Human population increases exponentially:

While humans may eventually define a logistic growth curve ; currently there is no evidence that this is the case. The only think that is demonstrable, as shown below, is that the rate of growth of the world's population is decreasing, but it's still exponential in nature. here is the data

In other words, the population doubling time is increasing, but there still is a characteristic doubling time. (70/n).

Its currently an open question if the whole world will also conform to the logistic growth curve that has been obtained by the developed world.

Often times exponential growth is plotted as a straight line on a semi-log plot. The Y-axis is logarithmic and the X-axis is linear.

Note:

e^{x} =y

ln (e^{x}) = ln y

ln y = x

We can use the Statistical Graphical Tool to help understand this.

Here is some data:

- 1 100
- 2 110
- 3 121
- 4 133
- 5 146
- 6 160
- 7 176
- 8 193
- 9 213
- 10 235

Then we will try an exponential fit with 10% growth rate to see its much better.

Population Growth and Finite Resources

Some Issues:

- Is population growth really exponential in nature?
- Can the Clock be Stopped? the reality of Zero
Population Growth
- Can new resources be found to replaced used resources?
- Can equilibrium be achieved through clever recycling?
- This is not my problem.

This depends on three factors:

- The average births per female note, the rise in teenage pregnancy severely impacts this estimate
- The average life expectancy
- The annual net immigration rate

Birth Rate | Expectancy | Immigration | Projections |
---|---|---|---|

1.9 | 75.3 | 350,000 | 293M (2030) 285M (2050) |

2.2 | 82.6 | 880,000 | 263(1995) 392M(2050) |

2.6 | 87.5 | 1.4 million | 522M(2050) |

In addition to this, the US population is undergoing a significant regional re-distribution

ZPG is only achieved in a model which limits immigration (a very politically sensitive issue), restricts birthrates to no more than 2 per female (more politics), and makes no attempt to increase life expectancy.

The fertility rate has gone up. In 1988 the rate was 1.8 per female, now its 2.0 this increase is entirely due to teenage pregnancy.

Equilibrium is difficult to achieve because exponential growth produces demographics such that they are substantially more people entering their reproductive years than those which are dying of old age.

What about ZPG for the World?

Worst case scenario assumes 2.5 children per couple 28 billion people by 2150

Best case assumes 1.7 7.8 billion in 2050 declining to 4.3 billion by 2150

Current fertility rate is 3.3. Fertility rate in 1970 was 4.5

The above calculations, however, do not adequately factor in increases in global life expectancy.

Population Growth Map

The above map shows the that the "industrialized" world has stabilized somewhat to population growth rates less than 1% a year. But 1% is still a doubling time of 70 years and the calculated growth rates are based, in this study, on only 5 years worth of data.

What does fertility rate depend on? Regression (to be discussed later) suggests that literacy is a key factor.

Education may be the single biggest contributor towards ZPG.

When did we know there would be a problem?

Historical Estimates of World Population (accurate to 10--20%)

So during this period of 650 years the world population was stable and fluctuated around a mean value of about 400 million.

Is this the "natural" carrying capacity of the planet?

Doubling time is approximately 150 years. Some scientists at the time began to express concern (e.g. Malthus )

Since 1900 the population of the world has grown by a factor of 4 (2 doubling times).

Summary of Population Data:

Clearly in the period 1200-1600 the population of the world was stabilized. Of course, the quality of life sucked big time.

Between 1900 and 1950 the world population rate grew at about 1% a year as shown below:

But projections based on that growth rate determined for 50 years of data would have been wrong.

Between 1950 and 1980 world population rate grew at a larger rate that 1% per year as shown below

In fact, the rate was closer to 1.8% per year which is almost a factor of 2 shorter doubling time!

Since 1980 there has been a small reduction in population growth down to about 1.65% per year.

Based on this data is fairly safe to assume, that if conditions do not change then the doubling time of the worlds population is 40--50 years.

The maximum rate ever observed, about 2.06%, occurred during the decade of the 60's. This was noticed and UN policy begin to dealt with the issue. Obviously, however, we are still growing exponentially at a significant rate.

What about other countries? Are there any that are approaching ZPG?

Sweden:

- 1950 7.014 M
- 1960 7.480 M
- 1970 8.043 M
- 1980 8.310 M
- 1990 8.559 M
- 2000 8.861 M

Growth rate: [ln (8.861/7.104)]/6 = 3.9% per decade; double = 180 years

Mexico:

- 1950 28.485 M
- 1960 38.579 M
- 1970 52.236 M
- 1980 68.686 M
- 1990 85.121 M
- 2000 95.772 M

Growth rate: [ln (95.772/28.485)]/6 = 20.2% per decade; double = 35 years

Egypt:

- 1950 21.198 M
- 1960 26.847 M
- 1970 33.574 M
- 1980 42.441 M
- 1990 56.106 M
- 2000 63.575 M

Growth rate: [ln (63.575/21.198)]/6 = 18.3% per decade; double = 38 years

Syria:

- 1950 3.495 M
- 1960 4.533 M
- 1970 6.258 M
- 1980 8.692 M
- 1990 12.620 M
- 2000 15.609 M

Growth rate: [ln (15.609/3.495)]/6 = 25% per decade; double = 28 years

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