The Global Energy System

Chapter

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Earth’s Atmosphere Earth’s Primordial Atmosphere Initially, the atmosphere probably consisted of helium (He) and hydrogen (H), with traces of ammonia (NH3) and methane (CH4). With time volcanoes added carbon dioxide (CO2), and lesser amounts of nitrogen (N2), sulphur dioxide (SO2), and water vapour (H2O).

Earth’s Atmosphere Earth’s Primordial Atmosphere Oxygen (O2) was slowly added to the atmosphere through photo-dissociation (ultraviolet radiation) of water molecules (~0.001 % of present concentrations). The subsequent rise in oxygen is attributed to photosynthesis.

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Earth’s Atmosphere Today’s Atmosphere Pure, dry air consists mainly of nitrogen, about 78% by volume, and oxygen, about 21%.

Other gases such as argon (Ar) and carbon dioxide, as well as water vapour and various pollutants (sulphur dioxide, halogens), account for the remaining 1%. Dusts and other fine particles are present, some of which play an important role in the formation of precipitation by acting as condensation nuclei.

Earth’s Atmosphere Ozone in the Upper Atmosphere A small, but important, constituent of the atmosphere is ozone (O3), a form of oxygen in which three oxygen atoms are bounded together. Ozone absorbs UV radiation, thus shielding plants and animals from its harmful effects. The concentration of ozone in the atmosphere is reported in Dobson units (DU), which measure the density of ozone in a column of atmosphere above a specified area of the Earth’s surface.

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Earth’s Atmosphere Ozone in the Upper Atmosphere Oxygen molecules (O2) absorb UV energy and split into two oxygen atoms (O + O). A free oxygen atom (O) then combines with an O2 molecule to form ozone (O3). Once formed, ozone can also be destroyed by UV radiation, which dissociates to form O2 + O. The net effect is that ozone (O3), molecular oxygen (O2), and atomic oxygen (O) are constantly formed, destroyed, and reformed in the ozone layer.

Earth’s Atmosphere Air Pollutants An air pollutant (gases, aerosols, particulates) is an unwanted substance injected into the atmosphere from the Earth’s surface by either natural or human activities. One group of pollutants is known collectively as greenhouse gases (carbon dioxide-CO2, methane-CH4, nitrous oxide-N2O, chlorofluorocarbons-CFCs). Although relatively scarce in the atmosphere, they are of special importance because of their ability to trap long-wave radiation.

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The Global Energy System Electromagnetic Radiation Electromagnetic radiation (the principal form in which energy is transported from the sun to the Earth) is characterized by its wavelength () and frequency (f). Electromagnetic waves of different wavelengths and frequencies form the electromagnetic spectrum (gamma rays to radio waves). The visible light portion of the electromagnetic spectrum corresponds to the wavelength range 400-700 nanometres (nm). The infrared region of the electromagnetic spectrum extends from the visible wavelenghts at 700 nm to about 1millimeter (mm).

The Global Energy System Radiation and Temperature There is an inverse relationship between the range of wavelengths that an object emits and the surface t temperature t off the th object. bj t The flow of radiant energy from the surface of an object is directly related to its absolute temperature (in Kelvin, K) raised to the fourth power.

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The Global Energy System Solar Radiation The sun (an average size star) has a surface temperature of about 5,800 K (5,500 ºC). The intensity of a flow of the sun’s sun s radiant energy that reaches the top of Earth’s atmosphere is 1,367 Watts per square metre (Wm-2). 1367 Wm-2 is the total solar irradiance. However, since the Earth is spherical in shape the average solar irradiance intercepted by the Earth (as a whole) is considerably less (~ 340 Wm-2).

The Global Energy System Effects of the Atmosphere on Solar Energy Energy emitted by the sun is predominantly in the visible UV and infrared wavelengths (0.2 to 4.0 m) collectively referred to as short-wave short wave radiation. radiation As solar radiation passes through the atmosphere, some wavelengths are absorbed, reflected or scattered as they encounter gas molecules and dust particles.

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The Global Energy System Effects of the Atmosphere on Solar Energy Absorption occurs when molecules and particles in the atmosphere intercept radiation at particular wavelengths. Reflection occurs when a light ray strikes a molecule or particle and bounces off again. Scattering differs from reflection in that light striking a molecule is redirected in all directions.

The Global Energy System Effects of the Atmosphere on Solar Energy Selective scattering, also known as Rayleigh scattering, is greatest in the shorter wavelengths (caused by atmospheric gases and particles which are smaller in di diameter t than th a particular ti l wavelength l th off radiation). di ti ) Rayleigh scattering for blue wavelengths is approximately 10 times as great as for the longer red wavelengths of sunlight (the scattered blue light reaches us from all directions, and the sky appears blue).

