Environmental Problems, Their Causes, and Sustainability 1

AtmosphereVegetationBiosphereand animalsSoilRockCrustFigure 4-7 Naturalcapital: general structureof the earth. Theearth’s surface consistsof a crust of rock floatingon a mantle of solidand partly melted rock,which surrounds an intenselyhot core.OceaniccrustCoreMantleHydrosphere(water)Lithosphere(crust, top of upper mantle)ContinentalcrustLithosphereUpper mantleAsthenosphereLower mantleAtmosphere(air)Crust (soiland rock)Biosphere(living and deadorganisms)How Does the Sun Help Sustain Lifeon Earth? A Distant Nuclear FusionReactorEnergy released by the gigantic nuclear fusionreactor we call the sun provides the light andheat needed to sustain the earth’s life.Energy from the sun supports most life on theearth by lighting and warming the planet. It supportsphotosynthesis, the process in which greenplants and some bacteria make compounds suchas carbohydrates that keep them alive and feedmost other organisms. And it powers the cyclingof matter and drives the climate and weathersystems that distribute heat and fresh water overthe earth’s surface.About one-billionth of the sun’s output ofenergy reaches the earth—a tiny sphere in thevastness of space—in the form of electromagneticwaves (Figure 3-10, p. 45). The amount ofenergy reaching the earth from the sun equalsthe amount of heat energy the earth reflects orradiates back into space. Otherwise, the earthwould heat up to temperatures too hot for life aswe know it.Much of the sun’s incoming energy is reflectedaway or absorbed by chemicals, dust,and clouds in the atmosphere as shown in Fig-What Sustains Life on Earth? Sun, Cycles,and GravitySolar energy, the cycling of matter, and gravity sustainthe earth’s life.Life on the earth depends on three interconnected factors,shown in Figure 4-8.■ The one-way flow of high-quality energy from the sun,through materials and living things in their feedinginteractions, into the environment as low-qualityenergy (mostly heat dispersed into air or water moleculesat a low temperature), and eventually back intospace as heat. No round-trips are allowed because energycannot be recycled.■ The cycling of matter (the atoms, ions, or moleculesneeded for survival by living organisms) throughparts of the biosphere. Because the earth is closed tosignificant inputs of matter from space, the earth’s essentiallyfixed supply of nutrients must be recycledagain and again for life to continue. Nutrient trips inecosystems are round-trips.■ Gravity, which allows the planet to hold onto itsatmosphere and causes the downward movement ofchemicals in the matter cycles.CarboncyclePhosphoruscycleBiosphereNitrogencycleHeat in the environmentWatercycleOxygencycleHeat Heat HeatFigure 4-8 Natural capital: life on the earth depends on the onewayflow of energy (dashed lines) from the sun through the biosphere,the cycling of crucial elements (solid lines around circles),and gravity, which keeps atmospheric gases from escaping intospace and draws chemicals downward in the matter cycles. Thissimplified model depicts only a few of the many cycling elements.60 CHAPTER 4 Ecosystems: What Are They and How Do They Work?