From 6 billion to 500 million: unask the question!

From: Charlie Stross (
Date: Tue Feb 06 2001 - 06:22:10 MST

On Wed, Jan 31, 2001 at 04:36:20AM +0100, denis bider wrote:
> I think we all dream of - or maybe at least feel highly of - a world like
> Asimov described in Aurora: a whole planet divided into a relatively small
> number of large properties, each of them governed by virtually
> self-sustained hominids and their robots.
Uh, no I don't. I *like* living in an extremely dense city where I can
walk anywhere I need to go. However, for the sake of argument ...

> Earth could be a place like that, but... with 6 billion people? I don't
> know. Can the planet sustain 6 billion individuals with high energy/space
> needs?

One estimate I've seen is that the earth has roughly 49 million square
miles of inhabitable land (total surface area of 197 million square
miles). That's actually 127 million square kilometres (inhabitable)
and 510 million square kilometres (total, including sea).

So. If we take six billion people and spread them evenly across the entire
planetary surface (glug, glug :) we get ten people per square kilometre,
or roughly ten hectares per person. There are 2.47 acres per hectare, so
we're talking about a roughly 25 acre plot per person -- divide by four
for the sea surface, and we still have 6 acres each of habitable land.

Just one acre is a pretty damn big urban plot: where I live, it's about
one block -- with maybe eighty households living on it. So, although you
might *think* that a population density of ten people per square kilometre
everywhere is high, it's actually about three orders of magnitude lower
than a density that I find comfortable.

Human cities currently tend towards population densities in the
1000-10,000 person/km^2 range, without the need for high-density vertical
accomodation (except in the more dense areas). And an increasing
proportion of the world's population lives in such cities: I believe
that we may have a situation where most people *prefer* city life.

So, even given the option of a six acre plot of dry land, or 25 acres
including mostly deep ocean, I figure a large proportion (20-80%) of
the world population would prefer urban living -- albeit probably with
a bit more space and better amenities than they have today.

If 50% of the population accept city life, they end up occupying maybe
1% of the planetary surface -- leaving 11.9 dry acres of land to everybody
else, or fifty acres of ocean. This is a highly sensitive ratio: if 80%
want to live in cities (as seems the case, most places) the country-
dwellers get a farm-sized estate each.

If we extrapolate in the opposite direction, at a density of 10,000 people
per square kilometre (same order as downtown Hong Kong) we have a carrying
capacity of 1.27 trillion people on Earth -- before we move into vertical
arcologies that add at least another order of magnitude. At the trillion-
citizen point we have an increasing problem with heat dissipation: just our
metabolic functions will pump out getting on for 0.5 x 10^15 watts. If we
allow for extremely efficient nanosystems to keep us fed, clothed, housed,
and amused, we can probably stick another order of magnitude onto that --
assuming a person consumes 5Kw of power (this highly insulated northern
European household typically draws about 10-20Kw and doesn't live on a soup
of nanomachines or include food production) we have a figure of 5 x 10^15

To put this in perspective, if we cover the entire land area with solar
cells that are 100% efficient, half of it is in sunlight at any time
-- so we have roughly 65 million km^2 of solar panels. Assuming 100
watts/square metre reaches ground level (a bit high, I think -- anyone
got the figures?) this gives us 6.5 x 10^15 watts of insolation.

So, with a population of 1.2 trillion people (200 times our current level)
we have just hit the buffers using solar power and a population density
no higher than a current-era highly affluent city (Hong Kong's quality of
life being not too dissimilar to that in urban Japan, in material terms).

I therefore conclude that there is *no* requirement to go for massive
population reduction. The earth can carry 200 times as many people before
things get sticky enough to require us to disrupt the planetary energy
budget and deal with our own waste heat, or overcrowd ourselves relative
to current high end population densities. Even with the current population,
there's no reason why everybody who wants it can't have a generous plot of

-- Charlie

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