COMPUTING SCIENCE Balanced on a Pencil Point Brian Hayes n 1972 a small paperbound book called T h e ment; there is one on every desk. Readers with Limits to Gro w t h was published with much fan- hands-on experience of these machines could If a re. Reporting on “Phase One of the Project on have brought to the book a more sophisticated the Predicament of Mankind,” the book warned understanding of the strengths and weaknesses that the world was steering a course for disaster. of computer modeling. More important, some Without a drastic change in direction, the human enterprising readers would have experimented population would run out of food and natural re- with the models themselves, or created new mod- s o u rces, or else would choke on its own pollution, els of their own. This kind of direct exploration is within 50 or 100 years. This gloomy message was clearly the best way to get a sense of how the g reeted with curious enthusiasm, at least in the models work and what their predictions mean. U.S. The book sold millions of copies; govern- As it happens, The Limits to Gro w t h has indeed ments took it seriously; conferences were con- just been published—in a sense. Members of the vened; the authors were awarded the German same group of workers have issued a new book, Peace Prize in 1974. There were many critics and Beyond the Limits, which makes essentially the skeptics, but the public was largely sympathetic. same arguments as the original one. More o v e r, America was in the mood for a jeremiad. And a the computer models themselves are available in year later when the gas pumps went dry, it a form that allows convenient experimentation, seemed the prophecies were being fulfilled. using readily available computer hard w a re. Now One factor that made The Limits to Gro w t h s o everyone can have doomsday on the desktop. persuasive was the book’s reliance on computer- aided mathematical modeling. The conclusions World Dynamics w e re not just opinions or personal interpre t a- The Project on the Predicament of Mankind was tions; they flowed ineluctably from the computer. s p o n s o red by the Club of Rome, a small org a n i z a- An introductory chapter noted two ways in tion whose members might best be described as which computer models are superior to mere concerned citizens of the world. In 1970 the club mental models: “First, every assumption we was looking for a quantitative model of what it make is written in a precise form so that it is had begun calling “the world p ro b l e m a t i q u e.” At a open to inspection and criticism by all. Second, meeting in Berne in 1970 Jay W. Forrester of the after the assumptions have been scrutinized, dis- Massachusetts Institute of Technology suggested a cussed, and revised to agree with our best cur- c o m p u t e r-based methodology for such a model. rent knowledge, their implications for the future F o r rester was already a formidable figure in the behavior of the world system can be traced with- history of computing—the inventor of magnetic- out error by a computer, no matter how compli- c o re memory and the architect of Project Whirl- cated they become.” The phrase “without erro r ” wind, perhaps the most ambitious machine in the was surely meant only to rule out mistakes in first generation of digital computers. Forrester had arithmetic, but for many readers the model must developed a technique of computer simulation have seemed infallible and irrefutable. In 1972 called system dynamics, which he had applied to the computer was still a marvelous oracular factory planning (in his book Industrial Dynamics) machine, kept behind glass walls and tended by and then to city planning (in Urban Dynamics) ; white-coated officiants. It predicted the weather; now he proposed a “world dynamics.” it predicted the outcome of presidential elections; F o r rester sketched out a preliminary version of why not have it predict the end of civilization? the model, called World1, on the flight home fro m If The Limits to Gro w t h had been published 20 Switzerland. A few weeks later members of the years later, the reception would have been diff e r- club visited MIT to see early runs of an expanded ent in certain details. The computer is no longer a model, World2. Forrester was invited to direct a mysterious, expensive and inaccessible instru- re s e a rch team that would further refine and extend the model, but he declined. Leadership of Brian Hayes, a former editor of American Scientist, has been the project went instead to Dennis L. Meadows, writing about computing for 10 years. Addre s s : 2 11 Dacian one of Forre s t e r ’s colleagues at MIT. (Although Avenue, Durham, NC 27701. Internet: [email protected]. F o r rester withdrew from the Club of Rome’s pro- 2 American Scientist, Volume 81 ject, he continued to work on his own global ple, one sector of the World3 model describes the model, and his book World Dynamics a p p e a red a global stock of arable land in terms of a re s e r v o i r year before The Limits to Gro w t h. It presents an with pipes flowing both in and out. The level of even grimmer vision of the future . ) the reservoir rises when raw land is developed For the club’s undertaking Dennis Meadows for agriculture; it falls when arable land erodes or re c ruited an international team of 17 young spe- is converted to urban and industrial uses. Each of cialists. The model they constructed was a dire c t these flows is regulated by a valve, whose setting o u t g row th of Forre s t e r ’s World2, and so they is determined by a complex web of other factors, called it World3. Four members of the gro u p including the world population, the amount of w e re listed as authors of The Limits to Gro w t h : food consumed per person and the level of Donella H. Meadows, Dennis Meadows, Jørg e n industrial output. Randers and William W. Behrens III. Later, vari- The dynamics of a system begin to get intere s t- ous members of the team published two more - ing when the connections between re s e r v o i r s , technical volumes, Toward Global Equilibrium a n d valves and other components include “loops” Dynamics of Growth in a Finite Wo r l d. The latter and thus introduce the possibility of feedback. A work constitutes the project’s final report and well-known instance is the growth of population. includes a complete program listing of the Because the number of births depends in part on World3 computer model. the number of people of child-bearing age, there Beyond the Limits, the most recent product of the is a positive feedback loop that can lead to expo- long collaboration, is by Donella and Dennis nential growth. More people have more childre n , Meadows and Jørgen Randers. They explain in who, after a delay of 20 years or so, have still their preface that they set out merely to update m o r e children. This process is re p resented in The Limits to Gro w t h for reissue on its 20th anniver- World3 by a feedback connection between a s a r y, but in the end they wrote a new book. In reservoir re p resenting the number of people many ways it is a better book than the original— aged 15 to 44 and the valve regulating flow into m o re carefully argued, less strident and, if not the human population. A similar mechanism quite optimistic, at least informed by a kinder and operates in the industrial sector: The total quanti- gentler Malthusianism. But the underlying mes- ty of industrial capital—factories, machinery, sage is the same: If we don’t stop burning the can- etc.—determines industrial output, but that out- dle at both ends, we shall soon be left in the dark. put in turn affects investment in new capital G row th in population and in consumption of equipment. The greater the industrial capacity of re s o u rces must be sharply curtailed. Again they a society, the faster that capacity can expand. rely on the World3 system dynamics model, Both of these feedback loops are positive, or slightly modified, to support these conclusions. s e l f - re i n f o rci ng; unless checked by some other f a c t o r, they give rise to unrestrained growth. The Plumbing and Wi r i n g World3 model also includes negative feedback System dynamics is all about plumbing and loops, in which the output of a process re d u c e s wiring. Stuff flows through pipes from sources to rather than re i n f o rces its own input. Perhaps the sinks. Along the way some of the stuff accumu- most fundamental example is the effect of over- lates in reservoirs. The flow into and out of the c rowding. If the World3 population gets too larg e , reservoirs is regulated by valves, which open and various factors connected with crowding incre a s e close in response to signals carried by wire s .