Environmental Problems, Their Causes, and Sustainability 1

are dying because the permafrost underneath them ismelting. According to University of Alaska scientists,Alaskan forests are also threatened by greatly increasedpopulations of the spruce-bark beetle (whichcan kill spruce trees) because of a lack of cold spellsthat help keep them under control.However, there are economic benefits from awarmer Alaska. They include a longer growing season,ice-free ports, more people moving to the state, andmore tourists visiting and spending money year-round.During the last 25 years glaciers have also beenmelting and shrinking at accelerating rates on manyof the world’s mountaintops. Only 27 of the 150 glaciersfound during the middle of the last century inMontana’s Glacier National Park remain. Tanzania’sMount Kilimanjaro—Africa’s tallest peak—may be icefreewithin 15 years. Other evidence indicates that 80%of South American glaciers could disappear within15 years.The disappearance of mountain glaciers means aloss of frozen water reservoirs that partially thaw outduring warm months and release water for use byfarms and city-dwellers in the valleys below. This isbad news for countries like Peru, Ecuador, and Bolivia,which rely on annual water release by mountain glaciersfor irrigation and household use.These modelers develop a three-dimensional representationof how energy, air masses, and moistureflow through the atmosphere, based on the laws ofphysics and the major factors affecting the earth’s temperatureand climate shown in Figure 21-9.Computer simulations begin by covering theearth’s surface with a grid of several hundred hugesquares (Figure 21-10, p. 471). Each square provides thebase for a stacked of gigantic imaginary cells, each severalhundred kilometers on a side and about 3 kilometers(2 miles) high. These layers of cells extend downinto the ocean and up into the atmosphere. Data onvariables such as solar energy, sunlight, air pressure,temperature, water vapor, and winds or currents thataffect climate in each cell are fed into the model. Then acomplex set of mathematical equations simulates flowsof matter and energy among the cells and the entire climatemodel is fed into a supercomputer. New climatedata can be added to the model to improve its accuracy.Such models provide scenarios of what is verylikely or likely to happen based on various assumptionsand data fed into the model. How well the results correspondto the real world depends on the assumptionsof the model (based on current knowledge about thesystems making up the earth, oceans, and atmosphere)and the accuracy of the data used.21-4 PROJECTING FUTURECHANGES IN THE EARTH’STEMPERATUREHow Do Scientists Model Changes in theEarth’s Temperature and Climate? ComputerModels as Crystal BallsScientists have developed complex mathematicalmodels of the earth’s climate systems, and they usethem to project future changes in the earth’s averagetemperature.To project the effects of increases in greenhouse gaseson average global temperature, scientists developmodels of how interactions among solar energy andthe earth’s land, oceans, ice, and greenhouse gases determinethe average temperature of the troposphere.Figure 21-9 ( p. 470) gives a greatly simplified summaryof some of these interactions. Trace the flows and connectionsin this figure.Scientists use this information to develop globalclimate models (also know as coupled global circulationmodels) that are applied to the atmosphere to projectthe effects of increases in greenhouse gases on averageglobal temperature. Currently 14 research laboratoriesare operating, evaluating, and improving coupledgeneral circulation models.What Is the Scientific Consensus aboutFuture Changes in the Earth’s Temperature?Hotter Times AheadMost climate scientists agree that human activitieshave influenced recent temperature increases andwill lead to further significant temperature increasesduring this century.In 1990, 1995, and 2001, the IPCC published reportsthat evaluate how global temperatures changed in thepast (Figure 21-2) and are likely to change during thiscentury. The IPCC reached its conclusions on the basisof scientific principles governing climate, data frompast events, human emissions of CO 2 and other greenhousegases, current temperature measurements, andglobal climate models.Here are three major findings of the 2001 report.■ Despite many uncertainties, the latest climatemodels match the records of global temperaturechanges since 1850 very closely.■ ”There is new and stronger evidence that most ofthe warming observed over the last 50 years is attributableto human activities.”■ It is very likely (90–99% probability) that theearth’s mean surface temperature will increase by1.4–5.8°C (2.5–10.4°F) between 2000 and 2100 (Figure21-11, p. 471).http://biology.brookscole.com/miller14469