My own opinion: i don't know the limit, but there definitely is one. And i'm not sure if we're preparing for reacting the right way if that happens (or has happened already).
That article focuses on climate change and per-capita CO2 pollution, making the point that it isn't merely population size that is the problem, but population size * per-capita environmental impact.
Climate change is the number one example of a planetary boundary that has been crossed from the "safe operating space for humanity" into an unsafe zone (Planetary boundaries, Rockström et al [1]).
One way of framing environmental impact is by the equation
I = PAT
where I is environmental impact, P is population, A is "affluence" (measured in affluence-units per capita), and T is "technology" (measured in environmental-impact-units per affluence-unit) [2]. This equation is fairly artificial in that it is an accounting identity, you start with I = I and pick some variables to factor the right hand side with. But it gives one way to start thinking about the problem.
Since this is hacker news, probably the factor people get excited about improving is the T factor, labelled "technology" -- i.e. reducing the pollution intensity of each unit of affluence -- think energy efficiency etc. Reducing the T factor is compatible with capitalism, compatible with a mindset that all problems have a technical fix, doesn't require talking about explicitly limiting anyone's freedom re: the P or A factors. On another hand, just focusing on improving efficiency doesn't necessarily result in an overall lower environmental impact once the whole economic system reacts (c.f. Jevon's paradox [3]). If you reduce T maybe P or A shoot up, giving you a greater I.
On a small scale, if you sit down and crunch the numbers, and focus on a time horizon of 100 years or so, you could ask which family & their descendants contributes the most CO2 pollution:
(a) a family in a low natural population growth, high CO2-pollution society; or
(b) a family in a high natural population growth, low CO2-pollution society .
At extremes, comparing e.g. (a) a couple from Australia (15.4 tons CO2/capita) with (b) a couple from Bangladesh (0.5 tons CO2/capita), the environmental impact of the relatively low-population-growth, high-per capita pollution couple is FAR worse, even if you say that all CO2 emissions of children, grandchildren, great-grandchildren etc over the time horizon are attributed to the original couple [4]. So in my mind, at least in the short run of 100 years, population isn't the concern, it's the population of individuals in high-pollution countries with high-pollution lifestyles (like myself).
On the other hand, there's nothing really special about climate change, if global human population continues to grow, if we figure out how to avoid or mitigate climate change, the interesting thing to watch will be how we deal with crossing the second global ecological boundary, or the third...
http://www.bbc.com/earth/story/20160311-how-many-people-can-...
My own opinion: i don't know the limit, but there definitely is one. And i'm not sure if we're preparing for reacting the right way if that happens (or has happened already).