Environmental Problems, Their Causes, and Sustainability 1

Winning nations are less likely to bring about controversialchanges or spend large sums of money to slowdown something that will benefit them. The catch: Wedo not know which countries and parts of countrieswill be winners and losers until it is too late to avoidharmful effects.Fourth, reducing greenhouse gas emissions will takeunprecedented international respoinse to a global problemthat is of uncertain magnitude. And this must be doneusing political and economic systems not designed todeal with long-term threats.Because of these characteristics, you can see whyso many analysts believe that responding to this threatis one of the most important and challenging dilemmaswe face.21-9 OZONE DEPLETIONIN THE STRATOSPHEREWhat Is the Threat from Ozone Depletion?A Clear DangerLess ozone in the stratosphere will allow moreharmful UV radiation to reach the earth’s surface.A layer of ozone in the lower stratosphere (Figure 20-2,p. 434, and Figure 20-3, p. 435) keeps about 95% of thesun’s harmful ultraviolet (UV) radiation from reachingthe earth’s surface. Measuring instruments on balloons,aircraft, and satellites show considerable seasonaldepletion (thinning) of ozone concentrations inthe stratosphere above Antarctica and the Arctic. Similarmeasurements reveal a lower overall loss of stratosphericozone everywhere except over the tropics.Based on these measurements and on mathematicaland chemical models, the overwhelming consensusof researchers in this field is that ozone depletion (thinning)in the stratosphere is a serious threat to humans,other animals, and some of the sunlight-driven primaryproducers (mostly plants) that support the earth’s food.What Causes Ozone Depletion? FromDream Chemicals to Nightmare ChemicalsWidespread use of a number of useful and longlivedchemicals has reduced ozone levels in thestratosphere.Thomas Midgley, Jr., a General Motors chemist, discoveredthe first chlorofluorocarbon (CFC) in 1930, andchemists developed similar compounds to create afamily of highly useful CFCs. The two most widelyused are CFC-11 (trichlorofluoromethane, CCl 3 F) andCFC-12 (dichlorodifluoromethane, CCl 2 F 2 ), known bytheir trade name, Freons.These chemically stable (nonreactive), odorless,nonflammable, nontoxic, and noncorrosive compoundsseemed to be dream chemicals. Inexpensive tomanufacture, they became popular as coolants in airconditioners and refrigerators (replacing toxic sulfurdioxide and ammonia), propellants in aerosol spraycans, cleaners for electronic parts such as computerchips, fumigants for granaries and ship cargo holds,and bubbles in plastic foam used for insulation andpackaging. Between 1960 and the early 1990s, CFCproduction rose sharply.But it turned out that CFCs were too good tobe true. In 1974, calculations by chemists SherwoodRowland and Mario Molina at the University ofCalifornia-Irvine indicated that CFCs were loweringthe average concentration of ozone in the stratosphere.They shocked both the scientific community and the$28-billion-per-year CFC industry by calling for an immediateban of CFCs in spray cans (for which substituteswere available).Rowland and Molina’s research led them to fourmajor conclusions. First, CFCs remain in the tropospherebecause they are insoluble in water and chemicallyunreactive. Second, over 11–20 years these heavier-than-airchemicals are lifted into the stratospheremostly through convection, random drift, and the turbulentmixing of air in the troposphere.Third, once they reach the stratosphere, the CFCmolecules break down under the influence of highenergyUV radiation. This releases highly reactivechlorine atoms (Cl), as well as atoms of fluorine (F),bromine (Br) and Iodine (I), which accelerate thebreakdown of ozone (O 3 ) into O 2 and O in a cyclicchain of chemical reactions, one of which is shown inFigure 21-21. This causes ozone in various parts of thestratosphere to be destroyed faster than it is formed.Finally, each CFC molecule can last in the stratospherefor 65–385 years, depending on its type. Duringthat time, each chlorine atom released from these moleculescan convert hundreds of molecules of O 3 to O 2 .Overall, according to Rowland and Molina’s calculationsand later models and atmospheric measurementsof CFCs in the stratosphere, these dream moleculeshad turned into global ozone destroyers.The CFC industry (led by DuPont), a powerful,well-funded adversary with a lot of profits and jobs atstake, attacked Rowland and Molina’s calculationsand conclusions. The researchers held their ground,expanded their research, and explained the meaningof their calculations to other scientists, elected officials,and the media. After 14 years of delaying tactics,DuPont officials acknowledged in 1988 that CFCswere depleting the ozone layer and agreed to stop producingthem once they found substitutes.In 1995, Rowland and Molina received the NobelPrize in chemistry for their work. In awarding theprize, the Royal Swedish Academy of Sciences saidthat they contributed to “our salvation from a globalenvironmental problem that could have catastrophicconsequences.”484 CHAPTER 21 Climate Change and Ozone Loss