Environmental Problems, Their Causes, and Sustainability 1

We are embarked on the most colossal ecological experimentof all time—doubling the concentration in the atmosphereof an entire planet of one of the most important gases in theearth’s atmosphere—and we really have little idea of whatmight happen.PAUL A. COLINVAUXThis chapter discusses how our activities are changingthe world’s climate and depleting ozone in the stratosphere,and what we can do about these threats. It addressesthe following questions:■■■■■■■How have the earth’s temperature and climatechanged in the past?How might the earth’s temperature change in thefuture?What factors can affect changes in the earth’saverage temperature?What are some possible beneficial and harmfuleffects of a warmer earth?What can we do to slow or adapt to projected increasesin the earth’s temperature?How have human activities depleted ozone in thestratosphere, and why should we care?What can we do to slow and eventually reverseozone depletion in the stratosphere caused byhuman activities?21-1 PAST CLIMATE CHANGEHow Have the Earth’s Temperature andClimate Changed in the Past? Climate ChangeIs Not NewTemperature and climate have been changingthroughout the earth’s history.The earth’s climate—determined mostly by its averagetemperature and average precipitation—is not fixed.Therefore, climate change is neither new nor unusual.Over the past 4.7 billion years it has shifted due to volcanicemissions, changes in solar input, continentsmoving as a result of shifting tectonic plates, strikes bylarge meteorites, and other factors.At some times (over hundreds to millions of years),the troposphere’s average temperature has changedgradually and at other times fairly quickly (over a fewdecades to 100 years) as shown in the Figure 21-2graphs. Over the past 900,000 years, the average temperatureof the troposphere has undergone prolongedperiods of global cooling and global warming (Figure 21-2,top left). These alternating cycles of freezing and thawingare known as glacial and interglacial (between iceages) periods.During each cold period, thick glacial ice coveredmuch of the earth’s surface for about 100,000 years.Most of it melted during a warmer interglacial periodlasting 10,000–12,500 years that followed each glacialperiod.For roughly 12,000 years, we have had the goodfortune to live in an interglacial period with a fairlystable climate and a moderate average global surfacetemperature (Figure 21-2, top right and bottom left).However, even during this generally stable period, regionalclimates have changed significantly. For example,about 7,000 years ago, most of the current Saharadesert received almost 20 times more annual rainfallthan it does today.How Do Scientists Study Climate Change?Drill Holes and Make MeasurementsGeologic records and atmospheric measurementsprovide a wealth of information about past atmospherictemperatures and climate.Scientific clues about the earth’s past temperaturesand climate are found deep within its glaciers and icecaps, such as those in Greenland and Antarctica. Scientistsdrill into these museums of atmospheric historyand extract long cores of ice (Figure 21-3). In 2004, datafrom cores drilled in antarctic ice indicated that thecurrent interglacial period could last for another15,000 years before a new ice age occurs—unless ouractivities seriously alter the earth’s climate.Scientists analyze air bubbles trapped in differentsegments of these ice cores to uncover informationabout past tropospheric composition, temperaturetrends such as those in Figure 21-2, greenhouse gasconcentrations, solar activity, snowfall, and forest firefrequency (from trapped layers of soot particles).Scientists also study past climates by drilling coresinto the bottoms of lakes, ponds, and swamps. Thenthey analyze different zones of the sediment for pollen,fossils, and other clues about what types of plantslived in the past and trends in plant life over time. Forthose who like detective work, finding out about theearth’s climate history is a fascinating activity.Scientists also make direct measurements to getcurrent information about tropospheric temperature,composition, and trends. They measure temperaturesusing thermometers on land and at sea and on weatherballoons at various altitudes. Direct temperature recordsgo back to 1861. Scientists have also been usinginfrared sensors on satellites to get temperature informationabout the troposphere.Finally, scientists collect air samples at different locationsand altitudes and analyze them to detectchanges in the chemical composition of the troposphere.For example, since 1958 environmental chemistCharles Keeling has analyzed CO 2 levels in the troposphereat the Mauna Loa observatory in Hawaii.Once they have accumulated a certain amount ofdata, scientists get together to try to reach a consensus.In 1988, the United Nations and the World Meteorolog-462 CHAPTER 21 Climate Change and Ozone Loss