Environmental Problems, Their Causes, and Sustainability 1

acid-loving mosses that can kill trees. These mosseshold enough water to drown tree roots and can killmycorrhizae fungi that help the roots absorb nutrients(Figure 8-10c, p. 155).A major problem is that acid deposition weakenstrees and other plants so they become more susceptibleto other types of damage such as severe cold, diseases,insect attacks, drought, and harmful mosses.Thus acid deposition rarely kills trees directly but canweaken them and make them more susceptible toother stresses.Mountaintop forests are the terrestrial areas hardesthit by acid deposition. These areas tend to havethin soils without much buffering capacity, and treeson mountaintops (especially conifers such as redspruce and balsam fir that keep their leaves yearround)are bathed almost continuously in very acidicfog and clouds.Note that most of the world’s forests and lakes are notbeing destroyed or seriously harmed by acid deposition. It is aregional problem that can harm forests and lakesdownwind from coal-burning facilities and from largecar-dominated cities without adequate pollution controls.Do you live in an area affected by acid deposition?How Serious Is Acid Deposition in theUnited States? Some Hopeful SignsMuch progress has been made inunderstanding and reducing acid depositionin the United States, but there is a long wayto go.Between 1980 and 1990, the federal government sponsoreda massive study called the National AcidPrecipitation Assessment Program (NAPAP). Its goalswere to coordinate government acid deposition researchand assess the costs, benefits, and effectivenessof the country’s acid deposition legislation and controlprograms.Good news. Acid deposition has not led to a declinein overall tree growth in the vast majority of forests inthe United States and Canada. Also, the 1990 amendmentsto the Clean Air Act have lead to significant reductionsin SO 2 and NO x emissions from coal-firedpower and industrial plants, and further reductionsare projected.Bad news. Acid deposition has accelerated theleaching of plant nutrients—such as ions of calciumand magnesium—from soils in some areas and thiscould eventually decrease tree growth. Acid depositionhas also increased concentrations of toxic forms ofaluminum in some soil and in lakes and streams.According to a 2001 study by Gene Likens andnine other acid deposition researchers, an additional80% reduction in SO 2 emissions from coal-burningpower and industrial plants in the midwestern UnitedStates (red dots in Figure 20-9) will be needed fornortheastern lakes, forests, and streams to recoverfrom past and projected effects of acid deposition.Bottom line. The 1990 amendments to the Clean AirAct have helped reduce some of the harmful impactsof acid deposition in the United States but there is stilla long way to go.What Can Be Done to Reduce AcidDeposition? Plenty, But There Is PoliticalOppositionA number of prevention and control methods canreduce acid deposition, but implementing thesesolutions is politically difficult.Figure 20-12 summarizes ways to reduce acid deposition.According to most scientists studying the problem,the best solutions are prevention approaches thatPreventionReduce airpollutionby improvingenergyefficiencyReduce coal useIncrease naturalgas useIncrease use ofrenewable energyresourcesBurn low-sulfurcoalRemove SO 2particulatesand NO x fromsmokestack gasesRemove NO x frommotor vehicularexhaustTax emissionsof SO 2SolutionsAcid DepositionCleanupAdd lime toneutralizeacidified lakesAdd phosphatefertilizer toneutralizeacidified lakesFigure 20-12 Solutions: methods for reducing acid depositionand its damage.448 CHAPTER 20 Air Pollution