can be worked through. Its aim is to solve global problems together, especially in terms of climate change. A site that includes basic information about the controls of climate and many other issues but with an emphasis on climate change. Allows a commentary and discussion from climate scientists about research activities and contentious issues. Fascinating to follow arguments on a wide range of climate subjects.