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Optimal Population?

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In his new book, Common Wealth [2], Jeffrey Sachs expresses his concern about population growth.  Worried by a U.N. prediction that global population will rise to 9.2 billion by the year 2050, from 6.6 billion today, Sachs says (on page 23 of his new book) the following about these additional 2.6 billion persons:

I will argue at some length that this is too many people to absorb safely, especially since most of the population increase is going to occur in today’s poorest countries.  We should be aiming….to stabilize the world’s population at 8 billion by midcentury.

(HT Karol Boudreaux)

Eight billion.  I’m not sure where Sachs got that number.  And, to be frank, I’m not curious about where he got it.  He could have dreamed it up in his sleep, or taken it from a multi-year study conducted by a lavishly funded committee made up of the world’s most accomplished economists, demographers, environmentalists, statisticians, physicians, and other Very Smart Experts.  No matter where the number comes from, it’s worthless.  There is simply no way to know how many persons the earth can "support" in the year 2050 (or any other year, for that matter).

First is the question: support at what standard of living?  Even if we grant the validity of the resources-are-very-tightly-limited supposition (upon which fear of population growth chiefly rests), there is no objective, scientifically determinable "optimal" number of people who can be alive at any one time.  According to the resources-are-very-tightly-limited supposition, the less that people consume, the greater are the amounts of resources that will be left for the future — the greater is the earth’s carrying capacity.  In this view, resources are simply ‘out there,’ waiting to be gathered, processed, and consumed by humans.  So more humans (or the same number of humans consuming more) will deplete resources faster than will fewer humans (or the same number of humans consuming less).

So on this resources-are-very-tightly-limited supposition, as people decrease their material standard of living, the earth can sustain a larger population.

How do we know today at what average standard of living persons alive in 2050 will seek to achieve?  We don’t.  It’s conceivable that the typical person alive in 2050 will have become so devoted to saving the earth that the prevalent culture and norms will dictate that most persons settle for material living standards lower than those that ordinary Americans enjoy today — or, perhaps even lower than ordinary Americans enjoyed in 1950.  If so, then surely the "optimal" global population in the year 2050 will be lower than it would be if most persons alive in 2050 will seek to achieve living standards much higher than ordinary Americans now enjoy.

A much deeper problem with Sachs’s eight-billion number is that, in calculating it, there is no way to predict how human creativity will alter the world during the next 42 years.  It’s ludicrous to pretend that we can know now what, say, the average MPG will be for internal-combustion engines in 2050.  Hell, we don’t even know if automobiles and lawnmowers and the like will still use such engines then.

Will another Norman Borlaug [3] arise, between now and 2050, to spark another green revolution?  Will someone invent a way to efficiently power automobiles with air?  Will someone develop new and better techniques for defining and enforcing private property rights in ocean-going fish stocks so that the tragedy of the commons called "over-fishing" is eliminated?  Will an enterprising entrepreneur invent a means for ordinary households to power their homes with mulch or autumn leaves or small fragments of fingernail clippings?

Think back 42 years to 1966.  Who in that year imagined personal computers in nearly every home in America?  The Internet?  Digital cameras?  Cell phones?  Quality wines sold in screw-top bottles?  Buying music with literally the click of a button (and not having to burn fossil fuels in driving to the record store).  Aluminum cans that contain only a fraction of the metal that cans contained back then?  The Kindle [4] (that will reduce the number of trees cut down to enable people to read books)?  Medical advances that make hip-replacements about as routine as getting cavities filled by the dentist?  Microfiber [5]?

There is no way — literally, no way — to know how technology and social institutions will change between now and 2050.  Given this impossibility — and given the fact that we can nevertheless predict with confidence that technology will advance and that social institutions will change — to assert that "optimal" population in the year 2050 will be eight-billion persons is ludicrous in the extreme.  It’s faux-science, and deserves only ridicule.