I think it is unhelpful to look at something like population in isolation from other deeply related issues, such as affluence, pollution per affluence, technology, unsustainable consumption of resources / production of pollution, changes in efficiency. For example, one simple way of modelling things is the `I=PAT` equation (see also the critique under "Reception") [0].
I'll quote from VHEMT [1]:
> For example, in terms of energy consumption,
> when a North American couple stops at two it’s
> about the same as an average East Indian couple
> stopping at 30, or a Bangaledesh couple stopping
> at 97.
Maybe energy consumption is a bit abstract - it is not immediately obvious what, if anything, is wrong with that. Instead we could consider per-capita CO2 pollution, taking the 2005 statistics from Wikipedia [2]:
United States: 19.3 tonnes CO2 / (person year)
Bangladesh: 0.3 tonnes CO2 / (person year)
From these numbers, a 1 additional US person has the same immediate short-term impact in terms of CO2 pollution as about 64 additional Bangladeshi people [2].
CO2 pollution produced by humans is primarily the cause of recent global warming. If the current trend in greenhouse gas pollution continues to track the high end of (conservative) IPCC baseline models [3] then it will not be surprising if we end up with a world that is +3.5C -- +4C -- or more higher than baseline, by the year 2100 (from memory, we're already at about +0.6C this year). It really doesn't look like we will limit the temperate increase to the "agreed 'safe'" +2C in the real world, outside of modelling scenarios[4].
It doesn't appear that the world has experienced a temperature of +4C for quite a long time (either hundreds of thousands or millions of years, depending on which graphs you look at)[5][6]. The last time it was that warm certainly pre-dates human agriculture (~12000 years ago) [7], and perhaps the human species (100s of thousands of years ago)[8]. I.e., we are rapidly pushing the climate toward a state where humans and human civilisation have never previously existed. The result will likely be unpleasant, in the sense that many people will suffer and die, due to heat stress, natural disasters, reduced crop yields, disease, and conflict caused by displaced people / resource shortages [9][10].
I have focused here on climate change in isolation, but that is just the first hurdle. Other serious concerns are human impacts upon biodiversity and the nitrogen cycle - and those are just the impacts that we're able to measure [11]. There's reasonable evidence that the world is currently experiencing a period of mass extinction [12].
So -- bringing it back to population. From the perspective of everyone else alive right now, assuming they are well informed and rational, should they think it is a good idea for me to have a child, particularly if I live in an affluent, polluting society[13]? I doubt it very much.
Of course, it isn't all grim. There is still time for us all to get better at being altruistic, thinking clearly, predicting the future, cooperating, changing our minds about what might be important, and modifying our environment (for better or worse). But, at best, it is obviously a race between positive and negative trends. There is not much time left. But, from historical evidence, so far the metrics of biodiversity and CO2 pollution are very much going in an undesirable direction, with no sign yet of change.
edit: I'll finish by quoting David MacKay out of context. He is talking about the UK's sustainable energy debate [14]:
> We need to choose a plan that adds up. It is possible
> to make a plan that adds up, but it’s not going to be
> easy.
>
> We need to stop saying no and start saying yes. We need
> to stop the Punch and Judy show and get building.
I pretty much agree with this sentiment in the realm of sustainable energy, and I'd also agree with the sentiment if it were extended much more broadly, beyond MacKay's intended context, to questions of population, affluence, equality, poverty, intergenerational fairness, and how economies should be run.
[0] https://en.wikipedia.org/wiki/I_%3D_PAT#Reception
[1] http://www.vhemt.org/demography.htm
[2] we're glossing over the fact that some of these additional people might choose to have children, or that annual per-capita CO2 pollution could change.
[3] For 2014 data, see figure 3 (a) of https://www.lakeheadu.ca/sites/default/files/uploads/53/outl...
[4] http://www.rtcc.org/2015/06/19/is-avoiding-2c-of-global-warm...
[5] http://earthobservatory.nasa.gov/Features/GlobalWarming/page...
[6] https://upload.wikimedia.org/wikipedia/commons/5/5f/All_pala...
[7] https://en.wikipedia.org/wiki/History_of_agriculture
[8] https://en.wikipedia.org/?title=Human
[9] http://www.worldbank.org/en/news/feature/2012/11/18/Climate-...
[10] http://www.eci.ox.ac.uk/4degrees/
[11] See figure 6 of: http://pdxscholar.library.pdx.edu/cgi/viewcontent.cgi?articl...
[12] http://www.researchgate.net/profile/Jenny_Mcguire/publicatio...
[13] assuming they don't have some kind of interest in driving down the cost of labour in the short term, to reduce the labour cost of producing this season's model of useless widget.
[14] http://www.withouthotair.com/c32/page_250.shtml
