A simple population model Harald&Rybka,&Jason&Wong,&Jorge&Rivas&RISE&(Research&Internship&of&Science&and&Engineering)&Intern&Johannes&Gutenberg&University,&Mainz&and&University&of&Maryland&AOSC,&2&September&2010&1. Introduction to VENSIM 2. Limits to Growth – World3 model 3. Basic New Model / Structure 4. Improved Basic Model  results – global 5. Improvements and Problems OUTLINE&9/2/2010& 2&− VENSIM – simulation software − used for analyzing and simulating dynamic feedback modeling − identifying leverage points and causal loops − similar to “STELLA” Results: − use “SyntheSim” mode − different effects can be simulated with sensitivity simulations INTRODUCTION&TO&VENSIM&9/2/2010& 3&INTRODUCTION&TO&VENSIM&─&AN&EXAMPLE&9/2/2010& 4&source&sink&stock&flow&flow&constant&/&auxiliary&variable&“SyntheSim”&sensi[vity&run&average&life[me:&5&–&25&years&average&life[me:&20&years&average&life[me:&6&years&─ created by Donella Meadows, Jørgen Randers, Dennis Meadows (authors of the book: Limits to Growth, 1972, 2004) ─ computer simulation of interactions between: ─ the population system ─ the industrial system ─ the food system (agriculture and food production) ─ the non-renewable resources system and ─ the pollution system ─ ten different scenarios present results from 1900 till 2100 ─ results varies from stabilization to collapse in the future ─ depending on several variables (e.g. nonrenewable resources) LIMITS&TO&GROWTH&–&WORLD3&MODEL&9/2/2010& 5&WORLD3&MODEL&–&STANDARD&RUN:&agrees&with&obs.&1970b2000&(Tu rner,&09)&9/2/2010& 6&popula[on&food&industrial&output&persistent&pollu[on&nonrenewable&&resources&&business-as-usual population collapses because of - decrease of food and health services - increase costs of nonrenewable resourcesWORLD3&MODEL&–&SCENARIO&2&9/2/2010& 7&doubled initial nonrenewable resources popula[on&food&industrial&output&persistent&pollu[on&nonrenewable&&resources&&nevertheless: population collapses about 20 years laterWORLD3&MODEL&–&OPTIMISTIC&(STABILIZED)&SCENARIO&9/2/2010& 8&popula[on&food&industrial&output&persistent&pollu[on&nonrenewable&&resources&&Compared to Standard Run: ─ doubled nonrenewable resources ─ increase of land yield ─ improved pollution control ─ reduced land erosion stabilized population with the right policiesWORLD3&–&TYPES&OF&POSSIBLE&SOLUTIONS&9/2/2010& 9&STABLE – no overshoot UNSTABLE - overshoot carrying capacity population continuous growth sigmoid approach to equilibrium overshoot & oscillation overshoot & collapseBASIC&NEW&MODEL&9/2/2010& 10&simulate World population via fertility and life expectancy assumption: carrying capacity of the world is unlimited!!!BASIC&NEW&MODEL&b&RESULT&9/2/2010& 11&year 2010 ~ population of 7 billion next we will add a capacity limit World Population United Nations demographic data Basic New ModelNow food production is limited! BASIC&NEW&MODEL&WITH&LIMITS&9/2/2010& 12&limiting factors9/2/2010& 13&BASIC&NEW&MODEL&WITH&LIMITS&b&RESULTS&population approaches 8.9 billion people Setup: food production = food consumption for a population of 8 billion consumption per capita is a constant population notes: overproduction has no positive effect on life expectancy life expectancy collapses after year 2030 because of malnutrition life expectancy in yearsbefore agriculture was proportional to the population now we add instead of agriculture a new variable: pollution BASIC&&NEW&MODEL&–&NEW&VARIABLE:&POLLUTION&9/2/2010& 14&Basic&Model&popula[on&system&collapse or oscillation depends on pollution impact on life expectancy COLLAPSE&OR&OSCILLATION?&9/2/2010& 15&POPULATIONImprovements: ─ demographic system divided in age-cohorts → more details about demographic characteristics ─ more complex agriculture system → based on FAO (Food and Agriculture Organization) data ─ many variables show a sigmoid trend → production of meat and cereals, increasing arable land, land yield ─ calculation of life expectancy restricted to food consumption → cutting-off connection between life expectancy and population ─ different scenarios can be simulated BASIC&MODEL&WITH&AGEbCOHORTS&AND&FAO&AGRICULTURE&DATA&9/2/2010& 16&BASIC&MODEL&–&DEMOGRAPHIC&SYSTEM&9/2/2010& 17&no direct connection between life expectancy and population anymore!BASIC&MODEL&–&AGRICULTURE&SYSTEM&9/2/2010& 18&→ independent parametersassumptions: - total food production increases ~ constantly - max. life expectancy 90 years - fertility rate does not increase in the future nevertheless: → population decreases WHY? BASIC&MODEL&–&OPTIMISTIC&SCENARIO&9/2/2010& 19&population total food production9/2/2010& 20&BASIC&MODEL&–&OPTIMISTIC&SCENARIO&reason for decreasing population: → age distribution change higher mortality rate for older people → increasing overall mortality rate age&0&to&14&age&15&to&44&age&45&to&64&age&65&plus&fer[lity&rate&mortality&rate&DEPENDENCY&RATIO&b&dependent/working&popula[on&9/2/2010& 21&popula[on&dependency&ra[o&dependency ratio = (population 0 to 14 + population 65 plus) / population 15 to 64 small variation of dependency ratio (0.65 – 0.75)BASIC&MODEL&–&PESSIMISTIC&SCENARIO&9/2/2010& 22&population cereals yield total food production cropland “Erosion, Urbanization” “Green Revolution” “Running out of oil”Required changes to simulate regions: - change variables into arrays → no need to make several models - most regions are not self-sufficient → need to include imports and exports - add migration rate → dividing migrants in age-cohorts FUTURE&IMPROVEMENTS&–&INTRODUCE&REGIONAL&POPULATION&MODELS&9/2/2010& 23&SCHEMATIC&OF&THE&REGIONAL&MODELS&9/2/2010& 24&9/2/2010& 25&SCHEMATIC&OF&THE&REGIONAL&MODELS&fraction of migrants─ migration should be related to economic and demographic status (not much data for age-distribution of migrants) ─ imports and exports should be related to economics and food requirement → Most important: we have not yet included an economic subsystem ISSUES&WE&HAVE&TO&DEAL&WITH&9/2/2010& 26&WE&NEED&TO&ADD&F EE DBACKS&SUCH&AS&9/2/2010& 27&Popula[on&Urbaniza[on&Land&Use&Cropland&Food&Produc[on&Popula[on&Food&Requirement&Intensifica[on&of&Agricultural&Produc[on&Soil&Deple[on&Land&Degrada[on&Food&Produc[on&─ display real data for comparisons ─ add new systems like