Population may not be the driving force behind many of the global world problems, but it’s certainly important: Basically, it is a multiplication factor for the environmental impact of certain actions. E.g. better environmental performance of some products has occasionally been offset by its much greater use (cf. population density). Of course, if a real innovation comes along, the environmental impact could be cut more drastically (which also happens, but counting on it may be risky).
The 20-80 story puts population in perspective: 20% of the world population uses approximately 80% of the worlds’ resources (dependent on the resource of course). That alone means that focusing on population isn’t where the shoe pinches in many cases: It’s the (over-)consumption in the rich areas that causes the most strain on the world’s resources.
On the other hand, I’ve understood that the reason that native cultures had relatively little impact on their environment is to a large extent due to their small population density. Burning a small piece of forest to use the land for food production may not be a great problem for the ecosystem if it only occurs sporadically, thereby not causing more disruption than the ecosystem can handle. It only becomes a problem when the magnitude increases above sustainable levels, which is intricately linked to population. There are plenty of examples in nature where too large numbers of a certain species causes stress on the ecosystem.
The Kaya identity shows that population is a multiplication factor, just as consumption is:
CO2 emission = population * GDP/capita * energy/GDP * CO2 emission/energy.
It would require a systemic analysis to see which factors are most responsible for a given problem, but it’s pretty clear that population is a factor that influences the total pressure on the system. The 80-20 ratio described above shows that consumption patterns by the rich cause the most strain on the world’s resources. I’d wager that the difference in consumption patterns between different parts of the world is (a lot) larger than the spread in population density, which would make the former most important. Population is not a factor that is easily or quickly influenced, but for the long term, it should be seriously considered as an important factor (especially because it has so much inertia).
Pointing fingers solely to, or firmly away from population, both misses the mark imho. reality is not black and white.
How many people the earth can sustain of course depends on the other factors in the various Kaya identities: If everyone were to have the consumption pattern of an average American, we would already have overshot the long term carrying capacity of the earth. If we all live a Buddhist lifestyle, we could probably do with a few more people. It’s a trade off, as always.
Don’t want to use (and pay for) sustainable energy (cf consumption pattern)? Then use less energy (cf population).
Don’t want to use less energy? Then use (and pay for) sustainable energy.
Don’t want to do either? Go find another planet.
This leads to a major moral dilemma: Developing nations also want to increase their material welfare, but them doing so by mimicking our current ways of production and consumption is a recipe for disaster. OTOH, we have no more moral right to the earth’s riches as they do. Something has to give, obviously.
See also this thought provoking article by Michael Tobis, where he takes on the other, even bigger taboo: economic growth. Bottom line:
A given economic growth rate can be sustainable only if the average impact per unit wealth declines at an equal or greater rate.
I.e. if the carbon and energy intensities decrease at least as fast as the GDP increases.
Attempting to reach equitable economic prosperity and allowing for normally projected increases in GPD and population, Tobis estimates that the impact per unit of wealth has to decrease roughly 50 fold by 2050.