chap08.ipynb

Kernel: Python 3

Modeling and Simulation in Python

Chapter 8

Copyright 2017 Allen Downey

License: Creative Commons Attribution 4.0 International

# Configure Jupyter so figures appear in the notebook
%matplotlib inline

# Configure Jupyter to display the assigned value after an assignment
%config InteractiveShell.ast_node_interactivity='last_expr_or_assign'

# import functions from the modsim.py module
from modsim import *

Functions from the previous chapter

def plot_results(census, un, timeseries, title):
    """Plot the estimates and the model.
    
    census: TimeSeries of population estimates
    un: TimeSeries of population estimates
    timeseries: TimeSeries of simulation results
    title: string
    """
    plot(census, ':', label='US Census')
    plot(un, '--', label='UN DESA')
    plot(timeseries, color='gray', label='model')
    
    decorate(xlabel='Year', 
             ylabel='World population (billion)',
             title=title)

def run_simulation(system, update_func):
    """Simulate the system using any update function.
    
    system: System object
    update_func: function that computes the population next year
    
    returns: TimeSeries
    """
    results = TimeSeries()
    results[system.t_0] = system.p_0
    
    for t in linrange(system.t_0, system.t_end):
        results[t+1] = update_func(results[t], t, system)
        
    return results

Reading the data

def read_table2(filename = 'data/World_population_estimates.html'):
    tables = pd.read_html(filename, header=0, index_col=0, decimal='M')
    table2 = tables[2]
    table2.columns = ['census', 'prb', 'un', 'maddison', 
                  'hyde', 'tanton', 'biraben', 'mj', 
                  'thomlinson', 'durand', 'clark']
    return table2

#table2 = read_table2()
#table2.to_csv('data/World_population_estimates2.csv')

table2 = pd.read_csv('data/World_population_estimates2.csv')
table2.index = table2.Year
table2.head()

un = table2.un / 1e9
census = table2.census / 1e9
plot(census, ':', label='US Census')
plot(un, '--', label='UN DESA')
    
decorate(xlabel='Year', 
             ylabel='World population (billion)',
             title='Estimated world population')

Running the quadratic model

Here's the update function for the quadratic growth model with parameters alpha and beta.

def update_func_quad(pop, t, system):
    """Update population based on a quadratic model.
    
    pop: current population in billions
    t: what year it is
    system: system object with model parameters
    """
    net_growth = system.alpha * pop + system.beta * pop**2
    return pop + net_growth

Extract the starting time and population.

t_0 = get_first_label(census)
t_end = get_last_label(census)
p_0 = get_first_value(census)

Initialize the system object.

system = System(t_0=t_0, 
                t_end=t_end,
                p_0=p_0,
                alpha=0.025,
                beta=-0.0018)

Run the model and plot results.

results = run_simulation(system, update_func_quad)
plot_results(census, un, results, 'Quadratic model')

Generating projections

To generate projections, all we have to do is change t_end

system.t_end = 2250
results = run_simulation(system, update_func_quad)
plot_results(census, un, results, 'World population projection')
savefig('figs/chap08-fig01.pdf')

The population in the model converges on the equilibrium population, -alpha/beta

results[system.t_end]

-system.alpha / system.beta

Exercise: What happens if we start with an initial population above the carrying capacity, like 20 billion? Run the model with initial populations between 1 and 20 billion, and plot the results on the same axes.

# Solution goes here

Comparing projections

We can compare the projection from our model with projections produced by people who know what they are doing.

def read_table3(filename = 'data/World_population_estimates.html'):
    tables = pd.read_html(filename, header=0, index_col=0, decimal='M')
    table3 = tables[3]
    table3.columns = ['census', 'prb', 'un']
    return table3

#table3 = read_table3()
#table3.to_csv('data/World_population_estimates3.csv')

table3 = pd.read_csv('data/World_population_estimates3.csv')
table3.index = table3.Year
table3.head()

NaN is a special value that represents missing data, in this case because some agencies did not publish projections for some years.

This function plots projections from the UN DESA and U.S. Census. It uses dropna to remove the NaN values from each series before plotting it.

def plot_projections(table):
    """Plot world population projections.
    
    table: DataFrame with columns 'un' and 'census'
    """
    census_proj = table.census / 1e9
    un_proj = table.un / 1e9
    
    plot(census_proj.dropna(), ':', color='C0', label='US Census')
    plot(un_proj.dropna(), '--', color='C1', label='UN DESA')

Run the model until 2100, which is as far as the other projections go.

system = System(t_0=t_0, 
                t_end=2100,
                p_0=p_0,
                alpha=0.025,
                beta=-0.0018)

results = run_simulation(system, update_func_quad)

plt.axvspan(1950, 2016, color='C0', alpha=0.05)
plot_results(census, un, results, 'World population projections')
plot_projections(table3)
savefig('figs/chap08-fig02.pdf')

People who know what they are doing expect the growth rate to decline more sharply than our model projects.

Exercises

Exercise: The net growth rate of world population has been declining for several decades. That observation suggests one more way to generate projections, by extrapolating observed changes in growth rate.

The modsim library provides a function, compute_rel_diff, that computes relative differences of the elements in a sequence.

Here's how we can use it to compute the relative differences in the census and un estimates:

alpha_census = compute_rel_diff(census)
plot(alpha_census, label='US Census')

alpha_un = compute_rel_diff(un)
plot(alpha_un, label='UN DESA')

decorate(xlabel='Year', label='Net growth rate')

Other than a bump around 1990, net growth rate has been declining roughly linearly since 1965. As an exercise, you can use this data to make a projection of world population until 2100.

1. Define a function, alpha_func, that takes t as a parameter and returns an estimate of the net growth rate at time t, based on a linear function alpha = intercept + slope * t. Choose values of slope and intercept to fit the observed net growth rates since 1965.

2. Call your function with a range of ts from 1960 to 2020 and plot the results.

3. Create a System object that includes alpha_func as a system variable.

4. Define an update function that uses alpha_func to compute the net growth rate at the given time t.

5. Test your update function with t_0 = 1960 and p_0 = census[t_0].

6. Run a simulation from 1960 to 2100 with your update function, and plot the results.

7. Compare your projections with those from the US Census and UN.

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Related viewing: You might be interested in this video by Hans Rosling about the demographic changes we expect in this century.