Environmental Problems, Their Causes, and Sustainability 1

21-2 THE EARTH’S NATURALGREENHOUSE EFFECTWhat Role Does the Natural GreenhouseEffect Play in the Earth’s Temperature andClimate? A Giver of LifeCertain gases in the atmosphere absorb heat andwarm the lower atmosphere.In addition to incoming sunlight, a natural processcalled the greenhouse effect (Figure 6-14, p. 110) warmsthe earth’s lower troposphere and surface. Some of theenergy from the sun warms the earth’s surface, causingit to radiate infrared energy back toward space.Clouds, water vapor, carbon dioxide, and other gasesin the lower troposphere are heated when they absorbsome of this outgoing infrared energy. These cloudsand gases (called greenhouse gases) then radiate heat aslonger-wavelength infrared radiation in all directions.Some of the released energy is radiated into space andsome warms the troposphere and the earth’s surface.Swedish chemist Svante Arrhenius first recognizedthis natural tropospheric heating effect in 1896.Since then numerous laboratory experiments andmeasurements of atmospheric temperatures at differentaltitudes have confirmed this relationship. As a result,it is one of the most widely accepted theories inthe atmospheric sciences.A natural cooling process also takes place at theearth’s surface. Large quantities of heat are absorbed bythe evaporation of liquid surface water, and the watervapor molecules rise, condense to form droplets inclouds, and release their stored heat higher in the troposphere(Figure 6-9, p. 107). Because of the impact of thisnatural heating and cooling, the earth’s average surfacetemperature is about 15°C (59°F).What Are the Major Greenhouse Gases?Two Important MoleculesThe two major greenhouse gases are water vaporand carbon dioxide.Table 21-1 shows the major sources, average time inthe troposphere, and relative warming potential ofvarious greenhouse gases in the troposphere. The twogreenhouse gases with the largest concentrations arewater vapor, controlled by the hydrologic cycle, and carbondioxide (CO 2 ), controlled by the carbon cycle. Carbondioxide is the greenhouse gas we have added tothe troposphere.The coal, oil, and natural gas that support theworld’s economy all contain carbon that plants andsunshine converted to organic compounds hundredsof millions of years ago. Under high pressures andtemperatures these buried organic compounds wereconverted to fossil fuels. Extracting and burning thesestorehouses of carbon releases carbon dioxide into theatmosphere.According to the measurements of CO 2 concentrationsin glacial ice, estimated changes in troposphericCO 2 levels correlate fairly closely with estimated variationsin the average global temperature near theTable 21-1 Major Greenhouse Gases from Human ActivitiesAverageRelativeTime in theWarming PotentialGreenhouse Gas Human Sources Troposphere (compared to CO 2 )Carbon dioxide (CO 2 ) Fossil fuel burning, especially coal (70–75%), 100–120 years 1deforestation, and plant burningMethane (CH 4 ) Rice paddies, guts of cattle and termites, landfills, 12–18 years 23coal production, coal seams, and natural gas leaksfrom oil and gas production and pipelinesNitrous oxide (N 2 O) Fossil fuel burning, fertilizers, livestock wastes, 114–120 years 296and nylon productionChlorofluorocarbons Air conditioners, refrigerators, plastic foams 11–20 years (65–110 years 900–8,300(CFCs)*in the stratosphere)Hydrochloro- Air conditioners, refrigerators, plastic foams 9–390 470–2,000fluorocarbons (HCFCs)Hydrofluorocarbons Air conditioners, refrigerators, plastic foams 15–390 130–12,700(HFCs)Halons Fire extinguishers 65 5,500Carbon tetrachloride Cleaning solvent 42 1,400*CFC use is being phased out, but they remain in the troposphere for 1–2 decades.464 CHAPTER 21 Climate Change and Ozone Loss