Editing World3 (section)

== Model ==
The model consisted of several interacting parts.  Each of these dealt with a different system of the model.  The main systems were 
*the [[food system]], dealing with agriculture and food production
*the industrial system
*the population system
*the non-renewable resources system
*the pollution system

=== Agricultural system ===
The simplest useful view of this system is that land and [[fertilizer]] are used for [[farm]]ing, and more of either will produce more food. In the context of the model, since land is finite, and industrial output required to produce fertilizer and other [[agricultural]] inputs can not keep up with demand, there necessarily will be a food collapse at some point in the future.

=== Nonrenewable resources system ===
The nonrenewable resource system starts with the assumption that the total amount of resources available is finite (about 110 times the consumption at 1990s rates for the World3/91 model). These resources can be extracted and then used for various purposes in other systems in the model. An important assumption that was made is that as the [[nonrenewable resources]] are extracted, the remaining resources are increasingly difficult to extract, thus diverting more and more industrial output to resource extraction.

The model combines all possible nonrenewable resources into one aggregate [[Variable (computer science)|variable]], {{var|nonrenewable_resources}}.<ref name="dynamics_of_growth">{{cite book |last1=Meadows |first1=Dennis L. |last2=Behrens, III |first2=William W. |last3=Meadows |first3=Donella H. |last4=Naill |first4=Roger F. |last5=Randers |first5=Jørgan |last6=Zahn |first6=Erich |title=Dynamics of Growth in a Finite World |date=1974 |publisher=Wright-Allen Press, Inc. |location=Cambridge, Massachusetts |isbn=0-9600294-4-3}}</ref>{{rp|387}}  This combines both energy resources and non-energy resources.  Examples of nonrenewable energy resources would include oil and coal.  Examples of material nonrenewable resources would include [[Aluminium|aluminum]] and [[zinc]].  This assumption allows costless substitution between any nonrenewable resource.  The model ignores differences between discovered resources and undiscovered resources.<ref name="dynamics_of_growth" />{{rp|381}}

The model assumes that as greater percentages of total nonrenewable resources are used, the amount of effort used to extract the nonrenewable resources will increase.
The way this cost is done is as a variable {{var|fraction_of_capital_allocated_to_obtaining_resources}}, or abbreviated {{var|fcaor}}.<ref name="dynamics_of_growth" />{{rp|393-8}}  The way this variable is used is in the equation that calculates industrial output.  Basically, it works  as {{code|code=effective_output = industrial_capital*other_factors*(1-fcaor)}}.  This causes the amount of resources expended to depend on the amount of industrial capital, and not on the amount of resources consumed.<ref name="dynamics_of_growth" />{{rp|390-3}}

The consumption of nonrenewable resources is determined by a nonlinear function of the per capita industrial output.  The higher the [[Per capita income|per capita]] industrial output, the higher the nonrenewable [[resource consumption]].

[[Image:World3 nonrenewable resource sector.svg]]

=== Reference run predictions ===
The ''Dynamics of Growth in a Finite World'' provides several different scenarios. The "reference run" is the one that "represent the most likely behavior mode of the system if the process of [[industrialization]] in the future proceeds in a way very similar to its progress in the past, and if technologies and value changes that have already been institutionalized continue to evolve."<ref name="Meadows et al.-1974">{{cite book|last1=Meadows|first1=Dennis L.|display-authors=etal|title=Dynamics of Growth in a Finite World|date=1974|publisher=MIT Press|location=Cambridge|isbn=0262131420|url=https://www.bookdepository.com/Dynamics-Growth-Finite-World-Dennis-L-Meadows/9780262131421|access-date=28 November 2017}}</ref>{{RP|502}} In this scenario, in 2000, the world population reaches six billion, and then goes on to peak at seven billion in 2030. After that population declines because of an increased death rate. In 2015, both industrial output per capita and food per capita peak at US$375 per person (1970s dollars, about ${{inflation|US|375|1972|fmt=c|r=-1}} today) and 500 vegetable-equivalent kilograms/person. Persistent pollution peaks in the year 2035 at 11 times 1970s levels.<ref name="Meadows et al.-1974"/>{{RP|500}}

[[File:Limits-to-growth-figure-35.svg|thumb|World Model Standard Run as shown in The Limits to Growth]]