Here's a shot at an upper limit (idea ripped off from an Isaac Asimov essay I read a long time ago--late '70s most likely, but I re-did the calculations myself since I couldn't remember his numbers).<p>Imagine all humanity packed into a very large sphere, with each human allocated a mere 1 cubic meter of space. As the population expands, we have to enlarge the sphere to make room.<p>Starting with the current population, how long can a 1% growth rate be sustained before the frontier of the human sphere would have to expand faster than the speed of light to make room for the new humans?<p>That's a pretty good hard upper bound on long 1% growth can be sustained. If my calculations are right, it's about 9000 years from now. (That fits with my recollection of Asimov's result).
One of the most annoying things things in science fiction is this plot line: the earth is running out of food, so several hundred thousand people get on a big ark ship to the nearest star. They then feed themselves in a few cubic kilometres simply using nuclear energy to power giant greenhouses.<p>Why couldn't they have done that on earth? Using nuclear power plants to desalinate water and power vertical greenhouses, back of the envelope calculations show the maximum population of the earth at somewhere around 1 TRILLION people.<p>Sure, that would make food more expensive, but given how dramatically the cost of food has dropped over time, we could deal with it.<p>For example, the price of a bushel of wheat has been within an order of magnitude of 1 UK pound for the last 1000 years. The price of everything else has increased over time, but wheat hasn't.
Even assuming a limit exists, without hard numbers this discussion is a waste of time. There is a huge practical difference between a limit of 8 billion and one of 20 billion; especially as these same UN population models, which the author mentions, predict that world population may stabilize, or even start declining, by 2050 (<a href="http://www.un.org/esa/population/publications/popnews/Newsltr_87.pdf" rel="nofollow">http://www.un.org/esa/population/publications/popnews/Newslt...</a>)
All developed nations are either right at replacement rates (USA) or well below (Japan, former Soviet nations etc). The <i>developing</i> nations (Thailand, Brazil, etc) are approaching replacement quickly. That leaves Africa and the poorer parts of large heterogeneous nations like India.<p>Some of us demographers think that as we continue to develop, population growth rates will go to under replacement and then sort of bounce around replacement after that due to social reasons.
Population <i>will</i> almost certainly run into hard limits before mid-century. The author points out resource limits in passing, but it's worse than he admits.<p>We run our civilization off of stored solar energy. That's what oil, natural gas and coal are. In a century we've used millions of years worth of stored solar energy. That's the energy that's powered the green revolution (along with non-renewable aquifers).<p>Nuclear and our small sources of renewable are the only exceptions. The rest of our power comes from stored energy. And nuclear/renewables are generally built using stored fossil energy.<p>Effectively, we're burning through capital, beyond a sustainable rate. If anyone has thought of a way out, please let me know.<p>Meanwhile, if you've been looking for something meaningful to build that will be in demand, this is it.
The practical limits are much more esoteric than Malthus imagined.<p>First limit: that population limit that some group of people with the power for force their beliefs on everyone else, thinks is ideal. Not like some group, of, oh, let's call them Elites, might decide the limit was radically less than the present population, and plan to use multiple means including mass contagion, war and economic collapse to force depopulation or anything. See: Georgia Guide Stones.<p>Second limit: The number of people alive at the point when some technological singularity occurs, making 'population' a moot concept.
For example: <a href="http://everist.org/texts/Fermis_Urbex_Paradox.txt" rel="nofollow">http://everist.org/texts/Fermis_Urbex_Paradox.txt</a>
This is bogus. Hate all this overpopulation nonsense. Every developed country in the world other than the US has a negative replacement rate (births - deaths). The developing world is rapidly slowly as well. If I remember right the world only has another 50 years or so of positive growth. Not only that but the demographic skew in the developing world in male-female will likely cause it to slow even faster. The slowing growth and shrinking populations will be the huge problem of this century because our social structure in its current form can't survive a shrinking population.
if to look at civilization development through energy harnessed/controlled, our civilization has been through the following stages:<p>1. at human body energy level by gathering/hunting (ie. what nature provided)<p>2. at human body energy level, managed/renewable. I.e. agriculture.<p>3. at industrial energy level by gathering - fossil fuels, nuclear fission.<p>we're at the end of the stage 3. with stage 4. "managed/renewable industrial level" (solar/wind and nuclear fusion) appearing on the horizon.<p>Increasing available energy levels allows for increased population growth. There is no limit in sight. The Moon and Mars are waiting :)
The only hard limits for human-style organisms on an earth-type planet are thermodynamic. Using 1970s technology, the earth could support hundreds of billions. e.g.:<p><a href="http://www.acceleratingfuture.com/michael/blog/2006/09/overpopulation-no-problem/" rel="nofollow">http://www.acceleratingfuture.com/michael/blog/2006/09/overp...</a><p>When you look at the simple figures that show that to be true, it somewhat puts the ridiculous debates over billions into context. i.e. they're not really debates over capacity and resources, they're debates over organization and (lack of) understanding the nature of progress and achievement.
If you take the title of this article at face value, why are we limiting ourselves to Earth? Seems like the limit of human population growth is likely at least a quadrillion based on current sky surveys and likely transportation mechanisms.