What will be the Earth’s maximum population?

The book by Joel E. Cohen, “How Many People Can the Earth Support?” in Appendix 3 (p.402-18) lists many authors’ answers to that unanswerable question. Perhaps a better question would be, looking back from the distant future after humanity has passed on into oblivion, “What was the maximum population of humans the Earth ever achieved?” Ultimately, that will be a very specific number which in fact may be known at some time in the not to distant future by people now living, whereas the book’s title has a huge number of imponderable variables built into its  question which can never be answered even after the fact. It was a poor question from a scientific point of view. Below is a list of various authors attempts to put a number to that question. Numbers are in billions. The current world population is approximately 6.8 billion.

Minimum	Maximum	Author	Year	Comment
Billion ~	Billion ~ 13.4	Leeuwenhoek	1679	First known estimate
6.3	12.5	King	1695
4.~	6.6	Sussmilch	1741
	13.9	Sussmilch	1765
	6.0	Ravenstein	1891
	8.1	Fircks	1898
	10.9	Pfaundler	1902
2.3	22.4	Ballod	1912
	132.~	Knibbs	1917
	5.2	Edward M. East	1924
	2.~	Pearl & Reed	1924
6.-	12.~	Wickens	1925
7.7	15.9	Penck	1925
	6.2	Fischer	1925
	5.7	Warren D. Smith	1935
	2.6	Pearl & Gould	1936
	13.3	Hollstein	1940
5.6	13.3	Boerman	1940
.9	2.8	Pearson & Harper	1945
7.0	8.6	Mukerjee	1946
	5.~	Salter	1946
6.5	10.~	Fawcett	1947
1.8	7.2	Spengler	1949
6.-	10~	C. Galton Darwin	1952
	50.-	Brown	1954
3.7	7.7	Brown, Bonner,W	1957
	28.~	Clark	1958
	30.~	Baade	1960
16.~	800.-	Kleiber	1961
	10×10 raised 18	Fremlin	1964	Heat dissipation limit
	10.~	Cepede	1964
	30.~	Schmitt	1965
	41.~	Zierhoffer	1966
47.~	157.~	Clark	1967	Americans/Japanese
79.-	1,022.-	De Wit	1967	Total usage
	1.-	Hulett	1970	American style
40.-	60.-	Austin & Brewer	1971
0.5	1.2	Ehrlich	1971	Permanent & stable
35	40.~	Muckenhausen	1973
	100.~	Lieth & Blaxter	1973
38.-	48.~	Revelle	1974
	6.7	Buringh	1975
5.~	7.~	Whittaker & Likens	1975
	40.~	Revelle	1976
	17	Eyre & Blaxter	1978
	1,000.~	Marchetti	1978
	14	Kovda	1980
	4.5	Mann	1981
2.0	3.9	Westing	1981
	12.~	Gates	1982
	7.5	Gilland	1979
4.0	32.8	Higgins	1983
	6.1	Ferrell, Sander, Vo	1984
	300	Hardin	1986
	22	Calvin  & Hudson	1986
9.8	19.3	Hudson	1989
2.8	5.5	Chen	1990
	5.3	Raven	1991
	7.7	Meadows	1992
	23.8	Tuckwell & Koziol	1992
	much less 5.5	Ehrlich	1993
12.-	14.~	Heilig	1993
	10+	Waggoner	1994
	3.~	Pimentel	1994
10.~	11.~	Smil	1994
11.~	44.~	Dutch Gov.	1994
—	—	—	—	—
	11.6	Daily & Ehrlich	1992
	~12.~	WikiAnswers	2008
	2.0	Pimentel	1999	At American style
	2.7	OPT	2008	Optimum Pop.

As you can see for yourself there is little agreement among these projections and that is why the question is flawed. If each of these researchers was asked “What is the maximum number the population the Earth would reach before retreating,” there would probably be a number much nearer Leeuwenhoek’s first prediction of 13.4 billion made back in 1697. He may have been looking through the first microscope but surely he had a reasonably clear distant vision also.

1. See also:
2. Population, Sustainability by Daily & Paul Ehrlich
3. How Many People Should the Earth Support? by Ross McCluney
4. Wikipedia article on – Overpopulation
   
5. World Prout Assembly gives a generalized smoothed version.
6. The chart above seems to indicate that the population will smoothly top out in 2040 and then smoothly drop back to a more sustainable number. This scenario would be possible if every person on the planet was reasonable but my modest experience has led to believe the exact opposite it closer to the truth.
7. 

   Doomsday population crash with food shortfall as a precursor

   Probaway’s Doomdsay chart above doesn’t predict an exact day for Doomsday or an exact population when that day comes but it does imply that the Earth’s population will be very near its maximum when that day does arrive.

8. 

Pick a number, any number at all from 0 to 1,000,000,000,000,000,000 for the maximum number that the Earth will support and there will be a scholarly supporter for that number. This is especially true if you fund a univiersity chair for that scholar.