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Chapter 2 The Earth’s Global Energy Balance This chapter focuses on solar radiation which flows through the atmosphere to the Earth’s surface. This energy is responsible for driving the Earth’s physical and biological systems. The Earth’s energy balance is the balance between the flow of energy reaching the Earth and the flow of energy leaving the Earth. Solar energy is the driving force for most natural phenomena at the Earth’s surface. Electromagnetic radiation is emitted from all objects as a collection of wavelengths traveling away from the surface of an object. Two principles that govern the emission of electromagnetic radiation are: An inverse relationship exists between the temperature of an object and the range of wavelengths that object emits as electromagnetic radiation. Hot objects radiate more energy than cooler objects. The sun is a star of average size with a surface temperature of 6000° C generated by nuclear fusion. The solar constant is the amount of energy received per square meter just outside the Earth’s atmosphere. The value is 1370 watts per square meter (1370 W/m2). The sun emits a large amount of energy, concentrated in the ultraviolet, visible, and shortwave infrared wavelengths. This is called short wave radiation. The Earth is much cooler than the sun. It therefore emits less energy and emits that energy as longwave radiation. Insolation, or incoming solar radiation, varies with the angle of the sun above the horizon and daylength. Locations between 23½° north and 23½° south of the equator experience two insolation maxima per year, while locations poleward of these latitudes experience only one insolation maximum. Locations poleward of the arctic and Antarctic circles experience daily insolation values of zero for part of the year. Daily insolation values are greatest at the pole during the summer solstice. The seasonal pattern of daily insolation is the basis for dividing the Earth into world latitude zones which include equatorial, tropical, subtropical, midlatitude, subarctic (subantarctic), arctic (antarctic), and north (south) polar zones. Although the Earth’s atmosphere extends to approximately 10,000 kilometers above the Earth, ninety-seven percent of the atmosphere lies within 30 kilometers of the Earth’s surface. Pure dry air consists of seventy-eight percent nitrogen and twenty-one percent oxygen by volume. Argon, CO2, and other trace gases make up the remaining one percent. CO2 is a very important gas due to its ability to absorb radiant heat and its role in photosynthesis. The ozone layer is found in the stratosphere, where it absorbs ultraviolet radiation and shields the Earth from the stratosphere’s harmful effects. Human activity has increased the amount of gases such as chloroflourocarbons, nitrous oxides, bromine oxides, and hydrogen oxides which are depleting the ozone layer. For every one percent decrease in global ozone, ultraviolet radiation may increase by two percent. Sensible heat is the quantity of heat held by an object that can be sensed by touch, measured by a thermometer, and transferred by conduction from warmer to cooler objects. Latent heat is energy that is absorbed and stored when a substance changes state from a liquid to a gas or a solid to a liquid. Latent heat is transferred when water evaporates from a land or water surface and is important in moving large amounts of energy from one region to another. As solar radiation flows through the atmosphere, energy is scattered and absorbed by gas molecules and dust particles in the air. Clouds are a major factor in determining how much energy reaches the Earth’s surface absorbing five to twenty percent and reflecting thirty to sixty percent of insolation. Albedo refers to the percentage of shortwave (SW) energy reflected by a surface. The albedo of the Earth is twenty-nine to thirty-four percent. CO2 and water vapor absorb incoming SW radiation and outgoing LW radiation from the Earth. They re-emit this radiation in all directions with part of it returning to the Earth’s surface in counterradiation, making the surface of the Earth warmer than it would otherwise be. Energy entering the Earth’s atmosphere is reflected by molecules, dust, clouds, and the surface and absorbed by molecules, dust, and clouds leaving only forty-nine percent of the incoming energy to be absorbed by the Earth’s land and water surfaces. The energy entering the Earth’s system must be balanced by energy leaving the Earth’s system. Energy leaves the Earth’s surface as longwave radiation as well as through transfers of sensible heat and latent heat. Human changes to the Earth that affect albedo, cloud cover, or other aspects of the energy transfers may have an impact on this balance. Net radiation is the difference between all incoming and all outgoing radiation. Although net radiation is zero for the Earth as a whole, it is positive between latitude 40° north and 40° south and negative poleward of these latitudes. As a result, global and atmospheric circulation systems transport energy from lower to higher latitudes.