Environmental Problems, Their Causes, and Sustainability 1

After building and testing a mathematical model,scientists use it to project what is likely to happen undera variety of conditions. In effect, they use mathematicalmodels to answer if–then questions: “If we do such andsuch, then what is likely to happen now and in the future?”This process can give us a variety of projectionsor scenarios of possible futures or outcomes based ondifferent assumptions.Despite its usefulness, a mathematical model isnothing more than a set of hypotheses or assumptionsabout how we think a certain system works. Mathematicalmodels (like all other models) are no better thanthe assumptions on which they are built and the datafed into them.How Do Feedback Loops Affect Systems?Changing DirectionOutputs of matter, energy, or information fed backinto a system can cause the system to do more ofwhat it was doing (positive feedback) or less(negative feedback).When someone asks you for feedback, they are askingfor information that they can feed back into their mentalprocesses to help them make a decision or carry outsome action. All systems undergo change as a result offeedback loops. A feedback loop occurs when an outputof matter, energy, or information is fed back intothe system as an input and leads to changes in thatsystem.A positive feedback loop causes a system tochange further in the same direction. One example involvesdepositing money in a bank at compoundinterest and leaving it there. The interest increases thebalance, which through a positive feedback loop leadsto more interest and an even higher balance.A negative, or corrective, feedback loop causes asystem to change in the opposite direction. An exampleis recycling aluminum cans. This involves meltingaluminum and feeding it back into an economic systemto make new aluminum products. This negativefeedback loop of matter reduces the need to find, extract,and process virgin aluminum ore. It also reducesthe flow of waste matter (discarded aluminum cans)into the environment.The temperature-regulating system of your body isan example of a system governed by feedback. Normallya negative feedback loop prevents your bodytemperature from going too high. If you get hot, yourbrain receives this information and causes your body tosweat. The evaporation of sweat on your skin removesheat and cools your body. However, if your body temperatureexceeds 42°C (108°F), your temperature controlsystem breaks down as your body produces moreheat than your sweat-dampened skin can get rid of.Then a positive feedback loop caused by overloadingthe system overwhelms the negative feedback loop.These conditions produce a net gain in body heat,which produces even more body heat, and so on, untilyou die from heatstroke.The tragedy on Easter Island discussed at the beginningof the chapter also involved the coupling ofpositive and negative feedback loops. As the abundanceof trees turned to a shortage of trees, a positivefeedback loop (more births than deaths) becameweaker as death rates rose. Eventually a negative feedbackloop (more deaths than births) dominated andcaused a dieback of the island’s human population.How Do Time Delays Affect ComplexSystems? Waiting for Something toKick InSometimes corrective feedback takes so long to workthat a system can cross a threshold and change itsnormal behavior.Complex systems often show time delays between theinput of a stimulus and the response to it. A long timedelay can mean that corrective action comes too late.For example, a smoker exposed to cancer-causingchemicals in cigarette smoke may not get lung cancerfor 20 years or more.Time delays allow a problem to build up slowlyuntil it reaches a threshold level and causes a fundamentalshift in the behavior of a system. Prolonged delaysdampen the negative feedback mechanisms that mightslow, prevent, or halt environmental problems. Examplesare population growth, leaks from toxic wastedumps, and degradation of forests from prolonged exposureto air pollutants.What Is Synergy, and How Can It AffectComplex Systems? One Plus One Can BeGreater Than TwoSometimes processes and feedbacks in a system caninteract to amplify the results.In arithmetic, 1 plus 1 always equals 2. However, insome of the complex systems found in nature, 1 plus 1may add up to more than 2 because of synergistic interactions.A synergistic interaction, or synergy, occurswhen two or more processes interact so that thecombined effect is greater than the sum of their separateeffects.Synergy can result when two people work togetherto accomplish a task. For example, suppose youand I need to move a 140-kilogram (300-pound) treethat has fallen across the road. By ourselves, each of uscan lift only, say, 45 kilograms (100 pounds). But if wework together and use our muscles properly, we canhttp://biology.brookscole.com/miller1437