solar energy is a very important factor in the world. Organisms at or near the surface of the earth are immersed in a radiation environment consisting of direct downward flowing solar radiation and long – wave heat radiation from nearby surfaces. Both contribute to the climatic regime that determines ‘’conditions of existence,’’ but only a small fraction of the direct solar component can be converted by photosynthesis to provide food energy for the biotic components of the ecosystem. Extra – terrestrial sunlight reaches the biosphere at a rate of 2 g- cal /cm2/ min. this quantity is known as the solar constant. Since the sun shines only for part of the day at any location, the amount coming in on a day or year basis is about half, more or less. On a square – meter basis this comes to about 14,400 kcal/ day or 5.25 million kcal/ year. This large flow is reduced exponentially as it passes through clouds, water vapor, and other gases of the atmosphere, so the amount actually reaching the autotrophic layer of ecosystems is on the order of 1.0 to 2.0 million kcal/ m^-2 year^-1       , depending on latitude, cloud cover, and 1 -5 percent of this converted to organic matter that structures and operates the solar – powered ecosystem.
solar energy radiation

       The sequence of energy of energy flow that we have just described, including further transfers to animals and man, is shown in the diagrammatic model of figure 3 -1. Quantities shown are much rounded off averages that are appropriate for a north temperate latitude such as midcontinent north America. On an annual basis we see how rapidly solar energy is lost into the heat ( I, ii, and so on in figure 3 –IB) as it passes through the atmosphere and the green belt. The organic food that plants are able to produce from sunlight is partly used by the plants themselves for their own maintenance and growth (with appropriate heat loss) and is partly passed on to the heterotrophy. In the diagram c1 represents the primary consumer or herbivore level and c2 the secondary or carnivore level. In the plant – animal portion of the energy flow chain about 80 to 90 percent of the energy is lost with each step. Or to put it another way, only 10 – 20 percent can be transferred to the next level. Thus, out of the millions of calories of solar energy coming into a column with a square – meter base, only a few hundreds are left to nourish a meat – eating, animal, or man. Two sets of figures are shown in the right – hand portion of figure3 – 1: (1) along the top of the line averages for biosphere as a whole, and (2) in parentheses below the line 10 times these figures for the favored for the favored solar powered ecosystem that receive supplemental energy.

       It is important to note that useful work is accomplished at each transfer, not just in the biological part but all along the chain. Thus, although we cannot eat much of it, or use it directly to run our machines, all of the incoming solar radiation is vital to the operation of the biosphere. For example, the dissipation of solar radiation (a and b, figure3 -1) as it passes into the atmosphere, the seas, and the green belts warms the biosphere of life – tolerable levels, drives the hydrological cycle and power weather systems. So delicate are the heat and other energy balances of the earth that meteorological models now show that only very small changes in the solar constant, or in the turbidity of the atmosphere (which would let more or less energy reach the surface of the earth) are needed to change the world’s climates. Just a little bit of decrease in heat brings on ice age, while a small increase brings on a tropical era, with a melting of all the polar ice raising the sea level to flood large areas of present continents. (Good – by new York and most of the world’s large coastal cities)

          In figure 3 -1 we introduce a symbolic ‘’energy language’’ which has been developed by Howard T. odum to facilitate communication between physical scientists and engineers on the one hand, and biologists and social scientists on the other (H. T. Odum, 1971,). In this and subsequent diagrams of this type circles signify energy sources, the sun in this instance. The heat sink symbol (I through v in figure 3 – 1B), shows where energy is lost in transformation from one form to another as required by the second law of thermodynamics. The heat sink symbol resembles an electrical ground symbol, but is one – way (as indicated by downward directed arrow). The bullet – shaped symbol represents an autotrophic system (or more broadly a unit capable of receiving Pire wave energy, such as light, and producing an energy – activated state, such as food, which can be deactivated to pass energy on to another step in a chain of energy flow). The hexagonal symbol represents a heterotrophic unit, or more broadly a self – maintaining component that is capable of receiving, storing, and feeding back energy received from an autotrophic, another heterotrophic, or another concentrated potential energy source. Additional symbols will be introduced in subsequent diagrams.
solar energy ultraviolet radiation

        The spectral, that is, the wavelength, distribution of sunlight is also altered as it passes through atmosphere, clouds, tater, and vegetation. The ozone belt of the upper atmosphere selectively absorbs the lethal short – wave ultraviolet radiation so that only about 10 percent reaches the earth’s surface on a clear day. Visible radiation (medium wavelength) on which photosynthesis depends is least attenuated as it passes through clouds and water, which means that photosynthesis can continue on cloud days, and at some depth in lakes and the sea (if they are not too turbid). Green plants sufficiently absorb the blue and visible red light that is most useful in photosynthesis and reject, as it were, the near infrared heat waves and thus avoid overheating. The long – wave infrared radiation, in general, which makes up the bulk of solar energy. Is absorbed and reradiated as heat in a complex manner by atmosphere, clouds, and various natural and man – made objects and surfaces. For more on these aspects, see gates (1963). Just because the world’s green belts convert only a small percentage of incoming solar energy to food energy, does not mean that they are inefficient actually. Photosynthesis is a very efficient process for tapping that small portion of sunlight that can readily be converted to high utility potential energy of organic matter.