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The Global Energy System Effects of the Atmosphere on Solar Energy Non-selective scattering, also known as Mie scattering, is caused by particles with diameters greater than ten times the wavelength of the solar radiation (dust particles and water t droplets). d l t ) Large particles cause all wavelengths of light to scatter more or less equally (no particular wavelength of light is preferentially scattered causing water droplet filled clouds or fog to appear white).

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The Global Energy System Long-Wave Radiation form the Earth Given the size of Earth and its distance from the sun and assuming there is no atmosphere, under radiative equilibrium the average surface temperature of the planet would be -18 ºC. However, the mean Earth surface temperature is 14 ºC (the increase is due to greenhouse gases such as water vapour and carbon dioxide – primary absorbers of long-wave radiation). At a mean temperature of 14 ºC, radiation from the Earth ranges from about 3 to 30 m and peaks at about 10 um in the thermal infrared region (long-wave radiation).

The Global Energy System Albedo The proportion of short-wave radiant energy scattered upward by a surface is termed albedo. A high albedo surface such as snow reflects most (50 to 85%) of the solar radiation reaching it. A low albedo surface such as black pavement (reflecting only 3% of the solar radiation reaching it) absorbs nearly all the incoming solar energy.

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The Global Energy System Counter-Radiation and the Greenhouse Effect Although the atmosphere is cooler than the surface of the Earth, it too emits long-wave radiation. It emits this radiation in all directions, some upward to space, and some toward the Earth’s surface. Since this downward flow is the opposite direction to longwave radiation leaving the surface, it is termed counterradiation.

The Global Energy System Counter-Radiation and the Greenhouse Effect Much of the long-wave radiation emitted upward by the Earth’s surface is absorbed by water vapour and carbon dioxide. The absorbed energy raises the temperature of the atmosphere, causing it to emit more counter-radiation. The mechanism, in which the atmosphere traps long-wave radiation and returns some of it to the surface, is termed the greenhouse effect.

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Planetary Insolation Although the flow of solar radiation from the sun remains more or less constant, the amount received on Earth varies both spatially and over time. Average g incoming g energy gy flow, measured in watts p per square metre over the course of a 24-hour day, is referred to as daily insolation (incoming solar radiation).

Planetary Insolation Assuming that the Earth is a uniformly spherical planet with no atmosphere, the amount of energy intercepted daily at a location depends on two factors: (1) the angle at which the sun’s rays strike the Earth, and (2) the length of time the location is directly exposed to the rays. Both of these factors are controlled by the latitude of the location and the time of year.

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World Latitude Zones The seasonal pattern of daily insolation can be used as a basis for dividing the globe into broad latitude zones: Equatorial zone Tropical zones Subtropical zones Midlatitude zones Subarctic and Subantarctic zones

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Sensible Heat and Latent Heat Transfer Sensible heat is heat energy held by an object or substance that can be measured by a thermometer. The store of sensible heat increases as the temperature of the object rises. When two objects of unlike temperature contact each other, heat energy moves by conduction from the warmer object to the cooler. This type of heat flow is referred to as sensible heat transfer. Sensible heat transfer can also occur in fluids by convection.

Sensible Heat and Latent Heat Transfer Latent heat is heat that is taken up and stored in the form of molecular motion. It is associated with a change of state of substance from a solid to a liquid, from a liquid to a gas, or from a solid directly to a gas. For example, energy is used to convert liquid water to water vapour and released when water vapour converts back to liquid water.

Sensible Heat and Latent Heat Transfer Latent heat transfer occurs when water evaporates from a moist land surface or an open water surface. This process transfers heat from the surface to the atmosphere.

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Global Energy Balance of the EarthAtmosphere System The pathways of energy flow between the sun, the Earth’s surface, and the atmosphere integrate all the dynamic processes that shape the planet. Ultimately, the amount of energy received by the Earth–atmosphere system equals the amount of energy returned to space space. Therefore, on a global scale, there is a radiation balance. A full accounting of all the energy flows among the sun, the atmosphere, the Earth’s surface, and space forms the global energy balance.

Global Energy Balance of the EarthAtmosphere System The global energy balance involves Incoming ShortWave Radiation, Energy Flows at the Earth Surface and Energy Flows to and from the Atmosphere. Changes in the processes and mechanisms that govern the global energy balance can have an influence on Climate and Environmental Change.

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Net Radiation, Latitude and Energy Balance Solar energy input varies strongly with latitude. In some locations, radiant energy flows in faster than it flows out; elsewhere, radiant energy flows out faster than it flows in. The difference between all incoming radiation and all outgoing radiation is termed net radiation.

Net Radiation, Latitude and Energy Balance Solar Energy The Earth intercepts solar energy at the rate of 1.77 × 1014 kilowatts per year. This quantity of energy is about 28,000 times as much as all the energy human society currently consumes each year. Thus, an enormous energy source is available for use (e.g., photovoltaic cells, direct and passive heating of buildings).

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A Look Ahead Chapter 4 discusses present-day temperatures at both local and regional scales, and examines how and why they vary from place to place as well as seasonally and over the course of a day.

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