The world's population should be at most 50 million.

[The Grand Nudger]

Let me lay out the basic case for population optimism.  It doesn't get much play, lol.  

Human beings are the ultimate human resource.  No other thing is as valuable to us as another thinking, problem solving human being.  There are never enough thinking, problem solving human beings working on any given problem.  You could always add another.  Historically, we've used population growth as a tool to combat resource availability issues, because population growth drives increased productivity.  This is the basis of a boserupian theory (the counter to malthusian theories).  Or, to use more generic terms, population pessimism and population optimism.  

Writing before the agricultural revolution..Malthus asserted that productivity (and specifically the productivity of land) was fixed, due to ag tech itself being fixed..which it largely was, in his pre-industrial timeframe.  An implication of that assumption was that a theoretical carrying capacity of any given chunk of rock could be derived.  His predictions did not come to pass for the simple fact that both the agricultural and industrial revolutions followed.  In truth, it began then, as a very important development in deep moldboard plows occurred in his lifetime.  His calculations, thusly..were off by orders of magnitude.  

Writing after the agricultural and industrial revolutions, chiefly interested in the history of agricultural intensification...Boserup noticed that the pull relationship that Malthus contentions depended on were also inaccurate.  Rather than population outstripping resource production..the reverse was true.  As population increased, resource production (real and potential) outstripped potential consumption, even though local shortages could be and were still common.  In effect, acknowledging that the data supported malthus contention at least insomuch as some present moment and some specific region, but denying that historical trends lent any credence to global malthusian effects.  

The pessimistic contention was rendered doa on account of being a failed prediction based upon shortsighted metrics and an inaccurate representation of the relationship between population and productivity in developing economies and methodologies.  That said.....nothing ensures that an optimistic theory will be true tomorrow, either.  

The pessimistic contention exploded again in the late 60's with the aptly titled book "The Population Bomb".  Warning us of mass starvation in the 70's and 80's.  Essentially rewarming the malthusian catastrophe argument from 1798.  It was, like it's predecessor..wrong, and wrong for the same reasons.  Unlike it's predecessor, however, it was more alarmist in it's tone and it's predictions were so immediate (and so immediately wrong) that it isn't looked at..today, with quite the same kindness that Malthus own interpretation was.  Nevertheless, it was this, more than anything else, that cemented the dire threat of overpopulation in the minds of the environmental movement.  This was reenforced in 09 when the erlichs stated that though they turned out to be wholly wrong, they still considered their argument fundamentally right..and that it had brought more people to the table of environmental consciousness..which was the ultimate goal.  To quote one of the most alarming statements, erlich wrote (and it was widely accepted at the time that he wrote it) that he couldn't see how india would be able to feed another 200million people by 1980.  It has since tripled it's population, and halved the ratio of malnourished citizens.  Corruption, incompetence, and global instability are now pointed to rather than overpopulation as the cause of local food shortage.  The optimistic contention was justified yet again in that exchange with the development of dwarf wheat coinciding with an intensification of productivity due to "surplus" population.  While this has been referred to as masked unemployment...ad that criticism has merit..it nevertheless entirely undercut the notion that india had a set carrying capacity, or that anyone knew what that was then or now.

Now, one of the stronger criticisms of optimistic population theory is that it may not accurately model relationships in a developed country.  The notion, again, being a rehashed malthusian catastrophe.  Those who offer this believe that while..yes, population increase can drive intensification and thus productivity, at some point the land has been "maximally intensified".  Innovation continues to prove this false on a yearly basis, but lets assume it were true.  Boserup had comments on this, while generally accepting that since the focus of her theory was development and intensification there may not be a one for one exchange in some other situation.  She noted, again, a historical trend of emigration.  A quick summary of her thoughts on the matter would be that in response to a local shortage, the population would breed labor and a deeper talent pool...drawing on this they would intensify to the limits of -their- ability...and upon reaching those limits, surplus population could and would emigrate to comparitively underdeveloped areas..leaving the native remnants in a decreasing state of fertility.  This has also turned out to be true in the developing (and now developed) world since the 50's.  Some localities, such as japan, have progressed so far along this path that they are now seeing sub replacement fertility levels, and a shrinking gdp, even as their base capacity metrics (by reference to some specific methodology) continue to rise.  The solution, for them..it's contended..is increased immigration - and immigrants have to come from somewhere, ultimately all immigrants come from the womb, lol. 

