Energy and Human Ambitions on a Finite Planet, 2021a

3 Population 31
expectations, then discuss the heralded “demographic transition” <strong>and</strong>
its implications.
3.1 Population History
Figure 3.2 shows a history of global population for the last 12,000 years.
Notice that for most of this time, the level is so far down as to be
essentially invisible. It is natural to be alarmed by the sharp rise in recent
times, which makes the current era seem wholly unusual: an aberration.
But wait—maybe it’s just a plain exponential function. All exponential
functions—ruthless as they are—would show this alarming rise at some
point, sometimes called a “hockey stick” plot. In order to peer deeper, we
plot population on a logarithmic vertical axis (Figure 3.3). Now we bring
the past into view, <strong>and</strong> can see whether a single exponential function
(which would have a constant slope in a logarithmic plot) captures the
story.
Wait, what? It still looks somewhat like a hockey stick (even more literally
so)! How can that be?! This can’t be good news. Peering more closely, we
can crudely break the history into two eras, each following exponential
growth (straight lines on the plot), but at different rates. The early phase
had a modest 0.044% growth rate. By the “rule of 70,” the corresponding
doubling time is about 1,600 years. In more recent times, a 1% rate is
more characteristic (70 year doubling). Indeed, we would be justified in
saying that recent centuries are anomalous compared to the first 10,000
years of the plot. If we extend the the 0.04% line <strong>and</strong> the 1% line, we find
that they intersect around the year 1700, which helps identify the era of
marked transition.
The recent rapid rise is a fascinating development, <strong>and</strong> begs for a closer
look. Figure 3.4 shows the last ∼1,000 years, for which we see several
exponential-looking segments at ever-increasing rates. The doubling
times associated with the four rates shown on the plot are presented in
Table 3.1.
An interpretation of the population history might go as follows. Not much
changed during the period following the Dark Ages. 2 The Renaissance
(∼1700) introduced scientific thinking so that we began to conquer
diseases, allowing an uptick in population growth. In the mid-19th
century (∼1870), the explosive expansion of fossil fuel usage permitted
industrialization at a large scale, <strong>and</strong> mechanized farming practices.
More people could be fed <strong>and</strong> supported, while our mastery over human
health continued to improve. In the mid-20th century (∼1950), the Green
Revolution [17] introduced a fossil-fuel-heavy diet of fertilizer <strong>and</strong> largescale
mechanization of agriculture, turning food production into an
industry. The combination of a qualitative change in the availability of
cheap nutrition <strong>and</strong> the march of progress on disease control cranked
the population rate even higher.
population (billions)
7
6
5
4
3
2
1
0
−10000−8000−6000−4000−2000 0 2000
year
Figure 3.2: Global population estimate, over
the modern human era, on a linear scale.
Figure 3.1 offers a recent close-up. [14, 15].
population (log scale)
10 10
10 9
10 8
10 7
0.04% growth
1.0% growth
1700
10 6
−10000−8000−6000−4000−2000 0 2000
year
Figure 3.3: Global population estimate, over
the modern human era, on a logarithmic
scale. [14, 15].
population (log scale)
10 10
10 9
0.12% growth
0.82% growth
0.41% growth
1700
1.7% growth
1870
1950
10 8
1000 1200 1400 1600 1800 2000
year
Figure 3.4: Global population estimate, over
recent centuries. On the logarithmic plot,
lines of constant slope are exponential in behavior.
Four such exponential segments can
be broken out in the plot, having increasing
growth rates. [14, 15].
2: ...exceptthatfamine <strong>and</strong> plague took a
toll in the 14th century
Table 3.1: Doubling times for Fig. 3.4.
Years % growth t 2
1000–1700 0.12% 600 yr
1700–1870 0.41% 170 yr
1870–1950 0.82% 85 yr
1950–2020 1.70% 40 yr
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.