With all of that in mind, the newest face of population pessimism revolves around climate change.  Again the same assumptions are made..which have turned out to be both predictively and factual untrue time and time again.  Now, you'd think that people would get tired of predicting the apocalypse by any name...but, obviously, we don't.  It's an ancient pastime.  These advocates point to localized food shortage as justification for their assumptions..but as we discussed above, the failure of those assumptions and the identification of the cause of those shortages has since become a known know..and it's not overall population.  We could assume, like malthus in 1798..that "climate appropriate tech" is essentially fixed, in order to derive a theoretical carrying capacity.  We could assume, like the erlichs in the 60's, that nothing can be done to avert crisis beyond depopulation.

The trouble, is that these contentions have been consistently wrong for two centuries. How long do we have to wait, and how many times must it be proven wrong... before we let go of a bad idea? Specific methodologies have capacity limits. We have no idea what the limits of the earth are, and our demographic trends strongly suggest that whatever those are, if they exist, our own habits will effect us long before we reach any conceptual wall. This is what is meant by the statement "earths carrying capacity is practically limitless". It's not to say that there isn't a number of people you could stuff in the earth to finally fill it up, nut to butt....with no room for another - it's the acknowledgement that we have no reason to believe that our population will ever outstrip it's ability to secure resources..in practice. We will self reduce. We will get sick. We will start wars. We will come up with some New Way™ to do x y or z. We will emigrate as productive realities constrain or empower us. Evemn though we find ourselves with so many more people today..we find ourselves in exactly the same predicament Malthus was in. He never would have believed that the uk could carry a fraction of it's pop..and the amusing thing is that we know that it's at least -possible- for it to carry many more than it does today. Will this take a reorganization of our technologies and priorities..yes..but we've done so for all of our history as human beings. Adapt or die.

(I can identify the silent malthusian assumptions in your data above, if you like, but I wonder if you might be able to identify them? Can you think of a reason why those statements might be wrong, today...or tomorrow? I think, for example, that your characterization of innovation being able to "stretch" the numbers is wildly understated. Historically, they certainly have been. Presently...I described a system to you with twice the primary productivity, an additional waste stream of edible protein completely absent from conventional production, no site specifications, and no parity of environmental impact - that is what intensification looks like. I note that, just as we've done in the past, we'll need to increase our population in order to achieve that intensification. We could do it through reproduction or through immigration. It's only an idea until we can secure enough human capital and labor. So, seeing as how the numbers above are based on a better utilization of current ag, I'd contend that we could safely double that number, at least..and that's just today - for the simple reason that if all currently used conventional land was used for this or it;s equivalents, instead, that would be the takeaway -and that;s ignoring the fact that the systrem I described doesn;t actually need to use that productive land..like I said, you can turn a parking lot into a farm nowadays. Tomorrow, we might be able to go even bigger. Any hard cap on our pop due to food and water is so distant and so difficult to pin down, that it makes it seem silly, to me, to give such focus to that instead of directly addressing those issues towards which pop reduction is a very dubious proposal. It's wrong..and it's the wrong message for the environmental movement to send, if we want it to succeed. We need to weed out those bits of antique cultishness so often repeated that they're taken for fact, blindsiding those advocates who are unaware of mutually exclusive and more well evidenced positions. Population pessimism is just pessimism. It's not scientific, and it's not a fact, and it's not a solution. It has it's roots and it's motivations in a political ideology - and I'm not saying that the ideology is all bad, or that they don't make valid points within it, mind..I'm just trying to give you a clearer picture of the field. )